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NAME | SYNOPSIS | DESCRIPTION | DOCUMENTATION OVERVIEW | OPTIONS | ARGUMENTS | SCRIPT LANGUAGE | PROCESSING | EXAMPLES | CACHING | SAFETY AND SECURITY | UNPRIVILEGED USERS | ALTERNATE RUNTIMES | EXIT STATUS | DEPRECATION | FILES | SEE ALSO | BUGS | COLOPHON |
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STAP(1) General Commands Manual STAP(1)
stap - systemtap script translator/driver
stap [ OPTIONS ] FILENAME [ ARGUMENTS ]
stap [ OPTIONS ] - [ ARGUMENTS ]
stap [ OPTIONS ] -e SCRIPT [ ARGUMENTS ]
stap [ OPTIONS ] -l PROBE [ ARGUMENTS ]
stap [ OPTIONS ] -L PROBE [ ARGUMENTS ]
stap [ OPTIONS ] --dump-probe-types
stap [ OPTIONS ] --dump-probe-aliases
stap [ OPTIONS ] --dump-functions
The stap program is the front-end to the Systemtap tool. It
accepts probing instructions written in a simple domain-specific
language, translates those instructions into C code, compiles this
C code, and loads the resulting module into a running Linux kernel
or a Dyninst user-space mutator, to perform the requested system
trace/probe functions. You can supply the script in a named file
(FILENAME), from standard input (use - instead of FILENAME), or
from the command line (using -e SCRIPT). The program runs until
it is interrupted by the user, or if the script voluntarily
invokes the exit() function, or by sufficient number of soft
errors.
The language, which is described the SCRIPT LANGUAGE section
below, is strictly typed, expressive, declaration free,
procedural, prototyping-friendly, and inspired by awk and C. It
allows source code points or events in the system to be associated
with handlers, which are subroutines that are executed
synchronously. It is somewhat similar conceptually to "breakpoint
command lists" in the gdb debugger.
systemtap comes with a variety of educational, documentation and
reference resources. They come online and/or packaged for offline
use. Some systemtap diagnostic warning/error messages specially
suggest reading a man page by including a string like
[man error::pass5]. For online documentation, see the project web
site, https://sourceware.org/systemtap/
┌───────────────────────────┬──────────────────────────────────────────────────────┐
│ man pages │ │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ stap (this page) │ language syntax, concepts, operation, options │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ error::* │ further explanation of error conditions │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ warning::* │ further explanation of warning conditions │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ stapprobes │ probe points and their $context variables │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ stapref │ quick reference to language syntax │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ stappaths │ list of directories, including books & references │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ stap-prep │ program to install auxiliary dependencies like ker‐ │
│ │ nel debuginfo │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ tapset::* │ generated list of tapsets │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ probe::* │ generated list of tapset probe aliases │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ function::* │ generated list of tapset functions │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ macro::* │ generated list of tapset macros │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ stapvars │ some of the tapset global variables │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ staprun, stapdyn, stapbpf │ programs for executing compiled systemtap scripts │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ systemtap │ initscript, boot-time probing │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ stap-server │ compilation server │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ stapex │ a few very basic script examples │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ books │ │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ Beginner's Guide │ tutorial book, language essentials, examples │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ Tutorial │ shorter tutorial, exercises │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ Language Reference │ detailed language manual, covers statistics/analysis │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ Tapset Reference │ the tapset man pages, reformatted into a book │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ references │ │
├───────────────────────────┼──────────────────────────────────────────────────────┤
│ example scripts │ over a hundred directly usable sysadmin tools, toys, │
│ │ hacks to learn from │
└───────────────────────────┴──────────────────────────────────────────────────────┘
The systemtap translator supports the following options. Any oth‐
er option prints a list of supported options. Options may be giv‐
en on the command line, as usual. If the file $SYSTEMTAP_DIR/rc
exist, options are also loaded from there and interpreted first.
($SYSTEMTAP_DIR defaults to $HOME/.systemtap if unset.)
In some cases, the default value of an option depends on particu‐
lar system configuration and thus can't be mentioned here direct‐
ly. In some of those cases running "stap --help" might display
the default.
- Use standard input instead of a given FILENAME as probe
language input, unless -e SCRIPT is given.
-h --help
Show help message.
-V --version
Show version message.
-p NUM Stop after pass NUM. The passes are numbered 1-5: parse,
elaborate, translate, compile, run. See the PROCESSING
section for details.
-v Increase verbosity for all passes. Produce a larger volume
of informative (?) output each time option repeated.
--vp ABCDE
Increase verbosity on a per-pass basis. For example,
"--vp 002" adds 2 units of verbosity to pass 3 only. The
combination "-v --vp 00004" adds 1 unit of verbosity for
all passes, and 4 more for pass 5.
-k Keep the temporary directory after all processing. This
may be useful in order to examine the generated C code, or
to reuse the compiled kernel object.
-g Guru mode. Enable parsing of unsafe expert-level con‐
structs like embedded C.
-P Prologue-searching mode. This is equivalent to --pro‐
logue-searching=always. Activate heuristics to work around
incorrect debugging information for function parameter
$context variables.
-u Unoptimized mode. Disable unused code elision and many
other optimizations during elaboration / translation.
-w Suppressed warnings mode. Disables all warning messages.
-W Treat all warnings as errors.
-b Use bulk mode (percpu files) for kernel-to-user data trans‐
fer. Use the stap-merge program to multiplex them back to‐
gether later.
-i --interactive
Interactive mode. Enable an interface to build the system‐
tap script incrementally and interactively.
-t Collect timing information on the number of times probe ex‐
ecutes and average amount of time spent in each probe-
point. Also shows the derivation for each probe-point.
-s NUM Use NUM megabyte buffers for kernel-to-user data transfer
per processor. The default is 16MB, or less on smaller
memory machines.
-I DIR Add the given directory to the tapset search directory.
See the description of pass 2 for details.
-D NAME=VALUE
Add the given C preprocessor directive to the module Make‐
file. These can be used to override limit parameters de‐
scribed below.
-B NAME=VALUE
In kernel-runtime mode, add the given make directive to the
kernel module build's make invocation. These can be used
to add or override kconfig options. For example, use
-B CONFIG_DEBUG_INFO=y
to add debugging information.
-B FLAG
In dyninst-runtime mode, add the given parameter to the
compiler CFLAGS used for building the dyninst shared li‐
brary. For example, use
-B -g
to add debugging information.
-a ARCH
Use a cross-compilation mode for the given target architec‐
ture. This requires access to the cross-compiler and the
kernel build tree, and goes along with the
-B CROSS_COMPILE=arch-tool-prefix-
and
-r /build/tree
options.
--modinfo NAME=VALUE
Add the name/value pair as a MODULE_INFO macro call to the
generated module. This may be useful to inform or override
various module-related checks in the kernel.
-G NAME=VALUE
Sets the value of global variable NAME to VALUE when
staprun is invoked. This applies to scalar variables de‐
clared global in the script/tapset.
-R DIR Look for the systemtap runtime sources in the given direc‐
tory. Your DIR default can be seen using "stap --help".
-r /DIR
Build for kernel in given build tree. Can also be set with
the SYSTEMTAP_RELEASE environment variable.
-r RELEASE
Build for kernel in build tree /lib/modules/RELEASE/build.
Can also be set with the SYSTEMTAP_RELEASE environment
variable.
-m MODULE
Use the given name for the generated kernel object module,
instead of a unique randomized name. The generated kernel
object module is copied to the current directory.
-d MODULE
Add symbol/unwind information for the given module into the
kernel object module. This may enable symbolic tracebacks
from those modules/programs, even if they do not have an
explicit probe placed into them.
--ldd Add symbol/unwind information for all user-space shared li‐
braries suspected by ldd to be necessary for user-space bi‐
naries being probed or listed with the -d option. Caution:
this can make the probe modules considerably larger. Note
that this option does not deal with kernel-space modules:
see instead --all-modules below.
--all-modules
Equivalent to specifying "-dkernel" and a "-d" for each
kernel module that is currently loaded. Caution: this can
make the probe modules considerably larger.
-o FILE
Send standard output to named file. In bulk mode, percpu
files will start with FILE_ (FILE_cpu with -F) followed by
the cpu number. This supports strftime(3) formats for
FILE.
-c CMD Start the probes, run CMD, and exit when CMD finishes.
This also has the effect of setting target() to the pid of
that process. Note that many probe types trigger indepen‐
dently of this setting. Consider including something like
this to focus your script.
probe FOO { if (pid() != target()) next; .... }
-x PID Sets target() to PID. The script runs independently of the
PID's lifespan.
-e SCRIPT
Run the given SCRIPT specified on the command line.
-E SCRIPT
Run the given SCRIPT specified. This SCRIPT is run in addi‐
tion to the main script specified, through -e, or as a
script file. This option can be repeated to run multiple
scripts, and can be used in listing mode (-l/-L).
-l PROBE
Instead of running a probe script, just list all available
probe points matching the given single probe point. The
pattern may include wildcards and aliases, but not comma-
separated multiple probe points. The process result code
will indicate failure if there are no matches.
% stap -e 'probe syscall.* { }'
[...]
% stap -l 'syscall.*'
syscall.accept
[...]
syscall.writev
-L PROBE
Similar to "-l", but list matching probe points plus their
available context variables. When -v is set with -L, the
output includes duplicate probe points which are distin‐
guished by their PC address.
% stap -L 'process("/lib64/libpython*.so.*").mark("*")'
process("/usr/lib64/libpython2.7.so.1.0").mark("function__entry") $arg1:long $arg2:long $arg3:long
process("/usr/lib64/libpython2.7.so.1.0").mark("function__return") $arg1:long $arg2:long $arg3:long
process("/usr/lib64/libpython3.6m.so.1.0").mark("function__entry") $arg1:long $arg2:long $arg3:long
process("/usr/lib64/libpython3.6m.so.1.0").mark("function__return") $arg1:long $arg2:long $arg3:long
process("/usr/lib64/libpython3.6m.so.1.0").mark("gc__done") $arg1:long
process("/usr/lib64/libpython3.6m.so.1.0").mark("gc__start") $arg1:long
process("/usr/lib64/libpython3.6m.so.1.0").mark("line") $arg1:long $arg2:long $arg3:long
-F Without -o option, load module and start probes, then de‐
tach from the module leaving the probes running. With -o
option, run staprun in background as a daemon and show its
pid.
-S size[,N]
Sets the maximum size of output file and the maximum number
of output files. If the size of output file will exceed
size megabytes, systemtap switches output file to the next
file. And if the number of output files exceed N , system‐
tap removes the oldest output file. You can omit the second
argument.
-T TIMEOUT
Exit the script after TIMEOUT seconds.
--skip-badvars
Ignore unresolvable or run-time-inaccessible context vari‐
ables and substitute with 0, without errors.
--prologue-searching[=WHEN]
Prologue-searching mode. Activate heuristics to work around
incorrect debugging information for function parameter
$context variables. WHEN can be either "never", "always",
or "auto" (i.e. enabled by heuristic). If WHEN is missing,
then "always" is assumed. If the option is missing, then
"auto" is assumed.
--suppress-handler-errors
Wrap all probe handlers into something like this
try { ... } catch { next }
block, which causes any runtime errors to be quietly sup‐
pressed. Suppressed errors do not count against MAXERRORS
limits. In this mode, the MAXSKIPPED limits are also sup‐
pressed, so that many errors and skipped probes may be ac‐
cumulated during a script's runtime. Any overall counts
will still be reported at shutdown.
--compatible VERSION
Suppress recent script language or tapset changes which are
incompatible with given older version of systemtap. This
may be useful if a much older systemtap script fails to
run. See the DEPRECATION section for more details.
--check-version
This option is used to check if the active script has any
constructs that may be systemtap version specific. See the
DEPRECATION section for more details.
--clean-cache
This option prunes stale entries from the cache directory.
This is normally done automatically after successful runs,
but this option will trigger the cleanup manually and then
exit. See the CACHING section for more details about cache
limits.
--color[=WHEN], --colour[=WHEN]
This option controls coloring of error messages. WHEN can
be either "never", "always", or "auto" (i.e. enable only if
at a terminal). If WHEN is missing, then "always" is as‐
sumed. If the option is missing, then "auto" is assumed.
Colors can be modified using the SYSTEMTAP_COLORS environ‐
ment variable. The format must be of the form
key1=val1:key2=val2:key3=val3 ...etc. Valid keys are
"error", "warning", "source", "caret", and "token". Values
constitute Select Graphic Rendition (SGR) parameter(s).
Consult the documentation of your terminal for the SGRs it
supports. As an example, the default colors would be ex‐
pressed as
error=01;31:warning=00;33:source=00;34:caret=01:token=01.
If SYSTEMTAP_COLORS is absent, the default colors will be
used. If it is empty or invalid, coloring is turned off.
--disable-cache
This option disables all use of the cache directory. No
files will be either read from or written to the cache.
--poison-cache
This option treats files in the cache directory as invalid.
No files will be read from the cache, but resulting files
from this run will still be written to the cache. This is
meant as a troubleshooting aid when stap's cached behavior
seems to be misbehaving. If it helped, there is a probably
a bug in systemtap that the developers would like you to
report.
--privilege[=stapusr | =stapsys | =stapdev]
This option instructs stap to examine the script looking
for constructs which are not allowed for the specified
privilege level (see UNPRIVILEGED USERS). Compilation
fails if any such constructs are used. If stapusr or stap‐
sys are specified when using a compile server (see
--use-server), the server will examine the script and, if
compilation succeeds, the server will cryptographically
sign the resulting kernel module, certifying that is it
safe for use by users at the specified privilege level.
If --privilege has not been specified, -pN has not been
specified with N < 5, and the invoking user is not root,
and is not a member of the group stapdev, then stap will
automatically add the appropriate --privilege option to the
options already specified.
--unprivileged
This option is equivalent to --privilege=stapusr.
--use-server[=HOSTNAME[:PORT] | =IP_ADDRESS[:PORT] | =CERT_SERIAL]
Specify compile-server(s) to be used for compilation and/or
in conjunction with --list-servers and --trust-servers (see
below) for listing. If no argument is supplied, then the
default in unprivileged mode (see --privilege) is to select
compatible servers which are trusted as SSL peers and as
module signers and currently online. Otherwise the default
is to select compatible servers which are trusted as SSL
peers and currently online. --use-server may be specified
more than once, in which case a list of servers is accumu‐
lated in the order specified. Servers may be specified by
host name, ip address, or by certificate serial number (ob‐
tained using --list-servers). The latter is most commonly
used when adding or revoking trust in a server (see
--trust-servers below). If a server is specified by host
name or ip address, then an optional port number may be
specified. This is useful for accessing servers which are
not on the local network or to specify a particular server.
IP addresses may be IPv4 or IPv6 addresses.
If a particular IPv6 address is link local and exists on
more than one interface, the intended interface may be
specified by appending the address with a percent sign (%)
followed by the intended interface name. For example,
"fe80::5eff:35ff:fe07:55ca%eth0".
In order to specify a port number with an IPv6 address, it
is necessary to enclose the IPv6 address in square brackets
([]) in order to separate the port number from the rest of
the address. For example,
"[fe80::5eff:35ff:fe07:55ca]:5000" or
"[fe80::5eff:35ff:fe07:55ca%eth0]:5000".
If --use-server has not been specified, -pN has not been
specified with N < 5, and the invoking user not root, is
not a member of the group stapdev, but is a member of the
group stapusr, then stap will automatically add --use-serv‐
er to the options already specified.
--use-server-on-error[=yes|=no]
Instructs stap to retry compilation of a script using a
compile server if compilation on the local host fails in a
manner which suggests that it might succeed using a server.
If this option is not specified, the default is no. If no
argument is provided, then the default is yes. Compilation
will be retried for certain types of errors (e.g. insuffi‐
cient data or resources) which may not occur during re-com‐
pilation by a compile server. Compile servers will be se‐
lected automatically for the re-compilation attempt as if
--use-server was specified with no arguments.
--list-servers[=SERVERS]
Display the status of the requested SERVERS, where SERVERS
is a comma-separated list of server attributes. The list of
attributes is combined to filter the list of servers dis‐
played. Supported attributes are:
all specifies all known servers (trusted SSL peers,
trusted module signers, online servers).
specified
specifies servers specified using --use-server.
online filters the output by retaining information about
servers which are currently online.
trusted
filters the output by retaining information about
servers which are trusted as SSL peers.
signer filters the output by retaining information about
servers which are trusted as module signers (see
--privilege).
compatible
filters the output by retaining information about
servers which are compatible with the current kernel
release and architecture.
If no argument is provided, then the default is specified.
If no servers were specified using --use-server, then the
default servers for --use-server are listed.
Note that --list-servers uses the avahi-daemon service to
detect online servers. If this service is not available,
then --list-servers will fail to detect any online servers.
In order for --list-servers to detect servers listening on
IPv6 addresses, the avahi-daemon configuration file
/etc/avahi/avahi-daemon.conf must contain an active "use-
ipv6=yes" line. The service must be restarted after adding
this line in order for IPv6 to be enabled.
--trust-servers[=TRUST_SPEC]
Grant or revoke trust in compile-servers, specified using
--use-server as specified by TRUST_SPEC, where TRUST_SPEC
is a comma-separated list specifying the trust which is to
be granted or revoked. Supported elements are:
ssl trust the specified servers as SSL peers.
signer trust the specified servers as module signers (see
--privilege). Only root can specify signer.
all-users
grant trust as an ssl peer for all users on the lo‐
cal host. The default is to grant trust as an ssl
peer for the current user only. Trust as a module
signer is always granted for all users. Only root
can specify all-users.
revoke revoke the specified trust. The default is to grant
it.
no-prompt
do not prompt the user for confirmation before car‐
rying out the requested action. The default is to
prompt the user for confirmation.
If no argument is provided, then the default is ssl. If no
servers were specified using --use-server, then no trust
will be granted or revoked.
Unless no-prompt has been specified, the user will be
prompted to confirm the trust to be granted or revoked be‐
fore the operation is performed.
--sign-module
Sign the module with a MOK (Machine Owner Key) on UEFI/Se‐
cureBoot systems. See the SECUREBOOT section for more de‐
tails.
--dump-probe-types
Dumps a list of supported probe types and exits. If --priv‐
ilege=stapusr is also specified, the list will be limited
to probe types available to unprivileged users.
--dump-probe-aliases
Dumps a list of all probe aliases found in library files
and exits.
--dump-functions
Dumps a list of all the public functions found in library
files and exits. Also includes their parameters and types.
A function of type 'unknown' indicates a function that does
not return a value. Note that not all function/parameter
types may be resolved (these are also shown by 'unknown').
This features is very memory-intensive and thus may not
work properly with --use-server if the target server impos‐
es an rlimit on process memory (i.e. through the ~stap-
server/.systemtap/rc configuration file, see
stap-server(8)).
--remote URL
Set the execution target to the given host. This option
may be repeated to target multiple execution targets.
Passes 1-4 are completed locally as normal to build the
script, and then pass 5 will copy the module to the target
and run it. Acceptable URL forms include:
[USER@]HOSTNAME, ssh://[USER@]HOSTNAME
This mode uses ssh, optionally using a username not
matching your own. If a custom ssh_config file is in
use, add SendEnv LANG to retain internationalization
functionality.
libvirt://DOMAIN, libvirt://DOMAIN/LIBVIRT_URI
This mode uses stapvirt to execute the script on a
domain managed by libvirt. Optionally, LIBVIRT_URI
may be specified to connect to a specific driver
and/or a remote host. For example, to connect to the
local privileged QEMU driver, use:
--remote libvirt://MyDomain/qemu:///system
See the page at <http://libvirt.org/uri.html> for
supported URIs. Also see stapvirt(1) for more infor‐
mation on how to prepare the domain for stap prob‐
ing.
unix:PATH
This mode connects to a UNIX socket. This can be
used with a QEMU virtio-serial port for executing
scripts inside a running virtual machine.
direct://
Special loopback mode to run on the local host.
--remote-prefix
Prefix each line of remote output with "N: ", where N is
the index of the remote execution target from which the
given line originated.
--download-debuginfo[=OPTION]
Enable, disable or set a timeout for the automatic debugin‐
fo downloading feature offered by abrt as specified by OP‐
TION, where OPTION is one of the following:
yes enable automatic downloading of debuginfo with no
timeout. This is the same as not providing an OPTION
value to --download-debuginfo
no explicitly disable automatic downloading of debugin‐
fo. This is the same as not using the option at all.
ask show abrt output, and ask before continuing down‐
load. No timeout will be set.
<timeout>
specify a timeout as a positive number to stop the
download if it is taking longer than <timeout> sec‐
onds.
--rlimit-as=NUM
Specify the maximum size of the process's virtual memory
(address space), in bytes.
--rlimit-cpu=NUM
Specify the CPU time limit, in seconds.
--rlimit-nproc=NUM
Specify the maximum number of processes that can be creat‐
ed.
--rlimit-stack=NUM
Specify the maximum size of the process stack, in bytes.
--rlimit-fsize=NUM
Specify the maximum size of files that the process may cre‐
ate, in bytes.
--sysroot=DIR
Specify sysroot directory where target files (executables,
libraries, etc.) are located. With -r RELEASE, the sys‐
root will be searched for the appropriate kernel build di‐
rectory. With -r /DIR, however, the sysroot will not be
used to find the kernel build.
--sysenv=VAR=VALUE
Provide an alternate value for an environment variable
where the value on a remote system differs. Path variables
(e.g. PATH, LD_LIBRARY_PATH) are assumed to be relative to
the directory provided by --sysroot, if provided.
--suppress-time-limits
Disable -DSTP_OVERLOAD related options as well as -DMAXAC‐
TION and -DMAXTRYLOCK. This option requires guru mode.
--runtime=MODE
Set the pass-5 runtime mode. Valid options are kernel (de‐
fault), dyninst and bpf. See ALTERNATE RUNTIMES below for
more information.
--dyninst
Shorthand for --runtime=dyninst.
--bpf Shorthand for --runtime=bpf.
--save-uprobes
On machines that require SystemTap to build its own uprobes
module (kernels prior to version 3.5), this option in‐
structs SystemTap to also save a copy of the module in the
current directory (creating a new "uprobes" directory
first).
--target-namespaces=PID
Allow for a set of target namespaces to be set based on the
namespaces the given PID is in. This is for namespace-aware
tapset functions. If the target namespaces was not set, the
target defaults to the stap process' namespaces.
--monitor=INTERVAL
Enables an interface to display status information about
the module(uptime, module name, invoker uid, memory sizes,
global variables, list of probes with their statistics). An
optional argument INTERVAL can be supplied to set the re‐
fresh rate in seconds of the status window. The module can
also be controlled by a list of commands using the follow‐
ing keys:
c Resets all global variables to their initial values
or zeroes them if they did not have an initial val‐
ue.
s Rotates the attribute used to sort the list of
probes.
t Brings up a prompt to allow toggling(on/off) of
probes by index. Probe points are still affected by
their conditions.
r Resumes the script by toggling on all probes.
p Pauses the script by toggling off all probes.
x Hides/shows the status window. This allows for more
output to be seen.
navigation-keys
The navigation keys can be used to scroll up and
down the windows.
Tab Toggle scrolling between status and output windows.
--example
This option is used to run example scripts without having
to enter the entire path to the script. Example scripts can
be found in the directory specified in the stappaths(7)
manual page.
--no-global-var-display
This option is used to disable the automatic logging of un‐
used global variables at the end of a stap session.
--language-server
Language server mode. Start a language server which will
communicate via stdio. The language server will respect
stap verbosity.
--build-as
Execute passes 1-4 under specified user to increase safety.
This approach may have limitations. Unprivileged users
might not have access to kallsyms or /proc. At the RPM in‐
stall time, the stapunpriv user is created, so stap can be
invoked with --build-as=stapunpriv.
Any additional arguments on the command line are passed to the
script parser for substitution. See below.
The systemtap script language resembles awk and C. There are two
main outermost constructs: probes and functions. Within these,
statements and expressions use C-like operator syntax and prece‐
dence.
GENERAL SYNTAX
Whitespace is ignored. Three forms of comments are supported:
# ... shell style, to the end of line, except for $# and @#
// ... C++ style, to the end of line
/* ... C style ... */
Literals are either strings enclosed in double-quotes (passing
through the usual C escape codes with backslashes, and with adja‐
cent string literals glued together, also as in C), or integers
(in decimal, hexadecimal, or octal, using the same notation as in
C). All strings are limited in length to some reasonable value (a
few hundred bytes). Integers are 64-bit signed quantities, al‐
though the parser also accepts (and wraps around) values above
positive 2**63.
In addition, script arguments given at the end of the command line
may be inserted. Use $1 ... $<NN> for insertion unquoted, @1 ...
@<NN> for insertion as a string literal. The number of arguments
may be accessed through $# (as an unquoted number) or through @#
(as a quoted number). These may be used at any place a token may
begin, including within the preprocessing stage. Reference to an
argument number beyond what was actually given is an error.
PREPROCESSING
A simple conditional preprocessing stage is run as a part of pars‐
ing. The general form is similar to the cond ? exp1 : exp2
ternary operator:
%( CONDITION %? TRUE-TOKENS %)
%( CONDITION %? TRUE-TOKENS %: FALSE-TOKENS %)
The CONDITION is either an expression whose format is determined
by its first keyword, or a string literals comparison or a numeric
literals comparison. It can be also composed of many alternatives
and conjunctions of CONDITIONs (meant as in previous sentence) us‐
ing || and && respectively. However, parentheses are not support‐
ed yet, so remembering that conjunction takes precedence over al‐
ternative is important.
If the first part is the identifier kernel_vr or kernel_v to refer
to the kernel version number, with ("2.6.13-1.322FC3smp") or with‐
out ("2.6.13") the release code suffix, then the second part is
one of the six standard numeric comparison operators <, <=, ==,
!=, >, and >=, and the third part is a string literal that con‐
tains an RPM-style version-release value. The condition is deemed
satisfied if the version of the target kernel (as optionally over‐
ridden by the -r option) compares to the given version string.
The comparison is performed by the glibc function strverscmp. As
a special case, if the operator is for simple equality (==), or
inequality (!=), and the third part contains any wildcard charac‐
ters (* or ? or [), then the expression is treated as a wildcard
(mis)match as evaluated by fnmatch.
If, on the other hand, the first part is the identifier arch to
refer to the processor architecture (as named by the kernel build
system ARCH/SUBARCH), then the second part is one of the two
string comparison operators == or !=, and the third part is a
string literal for matching it. This comparison is a wildcard
(mis)match.
Similarly, if the first part is an identifier like CONFIG_some‐
thing to refer to a kernel configuration option, then the second
part is == or !=, and the third part is a string literal for
matching the value (commonly "y" or "m"). Nonexistent or unset
kernel configuration options are represented by the empty string.
This comparison is also a wildcard (mis)match.
If the first part is the identifier systemtap_v, the test refers
to the systemtap compatibility version, which may be overridden
for old scripts with the --compatible flag. The comparison opera‐
tor is as is for kernel_v and the right operand is a version
string. See also the DEPRECATION section below.
If the first part is the identifier systemtap_privilege, the test
refers to the privilege level that the systemtap script is com‐
piled with. Here the second part is == or !=, and the third part
is a string literal, either "stapusr" or "stapsys" or "stapdev".
If the first part is the identifier guru_mode, the test refers to
if the systemtap script is compiled with guru_mode. Here the sec‐
ond part is == or !=, and the third part is a number, either 1 or
0.
If the first part is the identifier runtime, the test refers to
the systemtap runtime mode. See ALTERNATE RUNTIMES below for more
information on runtimes. The second part is one of the two string
comparison operators == or !=, and the third part is a string lit‐
eral for matching it. This comparison is a wildcard (mis)match.
Otherwise, the CONDITION is expected to be a comparison between
two string literals or two numeric literals. In this case, the
arguments are the only variables usable.
The TRUE-TOKENS and FALSE-TOKENS are zero or more general parser
tokens (possibly including nested preprocessor conditionals), and
are passed into the input stream if the condition is true or
false. For example, the following code induces a parse error un‐
less the target kernel version is newer than 2.6.5:
%( kernel_v <= "2.6.5" %? **ERROR** %) # invalid token sequence
The following code might adapt to hypothetical kernel version
drift:
probe kernel.function (
%( kernel_v <= "2.6.12" %? "__mm_do_fault" %:
%( kernel_vr == "2.6.13*smp" %? "do_page_fault" %:
UNSUPPORTED %) %)
) { /* ... */ }
%( arch == "ia64" %?
probe syscall.vliw = kernel.function("vliw_widget") {}
%)
PREPROCESSOR MACROS
The preprocessor also supports a simple macro facility, run as a
separate pass before conditional preprocessing.
Macros are defined using the following construct:
@define NAME %( BODY %)
@define NAME(PARAM_1, PARAM_2, ...) %( BODY %)
Macros, and parameters inside a macro body, are both invoked by
prefixing the macro name with an @ symbol:
@define foo %( x %)
@define add(a,b) %( ((@a)+(@b)) %)
@foo = @add(2,2)
Macro expansion is currently performed in a separate pass before
conditional compilation. Therefore, both TRUE- and FALSE-tokens in
conditional expressions will be macroexpanded regardless of how
the condition is evaluated. This can sometimes lead to errors:
// The following results in a conflict:
%( CONFIG_UPROBE == "y" %?
@define foo %( process.syscall %)
%:
@define foo %( **ERROR** %)
%)
// The following works properly as expected:
@define foo %(
%( CONFIG_UPROBE == "y" %? process.syscall %: **ERROR** %)
%)
The first example is incorrect because both @defines are evaluated
in a pass prior to the conditional being evaluated.
Normally, a macro definition is local to the file it occurs in.
Thus, defining a macro in a tapset does not make it available to
the user of the tapset. Publically available library macros can be
defined by including .stpm files on the tapset search path. These
files may only contain @define constructs, which become visible
across all tapsets and user scripts. Optionally, within the .stpm
files, a public macro definition can be surrounded by a preproces‐
sor conditional as described above.
CONSTANTS
Tapsets or guru-mode user scripts can access header file constant
tokens, typically macros, using built-in @const() operator. The
respective header file inclusion is possible either via the tapset
library, or using a top-level guru mode embedded-C construct.
This results in appropriate embedded C pragma comments setting.
@const("STP_SKIP_BADVARS")
VARIABLES
Identifiers for variables and functions are an alphanumeric se‐
quence, and may include _ and $ characters. They may not start
with a plain digit, as in C. Each variable is by default local to
the probe or function statement block within which it is men‐
tioned, and therefore its scope and lifetime is limited to a par‐
ticular probe or function invocation.
Scalar variables are implicitly typed as either string or integer.
Associative arrays also have a string or integer value, and a tu‐
ple of strings and/or integers serving as a key. Here are a few
basic expressions.
var1 = 5
var2 = "bar"
array1 [pid()] = "name" # single numeric key
array2 ["foo",4,i++] += 5 # vector of string/num/num keys
if (["hello",5,4] in array2) println ("yes") # membership test
The translator performs type inference on all identifiers, includ‐
ing array indexes and function parameters. Inconsistent type-re‐
lated use of identifiers signals an error.
Variables may be declared global, so that they are shared amongst
all probes and functions and live as long as the entire systemtap
session. There is one namespace for all global variables, regard‐
less of which script file they are found within. Concurrent ac‐
cess to global variables is automatically protected with locks,
see the SAFETY AND SECURITY section for more details. A global
declaration may be written at the outermost level anywhere, not
within a block of code. Global variables which are written but
never read will be displayed automatically at session shutdown.
The translator will infer for each its value type, and if it is
used as an array, its key types. Optionally, scalar globals may
be initialized with a string or number literal. The following de‐
claration marks variables as global.
global var1, var2, var3=4
Global variables can also be set as module options. One can do
this by either using the -G option, or the module must first be
compiled using stap -p4. Global variables can then be set on the
command line when calling staprun on the module generated by stap
-p4. See staprun(8) for more information.
The scope of a global variable may be limited to a tapset or user
script file using private keyword. The global keyword is optional
when defining a private global variable. Following declaration
marks var1 and var2 private globals.
private global var1=2
private var2
Arrays are limited in size by the MAXMAPENTRIES variable -- see
the SAFETY AND SECURITY section for details. Optionally, global
arrays may be declared with a maximum size in brackets, overriding
MAXMAPENTRIES for that array only. Note that this doesn't indi‐
cate the type of keys for the array, just the size.
global tiny_array[10], normal_array, big_array[50000]
Arrays may be configured for wrapping using the '%' suffix. This
causes older elements to be overwritten if more elements are in‐
serted than the array can hold. This works for both associative
and statistics typed arrays.
global wrapped_array1%[10], wrapped_array2%
Many types of probe points provide context variables, which are
run-time values, safely extracted from the kernel or userspace
program being probed. These are prefixed with the $ character.
The CONTEXT VARIABLES section in stapprobes(3stap) lists what is
available for each type of probe point. These context variables
become normal string or numeric scalars once they are stored in
normal script variables. See the TYPECASTING section below on how
to to turn them back into typed pointers for further processing as
context variables. There is some automation to help!
STATEMENTS
Statements enable procedural control flow. They may occur within
functions and probe handlers. The total number of statements exe‐
cuted in response to any single probe event is limited to some
number defined by the MAXACTION macro in the translated C code,
and is in the neighbourhood of 1000.
EXP Execute the string- or integer-valued expression and throw
away the value.
{ STMT1 STMT2 ... }
Execute each statement in sequence in this block. Note
that separators or terminators are generally not necessary
between statements.
; Null statement, do nothing. It is useful as an optional
separator between statements to improve syntax-error detec‐
tion and to handle certain grammar ambiguities.
if (EXP) STMT1 [ else STMT2 ]
Compare integer-valued EXP to zero. Execute the first
(non-zero) or second STMT (zero).
while (EXP) STMT
While integer-valued EXP evaluates to non-zero, execute
STMT.
for (EXP1; EXP2; EXP3) STMT
Execute EXP1 as initialization. While EXP2 is non-zero,
execute STMT, then the iteration expression EXP3.
foreach (VAR in ARRAY [ limit EXP ]) STMT
Loop over each element of the named global array, assigning
current key to VAR. The array may not be modified within
the statement. By adding a single + or - operator after
the VAR or the ARRAY identifier, the iteration will proceed
in a sorted order, by ascending or descending index or val‐
ue. If the array contains statistics aggregates, adding
the desired @operator between the ARRAY identifier and the
+ or - will specify the sorting aggregate function. See
the STATISTICS section below for the ones available. De‐
fault is @count. Using the optional limit keyword limits
the number of loop iterations to EXP times. EXP is evalu‐
ated once at the beginning of the loop.
foreach ([VAR1, VAR2, ...] in ARRAY [ limit EXP ]) STMT
Same as above, used when the array is indexed with a tuple
of keys. A sorting suffix may be used on at most one VAR
or ARRAY identifier.
foreach ([VAR1, VAR2, ...] in ARRAY [INDEX1, INDEX2, ...] [ limit
EXP ]) STMT
Same as above, where iterations are limited to elements in
the array where the keys match the index values specified.
The symbol * can be used to specify an index and will be
treated as a wildcard.
foreach (VAR0 = VAR in ARRAY [ limit EXP ]) STMT
This variant of foreach saves current value into VAR0 on
each iteration, so it is the same as ARRAY[VAR]. This also
works with a tuple of keys. Sorting suffixes on VAR0 have
the same effect as on ARRAY.
foreach (VAR0 = VAR in ARRAY [INDEX1, INDEX2, ...] [ limit EXP ])
STMT
Same as above, where iterations are limited to elements in
the array where the keys match the index values specified.
The symbol * can be used to specify an index and will be
treated as a wildcard.
break, continue
Exit or iterate the innermost nesting loop (while or for or
foreach) statement.
return EXP
Return EXP value from enclosing function. If the func‐
tion's value is not taken anywhere, then a return statement
is not needed, and the function will have a special "un‐
known" type with no return value.
next Return now from enclosing probe handler. This is especial‐
ly useful in probe aliases that apply event filtering pred‐
icates. When used in functions, the execution will be imme‐
diately transferred to the next overloaded function.
try { STMT1 } catch { STMT2 }
Run the statements in the first block. Upon any run-time
errors, abort STMT1 and start executing STMT2. Any errors
in STMT2 will propagate to outer try/catch blocks, if any.
try { STMT1 } catch(VAR) { STMT2 }
Same as above, plus assign the error message to the string
scalar variable VAR.
delete ARRAY[INDEX1, INDEX2, ...]
Remove from ARRAY the element specified by the index tuple.
If the index tuple contains a * in place of an index, the *
is treated as a wildcard and all elements with keys that
match the index tuple will be removed from ARRAY. The val‐
ue will no longer be available, and subsequent iterations
will not report the element. It is not an error to delete
an element that does not exist.
delete ARRAY
Remove all elements from ARRAY.
delete SCALAR
Removes the value of SCALAR. Integers and strings are
cleared to 0 and "" respectively, while statistics are re‐
set to the initial empty state.
EXPRESSIONS
Systemtap supports a number of operators that have the same gener‐
al syntax, semantics, and precedence as in C and awk. Arithmetic
is performed as per typical C rules for signed integers. Division
by zero or overflow is detected and results in an error.
binary numeric operators
* / % + - >> << & ^ | && ||
binary string operators
. (string concatenation)
numeric assignment operators
= *= /= %= += -= >>= <<= &= ^= |=
string assignment operators
= .=
unary numeric operators
+ - ! ~ ++ --
binary numeric, string comparison or regex matching operators
< > <= >= == != =~ !~
ternary operator
cond ? exp1 : exp2
grouping operator
( exp )
function call
fn ([ arg1, arg2, ... ])
array membership check
exp in array
[exp1, exp2, ... ] in array
[*, *, ... ] in array
REGULAR EXPRESSION MATCHING
The scripting language supports regular expression matching. The
basic syntax is as follows:
exp =~ regex
exp !~ regex
(The first operand must be an expression evaluating to a string;
the second operand must be a string literal containing a syntacti‐
cally valid regular expression.)
The regular expression syntax supports POSIX Extended Regular Ex‐
pression features as documented in grep(1) except for subexpres‐
sion reuse ("\1") functionality.
After a successful match, the contents of the matched string and
subexpressions can be extracted using the matched() and ngroups()
tapset functions as follows:
if ("an example string" =~ "str(ing)") {
matched(0) // -> returns "string", the matched substring
matched(1) // -> returns "ing", the 1st matched subexpression
ngroups() // -> returns 2, the number of matched groups
}
PROBES
The main construct in the scripting language identifies probes.
Probes associate abstract events with a statement block ("probe
handler") that is to be executed when any of those events occur.
The general syntax is as follows:
probe PROBEPOINT [, PROBEPOINT] { [STMT ...] }
probe PROBEPOINT [, PROBEPOINT] if (CONDITION) { [STMT ...] }
Events are specified in a special syntax called "probe points".
There are several varieties of probe points defined by the trans‐
lator, and tapset scripts may define further ones using aliases.
Probe points may be wildcarded, grouped, or listed in preference
sequences, or declared optional. More details on probe point syn‐
tax and semantics are listed on the stapprobes(3stap) manual page.
The probe handler is interpreted relative to the context of each
event. For events associated with kernel code, this context may
include variables defined in the source code at that spot. These
"context variables" are presented to the script as variables whose
names are prefixed with "$". They may be accessed only if the
kernel's compiler preserved them despite optimization. This is
the same constraint that a debugger user faces when working with
optimized code. In addition, the objects must exist in paged-in
memory at the moment of the systemtap probe handler's execution,
because systemtap must not cause (suppresses) any additional pag‐
ing. Some probe types have very little context. See the
stapprobes(3stap) man pages to see the kinds of context variables
available at each kind of probe point. As of systemtap version
4.3, functions called from the handlers of some probe point types
may also refer to context variables. These are treated as if a
clone of that function was inlined into the calling probe handler
and $variables evaluated in its context.
Probes may be decorated with an arming condition, consisting of a
simple boolean expression on read-only global script variables.
While disarmed (inactive, condition evaluates to false), some
probe types reduce or eliminate their run-time overheads. When an
arming condition evaluates to true, probes will be soon re-armed,
and their probe handlers will start getting called as the events
fire. (Some events may be lost during the arming interval. If
this is unacceptable, do not use arming conditions for those
probes.) Example of the syntax:
probe timer.us(TIMER) if (enabled) {
}
New probe points may be defined using "aliases". Probe point
aliases look similar to probe definitions, but instead of activat‐
ing a probe at the given point, it just defines a new probe point
name as an alias to an existing one. There are two types of alias,
i.e. the prologue style and the epilogue style which are identi‐
fied by "=" and "+=" respectively.
For prologue style alias, the statement block that follows an
alias definition is implicitly added as a prologue to any probe
that refers to the alias. While for the epilogue style alias, the
statement block that follows an alias definition is implicitly
added as an epilogue to any probe that refers to the alias. For
example:
probe syscall.read = kernel.function("sys_read") {
fildes = $fd
if (execname() == "init") next # skip rest of probe
}
defines a new probe point syscall.read, which expands to
kernel.function("sys_read"), with the given statement as a pro‐
logue, which is useful to predefine some variables for the alias
user and/or to skip probe processing entirely based on some condi‐
tions. And
probe syscall.read += kernel.function("sys_read") {
if (tracethis) println ($fd)
}
defines a new probe point with the given statement as an epilogue,
which is useful to take actions based upon variables set or left
over by the the alias user. Please note that in each case, the
statements in the alias handler block are treated ordinarily, so
that variables assigned there constitute mere initialization, not
a macro substitution.
Aliases can also be defined to include both a prologue and an epi‐
logue.
probe syscall.read = kernel.function("sys_read") {
fildes = $fd
if (execname() == "init") next
},{
if (tracethis) println ($fd)
}
An alias is used just like a built-in probe type.
probe syscall.read {
printf("reading fd=%d\n", fildes)
if (fildes > 10) tracethis = 1
}
Probes with an alias can make use of the @probewrite predicate.
This check is used to detect whether a script variable or target
variable has been written to in the probe handler body.
@probewrite(var)
expands to 1 iff var has been written to in the probe han‐
dler body, otherwise it expands to 0.
In the following example, @probewrite(var) expands to 1 because
var has been written to in the probe handler body and consequent‐
ly, the conditional statement will run.
probe foo = begin { var = 0 }, { if (@probewrite(var)) println(var) }
probe foo {
var = 1
}
FUNCTIONS
Systemtap scripts may define subroutines to factor out common
work. Functions take any number of scalar (integer or string) ar‐
guments, and must return a single scalar (integer or string). An
example function declaration looks like this:
function thisfn (arg1, arg2) {
return arg1 + arg2
}
Note the general absence of type declarations, which are instead
inferred by the translator. However, if desired, a function defi‐
nition may include explicit type declarations for its return value
and/or its arguments. This is especially helpful for embedded-C
functions. In the following example, the type inference engine
need only infer type type of arg2 (a string).
function thatfn:string (arg1:long, arg2) {
return sprint(arg1) . arg2
}
Functions may call others or themselves recursively, up to a fixed
nesting limit. This limit is defined by the MAXNESTING macro in
the translated C code and is in the neighbourhood of 10.
Functions may be marked private using the private keyword to limit
their scope to the tapset or user script file they are defined in.
An example definition of a private function follows:
private function three:long () { return 3 }
Functions terminating without reaching an explicit return state‐
ment will return an implicit 0 or "", determined by type infer‐
ence.
Functions may be overloaded during both runtime and compile time.
Runtime overloading allows the executed function to be selected
while the module is running based on runtime conditions and is
achieved using the "next" statement in script functions and
STAP_NEXT macro for embedded-C functions. For example,
function f() { if (condition) next; print("first function") }
function f() %{ STAP_NEXT; print("second function") %}
function f() { print("third function") }
During a functioncall f(), the execution will transfer to the
third function if condition evaluates to true and print "third
function". Note that the second function is unconditionally
nexted.
Parameter overloading allows the function to be executed to be se‐
lected at compile time based on the number of arguments provided
to the functioncall. For example,
function g() { print("first function") }
function g(x) { print("second function") }
g() -> "first function"
g(1) -> "second function"
Note that runtime overloading does not occur in the above example,
as exactly one function will be resolved for the functioncall. The
use of a next statement inside a function while no more overloads
remain will trigger a runtime exception Runtime overloading will
only occur if the functions have the same arity, functions with
the same name but different number of parameters are completely
unrelated.
Execution order is determined by a priority value which may be
specified. If no explicit priority is specified, user script
functions are given a higher priority than library functions. User
script functions and library functions are assigned a default pri‐
ority value of 0 and 1 respectively. Functions with the same pri‐
ority are executed in declaration order. For example,
function f():3 { if (condition) next; print("first function") }
function f():1 { if (condition) next; print("second function") }
function f():2 { print("third function") }
Since the second function has highest priority, it is executed
first. The first function is never executed as there no "next"
statements in the third function to transfer execution.
PRINTING
There are a set of function names that are specially treated by
the translator. They format values for printing to the standard
systemtap output stream in a more convenient way (note that data
generated in the kernel module need to get transferred to user-
space in order to get printed).
The sprint* variants return the formatted string instead of
printing it.
print, sprint
Print one or more values of any type, concatenated directly
together.
println, sprintln
Print values like print and sprint, but also append a new‐
line.
printd, sprintd
Take a string delimiter and two or more values of any type,
and print the values with the delimiter interposed. The
delimiter must be a literal string constant.
printdln, sprintdln
Print values with a delimiter like printd and sprintd, but
also append a newline.
printf, sprintf
Take a formatting string and a number of values of corre‐
sponding types, and print them all. The format must be a
literal string constant.
The printf formatting directives similar to those of C, except
that they are fully type-checked by the translator:
%b Writes a binary blob of the value given, instead of
ASCII text. The width specifier determines the num‐
ber of bytes to write; valid specifiers are %b %1b
%2b %4b %8b. Default (%b) is 8 bytes.
%c Character.
%d,%i Signed decimal.
%m Safely reads kernel (without #) or user (with #)
memory at the given address, outputs its content.
The optional precision specifier (not field width)
determines the number of bytes to read - default is
1 byte. %10.4m prints 4 bytes of the memory in a
10-character-wide field. Note, on some architec‐
tures user memory can still be read without #.
%M Same as %m, but outputs in hexadecimal. The minimal
size of output is double the optional precision
specifier - default is 1 byte (2 hex chars). %10.4M
prints 4 bytes of the memory as 8 hexadecimal char‐
acters in a 10-character-wide field. %.*M hex-
dumps a given number of bytes from a given buffer.
%o Unsigned octal.
%p Unsigned pointer address.
%s String.
%u Unsigned decimal.
%x Unsigned hex value, in all lower-case.
%X Unsigned hex value, in all upper-case.
%% Writes a %.
The # flag selects the alternate forms. For octal, this prefixes
a 0. For hex, this prefixes 0x or 0X, depending on case. For
characters, this escapes non-printing values with either C-like
escapes or raw octal. In the case of %#m/%#M, this safely access‐
es user space memory rather than kernel space memory.
Examples:
a = "alice", b = "bob", p = 0x1234abcd, i = 123, j = -1, id[a] = 1234, id[b] = 4567
print("hello")
Prints: hello
println(b)
Prints: bob\n
println(a . " is " . sprint(16))
Prints: alice is 16
foreach (name in id) printdln("|", strlen(name), name, id[name])
Prints: 5|alice|1234\n3|bob|4567
printf("%c is %s; %x or %X or %p; %d or %u\n",97,a,p,p,p,j,j)
Prints: a is alice; 1234abcd or 1234ABCD or 0x1234abcd; -1 or 18446744073709551615\n
printf("2 bytes of kernel buffer at address %p: %2m", p, p)
Prints: 2 byte of kernel buffer at address 0x1234abcd: <binary data>
printf("%4b", p)
Prints (these values as binary data): 0x1234abcd
printf("%#o %#x %#X\n", 1, 2, 3)
Prints: 01 0x2 0X3
printf("%#c %#c %#c\n", 0, 9, 42)
Prints: \000 \t *
STATISTICS
It is often desirable to collect statistics in a way that avoids
the penalties of repeatedly exclusive locking the global variables
those numbers are being put into. Systemtap provides a solution
using a special operator to accumulate values, and several pseudo-
functions to extract the statistical aggregates.
The aggregation operator is <<<, and resembles an assignment, or a
C++ output-streaming operation. The left operand specifies a
scalar or array-index lvalue, which must be declared global. The
right operand is a numeric expression. The meaning is intuitive:
add the given number to the pile of numbers to compute statistics
of. (The specific list of statistics to gather is given separate‐
ly, by the extraction functions.)
foo <<< 1
stats[pid()] <<< memsize
The extraction functions are also special. For each appearance of
a distinct extraction function operating on a given identifier,
the translator arranges to compute a set of statistics that satis‐
fy it. The statistics system is thereby "on-demand". Each execu‐
tion of an extraction function causes the aggregation to be com‐
puted for that moment across all processors.
Here is the set of extractor functions. The first argument of
each is the same style of lvalue used on the left hand side of the
accumulate operation. The @count(v), @sum(v), @min(v), @max(v),
@avg(v), @variance(v[, b]) extractor functions compute the num‐
ber/total/minimum/maximum/average/variance of all accumulated val‐
ues. The resulting values are all simple integers. Arrays con‐
taining aggregates may be sorted and iterated. See the foreach
construct above.
Variance uses Welford's online algorithm. The calculations are
based on integer arithmetic, and so may suffer from low precision
and overflow. To improve this, @variance(v[, b]) accepts an op‐
tional parameter b, the bit-shift, ranging from 0 (default) to 62,
for internal scaling. Only one value of bit-shift may be used
with given global variable. A larger bitshift value increases
precision, but increases the likelihood of overflow.
$ stap -e \
> 'global x probe oneshot { for(i=1;i<=5;i++) x<<<i println(@variance(x)) }'
12
$ stap -e \
> 'global x probe oneshot { for(i=1;i<=5;i++) x<<<i println(@variance(x,1)) }'
2
$ python3 -c 'import statistics; print(statistics.variance([1, 2, 3, 4, 5]))'
2.5
$
Overflow (from internal multiplication of large numbers) may occur
and may cause a negative variance result. Consider normalizing
your input data. Adding or subtracting a fixed value from all
variance inputs preserves the original variance. Dividing the
variance inputs by a fixed value shrinks the original variance by
that value squared.
Histograms are also available, but are more complicated because
they have a vector rather than scalar value. @hist_lin‐
ear(v,start,stop,interval) represents a linear histogram from
"start" to "stop" (inclusive) by increments of "interval". The
interval must be positive. Similarly, @hist_log(v) represents a
base-2 logarithmic histogram. Printing a histogram with the print
family of functions renders a histogram object as a tabular "ASCII
art" bar chart.
probe timer.profile {
x[1] <<< pid()
x[2] <<< uid()
y <<< tid()
}
global x // an array containing aggregates
global y // a scalar
probe end {
foreach ([i] in x @count+) {
printf ("x[%d]: avg %d = sum %d / count %d\n",
i, @avg(x[i]), @sum(x[i]), @count(x[i]))
println (@hist_log(x[i]))
}
println ("y:")
println (@hist_log(y))
}
The counts of each histogram bucket may be individually accessed
via the [index] operator. Each bucket is addressed from 1 through
N (for each natural bucket). In addition bucket #0 counts all the
samples beneath the start value, and bucket #N+1 counts all the
samples above the stop value. Histogram buckets (including the
two out-of-range buckets) may also be iterated with foreach.
global x
probe oneshot {
x <<< -100
x <<< 1
x <<< 2
x <<< 3
x <<< 100
foreach (bucket in @hist_linear(x,1,3,1))
// expecting 1 out-of-range-low bucket
// 3 payload buckets
// 1 out-of-range-high bucket
printf("bucket %d count %d\n",
bucket, @hist_linear(x,1,3,1)[bucket])
}
TYPECASTING
Once a pointer (see the CONTEXT VARIABLES section of
stapprobes(3stap)) has been saved into a script integer variable,
the translator attempts to keep the type information necessary to
access members from that pointer.
The translator attempts to track DWARF typing associated with
script variables assigned from addresses of context $variables,
@cast or @var operators. Depending on the complexity of the
script code, this association may pass to related variables, so
that -> and [] operators may be used on them, just as on the orig‐
inal context variable. For example:
foo = $param->foo; printf("x:%d y:%d\n", foo->x, foo->y)
printf("my value is %d\n", ($type == 42 ? $foo : $bar)->value)
printf("my parent pid is %d\n", task_parent(task_current())->tgid)
However, if this association heuristic doesn't work for a script,
using the @cast() operator tells the translator how to interpret
the number as a typed pointer.
@cast(p, "type_name"[, "module"])->member
This will interpret p as a pointer to a struct/union named
type_name and dereference the member value. Further ->subfield
expressions may be appended to dereference more levels. Note that
for direct dereferencing of a pointer {ker‐
nel,user}_{char,int,...}($p) should be used. (Refer to stap‐
funcs(5) for more details.) NOTE: the same dereferencing operator
-> is used to refer to both direct containment or pointer indirec‐
tion. Systemtap automatically determines which. The optional
module tells the translator where to look for information about
that type. Multiple modules may be specified as a list with :
separators. If the module is not specified, it will default ei‐
ther to the probe module for dwarf probes, or to "kernel" for
functions and all other probes types.
Previously up to systemtap version 4.2, "kernel" was inferred if
unspecified. Use --compatible=4.2 to activate this default.
The translator can create its own module with type information
from a header surrounded by angle brackets, in case normal debug‐
info is not available. For kernel headers, prefix it with "ker‐
nel" to use the appropriate build system. All other headers are
built with default GCC parameters into a user module. Multiple
headers may be specified in sequence to resolve a codependency.
@cast(tv, "timeval", "<sys/time.h>")->tv_sec
@cast(task, "task_struct", "kernel<linux/sched.h>")->tgid
@cast(task, "task_struct",
"kernel<linux/sched.h><linux/fs_struct.h>")->fs->umask
Values acquired by @cast may be pretty-printed by the $ and $$
suffix operators, the same way as described in the CONTEXT VARI‐
ABLES section of the stapprobes(3stap) manual page.
When in guru mode, the translator will also allow scripts to as‐
sign new values to members of typecasted pointers.
Typecasting is also useful in the case of void* members whose type
may be determinable at runtime.
probe foo {
if ($var->type == 1) {
value = @cast($var->data, "type1")->bar
} else {
value = @cast($var->data, "type2")->baz
}
print(value)
}
EMBEDDED C
When in guru mode, the translator accepts embedded C code in the
top level of the script. Such code is enclosed between %{ and %}
markers, and is transcribed verbatim, without analysis, in some
sequence, into the top level of the generated C code. At the out‐
ermost level, this may be useful to add #include instructions, and
any auxiliary definitions for use by other embedded code.
Another place where embedded code is permitted is as a function
body. In this case, the script language body is replaced entirely
by a piece of C code enclosed again between %{ and %} markers.
This C code may do anything reasonable and safe. There are a num‐
ber of undocumented but complex safety constraints on atomicity,
concurrency, resource consumption, and run time limits, so this is
an advanced technique.
The memory locations set aside for input and output values are
made available to it using macros STAP_ARG_* and STAP_RETVALUE.
Errors may be signalled with STAP_ERROR. Output may be written
with STAP_PRINTF. The function may return early with STAP_RETURN.
Here are some examples:
function integer_ops (val) %{
STAP_PRINTF("%d\n", STAP_ARG_val);
STAP_RETVALUE = STAP_ARG_val + 1;
if (STAP_RETVALUE == 4)
STAP_ERROR("wrong guess: %d", (int) STAP_RETVALUE);
if (STAP_RETVALUE == 3)
STAP_RETURN(0);
STAP_RETVALUE ++;
%}
function string_ops (val) %{
strlcpy (STAP_RETVALUE, STAP_ARG_val, MAXSTRINGLEN);
strlcat (STAP_RETVALUE, "one", MAXSTRINGLEN);
if (strcmp (STAP_RETVALUE, "three-two-one"))
STAP_RETURN("parameter should be three-two-");
%}
function no_ops () %{
STAP_RETURN(); /* function inferred with no return value */
%}
The function argument and return value types have to be inferred
by the translator from the call sites in order for this to work.
The user should examine C code generated for ordinary script-lan‐
guage functions in order to write compatible embedded-C ones.
The last place where embedded code is permitted is as an expres‐
sion rvalue. In this case, the C code enclosed between %{ and %}
markers is interpreted as an ordinary expression value. It is as‐
sumed to be a normal 64-bit signed number, unless the marker /*
string */ is included, in which case it's treated as a string.
function add_one (val) {
return val + %{ 1 %}
}
function add_string_two (val) {
return val . %{ /* string */ "two" %}
}
@define SOME_STAP_MACRO %( %{ SOME_C_MACRO %} %)
probe begin {
printf("SOME_C_MACRO has value: %d\n", @SOME_STAP_MACRO);
}
The embedded-C code may contain markers to assert optimization and
safety properties.
/* pure */
means that the C code has no side effects and may be elided
entirely if its value is not used by script code.
/* stable */
means that the C code always has the same value (in any
given probe handler invocation), so repeated calls may be
automatically replaced by memoized values. Such functions
must take no parameters, and also be pure.
/* unprivileged */
means that the C code is so safe that even unprivileged
users are permitted to use it.
/* myproc-unprivileged */
means that the C code is so safe that even unprivileged
users are permitted to use it, provided that the target of
the current probe is within the user's own process.
/* guru */
means that the C code is so unsafe that a systemtap user
must specify -g (guru mode) to use this. (Tapsets are per‐
mitted and presumed to call them safely.)
/* unmangled */
in an embedded-C function, means that the legacy (pre-1.8)
argument access syntax should be made available inside the
function. Hence, in addition to STAP_ARG_foo and STAP_RET‐
VALUE one can use THIS->foo and THIS->__retvalue respec‐
tively inside the function. This is useful for quickly mi‐
grating code written for SystemTap version 1.7 and earlier.
/* pragma:suffix */
Emit the file-level containing C code near the end of the
resulting kernel module C source file.
/* unmodified-fnargs */
in an embedded-C function, means that the function argu‐
ments are not modified inside the function body.
/* string */
in embedded-C expressions only, means that the expression
has const char * type and should be treated as a string
value, instead of the default long numeric.
Script level global variables may be accessed in embedded-C func‐
tions and blocks. To read or write the global variable var , the
/* pragma:read:var */ or /* pragma:write:var */ marker must be
first placed in the embedded-C function or block. This provides
the macros STAP_GLOBAL_GET_* and STAP_GLOBAL_SET_* macros to allow
reading and writing, respectively. For example:
global var
global var2[100]
function increment() %{
/* pragma:read:var */ /* pragma:write:var */
/* pragma:read:var2 */ /* pragma:write:var2 */
STAP_GLOBAL_SET_var(STAP_GLOBAL_GET_var()+1); //var++
STAP_GLOBAL_SET_var2(1, 1, STAP_GLOBAL_GET_var2(1, 1)+1); //var2[1,1]++
%}
Variables may be read and set in both embedded-C functions and ex‐
pressions. Strings returned from embedded-C code are decayed to
pointers. Variables must also be assigned at script level to allow
for type inference. Map assignment does not return the value writ‐
ten, so chaining does not work.
BUILT-INS
A set of builtin probe point aliases are provided by the scripts
installed in the directory specified in the stappaths(7) manual
page. The functions are described in the stapprobes(3stap) manual
page.
DEREFERENCING
Integers can be dereferenced from pointers saved as a script inte‐
ger variables using the @kderef() or @uderef() operators.
@kderef() is used for kernel space addresses and @uderef() is used
for user space addresses.
@kderef(SIZE, addr)
@uderef(SIZE, addr)
This will interpret addr as a kernel/user address and read SIZE
bytes starting at that address. SIZE should be either 1, 2, 4 or
8 bytes.
REGISTERS
The value stored within a register can be accessed using the
@kregister() or @uregister() operators. @kregister() is used for
kernel space registers and @uregister() is used for user space
registers. The register of interest is specified using its DWARF
number.
@kregister(0)
@uregister(5)
The translator begins pass 1 by parsing the given input script,
and all scripts (files named *.stp) found in a tapset directory.
The directories listed with -I are processed in sequence, each
processed in "guru mode". For each directory, a number of
subdirectories are also searched. These subdirectories are
derived from the selected kernel version (the -R option), in order
to allow more kernel-version-specific scripts to override less
specific ones. For example, for a kernel version 2.6.12-23.FC3
the following patterns would be searched, in sequence:
2.6.12-23.FC3/*.stp, 2.6.12/*.stp, 2.6/*.stp, and finally *.stp.
Stopping the translator after pass 1 causes it to print the parse
trees.
In pass 2, the translator analyzes the input script to resolve
symbols and types. References to variables, functions, and probe
aliases that are unresolved internally are satisfied by searching
through the parsed tapset script files. If any tapset script file
is selected because it defines an unresolved symbol, then the
entirety of that file is added to the translator's resolution
queue. This process iterates until all symbols are resolved and a
subset of tapset script files is selected.
Next, all probe point descriptions are validated against the wide
variety supported by the translator. Probe points that refer to
code locations ("synchronous probe points") require the
appropriate kernel debugging information to be installed. In the
associated probe handlers, target-side variables (whose names
begin with "$") are found and have their run-time locations
decoded.
Next, all probes and functions are analyzed for optimization
opportunities, in order to remove variables, expressions, and
functions that have no useful value and no side-effect. Embedded-
C functions are assumed to have side-effects unless they include
the magic string /* pure */. Since this optimization can hide
latent code errors such as type mismatches or invalid $context
variables, it sometimes may be useful to disable the optimizations
with the -u option.
Finally, all variable, function, parameter, array, and index types
are inferred from context (literals and operators). Stopping the
translator after pass 2 causes it to list all the probes,
functions, and variables, along with all inferred types. Any
inconsistent or unresolved types cause an error.
In pass 3, the translator writes C code that represents the
actions of all selected script files, and creates a Makefile to
build that into a kernel object. These files are placed into a
temporary directory. Stopping the translator at this point causes
it to print the contents of the C file.
In pass 4, the translator invokes the Linux kernel build system to
create the actual kernel object file. This involves running make
in the temporary directory, and requires a kernel module build
system (headers, config and Makefiles) to be installed in the
usual spot /lib/modules/VERSION/build. Stopping the translator
after pass 4 is the last chance before running the kernel object.
This may be useful if you want to archive the file.
In pass 5, the translator invokes the systemtap auxiliary program
staprun program for the given kernel object. This program
arranges to load the module then communicates with it, copying
trace data from the kernel into temporary files, until the user
sends an interrupt signal. Any run-time error encountered by the
probe handlers, such as running out of memory, division by zero,
exceeding nesting or runtime limits, results in a soft error
indication. Soft errors in excess of MAXERRORS block of all
subsequent probes (except error-handling probes), and terminate
the session. Finally, staprun unloads the module, and cleans up.
ABNORMAL TERMINATION
One should avoid killing the stap process forcibly, for example
with SIGKILL, because the stapio process (a child process of the
stap process) and the loaded module may be left running on the
system. If this happens, send SIGTERM or SIGINT to any remaining
stapio processes, then use rmmod to unload the systemtap module.
See the stapex(3stap) manual page for a brief collection of
samples, or a large set of installed samples under the systemtap
documentation/testsuite directories. See stappaths(7stap) for the
likely location of these on the system.
The systemtap translator caches the pass 3 output (the generated C
code) and the pass 4 output (the compiled kernel module) if pass 4
completes successfully. This cached output is reused if the same
script is translated again assuming the same conditions exist
(same kernel version, same systemtap version, etc.). Cached files
are stored in the $SYSTEMTAP_DIR/cache directory. The cache can be
limited by having the file cache_mb_limit placed in the cache
directory (shown above) containing only an ASCII integer
representing how many MiB the cache should not exceed. In the
absence of this file, a default will be created with the limit set
to 256MiB. This is a 'soft' limit in that the cache will be
cleaned after a new entry is added if the cache clean interval is
exceeded, so the total cache size may temporarily exceed this
limit. This interval can be specified by having the file
cache_clean_interval_s placed in the cache directory (shown above)
containing only an ASCII integer representing the interval in
seconds. In the absence of this file, a default will be created
with the interval set to 300 s.
Systemtap may be used as a powerful administrative tool. It can
expose kernel internal data structures and potentially private
user information. (In dyninst runtime mode, this is not the case,
see the ALTERNATE RUNTIMES section below.)
The translator asserts many safety constraints during compilation
and more during run-time. It aims to ensure that no handler
routine can run for very long, allocate boundless memory, perform
unsafe operations, or in unintentionally interfere with the
system. Uses of script global variables are automatically
read/write locked as appropriate, to protect against manipulation
by concurrent probe handlers. Locks are taken so as to run the
global-variable manipulation portion of probe handlers atomically
(locks are taken all-or-none). Deadlocks are detected with
timeouts. Use the -t flag to receive reports of excessive lock
contention. Experimenting with scripts is therefore generally
safe. The guru-mode -g option allows administrators to bypass
most safety measures, which permits invasive or state-changing
operations, embedded-C code, and increases the risk of upset. By
default, overload prevention is turned on for all modules. If you
would like to disable overload processing, use the
--suppress-time-limits option.
Errors that are caught at run time normally result in a clean
script shutdown and a pass-5 error message. The
--suppress-handler-errors option lets scripts tolerate soft errors
without shutting down.
PERMISSIONS
For the normal linux-kernel-module runtime, to run the kernel
objects systemtap builds, a user must be one of the following:
• the root user;
• a member of the stapdev and stapusr groups;
• a member of the stapsys and stapusr groups; or
• a member of the stapusr group.
The root user or a user who is a member of both the stapdev and
stapusr groups can build and run any systemtap script.
A user who is a member of both the stapsys and stapusr groups can
only use pre-built modules under the following conditions:
• The module has been signed by a trusted signer. Trusted
signers are normally systemtap compile-servers which sign
modules when the --privilege option is specified by the
client. See the stap-server(8) manual page for more
information.
• The module was built using the --privilege=stapsys or the
--privilege=stapusr options.
Members of only the stapusr group can only use pre-built modules
under the following conditions:
• The module is located in the /lib/modules/VERSION/systemtap
directory. This directory must be owned by root and not be
world writable.
or
• The module has been signed by a trusted signer. Trusted
signers are normally systemtap compile-servers which sign
modules when the --privilege option is specified by the
client. See the stap-server(8) manual page for more
information.
• The module was built using the --privilege=stapusr option.
The kernel modules generated by stap program are run by the
staprun program. The latter is a part of the Systemtap package,
dedicated to module loading and unloading (but only in the white
zone), and kernel-to-user data transfer. Since staprun does not
perform any additional security checks on the kernel objects it is
given, it would be unwise for a system administrator to add
untrusted users to the stapdev or stapusr groups.
SECUREBOOT
If the current system has SecureBoot turned on in the UEFI
firmware, all kernel modules must be signed. (Some kernels may
allow disabling SecureBoot long after booting with a key sequence
such as SysRq-X, making it unnecessary to sign modules.) There
are two ways to sign a systemtap module. The systemtap compile
server can sign modules with a MOK (Machine Owner Key) that it has
in common with a client system. For example:
stap --use-server=HOSTNAME:PORT -e 'SCRIPT'
# If there is no mok key in common with the server's systemtap mok key
# list and the client's mok database then the user is directed by stap
# to invoke:
sudo mokutil --import signing_key.x509
# then after rebooting the system:
stap --use-server=HOSTNAME:PORT -e 'SCRIPT'
# will use the server to build and sign the module and the module will run
# on the client
Another way to sign modules is to use the stap --sign-module op‐
tion, which uses a MOK on the client system without using a serv‐
er. For example:
stap --sign-module -e 'SCRIPT'
# If there is no systemtap mok key in the system mok database
# then the user is directed by stap to invoke:
sudo mokutil --import /home/USER/.systemtap/ssl/server/moks/FINGERPRINT/signing_key.x509
# then after rebooting the system:
stap --sign-module -e 'SCRIPT'
# will sign and run the module
See the following wiki page for more details:
https://sourceware.org/systemtap/wiki/SecureBoot
Some kernels do not let systemtap guess whether module module
signing is in effect. On such machines, set the SYSTEMTAP_SIGN
environment variable to any value while running stap.
RESOURCE LIMITS
Many resource use limits are set by macros in the generated C
code. These may be overridden with -D flags. A selection of
these is as follows:
MAXNESTING
Maximum number of nested function calls. Default deter‐
mined by script analysis, with a bonus 10 slots added for
recursive scripts.
MAXSTRINGLEN
Maximum length of strings, default 128.
MAXTRYLOCK
Maximum number of iterations to wait for locks on global
variables before declaring possible deadlock and skipping
the probe, default 1000.
MAXACTION
Maximum number of statements to execute during any single
probe hit (with interrupts disabled), default 1000. Note
that for straight-through probe handlers lacking loops or
recursion, due to optimization, this parameter may be in‐
terpreted too conservatively.
MAXACTION_INTERRUPTIBLE
Maximum number of statements to execute during any single
probe hit which is executed with interrupts enabled (such
as begin/end probes), default (MAXACTION * 10).
MAXBACKTRACE
Maximum number of stack frames that will be be processed by
the stap runtime unwinder as produced by the backtrace
functions in the [u]context-unwind.stp tapsets, default 20.
MAXMAPENTRIES
Maximum number of rows in any single global array, default
2048. Individual arrays may be declared with a larger or
smaller limit instead:
global big[10000],little[5]
or denoted with % to make them wrap-around (replace old en‐
tries) automatically, as in
global big%
or both.
MAPHASHBIAS
The number of powers-of-two to add or subtract from the
natural size of the hash table backing each global associa‐
tive array. Default is 0. Try small positive numbers to
get extra performance at the cost of more memory consump‐
tion, because that should reduce hash table collisions.
Try small negative numbers for the opposite tradeoff.
MAXERRORS
Maximum number of soft errors before an exit is triggered,
default 0, which means that the first error will exit the
script. Note that with the --suppress-handler-errors op‐
tion, this limit is not enforced.
MAXSKIPPED
Maximum number of skipped probes before an exit is trig‐
gered, default 100. Running systemtap with -t (timing)
mode gives more details about skipped probes. With the de‐
fault -DINTERRUPTIBLE=1 setting, probes skipped due to
reentrancy are not accumulated against this limit. Note
that with the --suppress-handler-errors option, this limit
is not enforced.
MINSTACKSPACE
Minimum number of free kernel stack bytes required in order
to run a probe handler, default 1024. This number should
be large enough for the probe handler's own needs, plus a
safety margin.
MAXUPROBES
Maximum number of concurrently armed user-space probes (up‐
robes), default somewhat larger than the number of user-
space probe points named in the script. This pool needs to
be potentially large because individual uprobe objects
(about 64 bytes each) are allocated for each process for
each matching script-level probe.
STP_MAXMEMORY
Maximum amount of memory (in kilobytes) that the systemtap
module should use, default unlimited. The memory size in‐
cludes the size of the module itself, plus any additional
allocations. This only tracks direct allocations by the
systemtap runtime. This does not track indirect alloca‐
tions (as done by kprobes/uprobes/etc. internals).
STP_OVERLOAD_THRESHOLD, STP_OVERLOAD_INTERVAL
Maximum number of machine cycles spent in probes on any cpu
per given interval, before an overload condition is de‐
clared and the script shut down. The defaults are 500 mil‐
lion and 1 billion, so as to limit stap script cpu consump‐
tion at around 50%.
STP_PROCFS_BUFSIZE
Size of procfs probe read buffers (in bytes). Defaults to
MAXSTRINGLEN. This value can be overridden on a per-procfs
file basis using the procfs read probe .maxsize(MAXSIZE)
parameter.
With scripts that contain probes on any interrupt path, it is pos‐
sible that those interrupts may occur in the middle of another
probe handler. The probe in the interrupt handler would be
skipped in this case to avoid reentrance. To work around this is‐
sue, execute stap with the option -DINTERRUPTIBLE=0 to mask inter‐
rupts throughout the probe handler. This does add some extra
overhead to the probes, but it may prevent reentrance for common
problem cases. However, probes in NMI handlers and in the call‐
path of the stap runtime may still be skipped due to reentrance.
In case something goes wrong with stap or staprun after a probe
has already started running, one may safely kill both user
processes, and remove the active probe kernel module with rmmod.
Any pending trace messages may be lost.
Systemtap exposes kernel internal data structures and potentially
private user information. Because of this, use of systemtap's full
capabilities are restricted to root and to users who are members
of the groups stapdev and stapusr.
However, a restricted set of systemtap's features can be made
available to trusted, unprivileged users. These users are members
of the group stapusr only, or members of the groups stapusr and
stapsys. These users can load systemtap modules which have been
compiled and certified by a trusted systemtap compile-server. See
the descriptions of the options --privilege and --use-server. See
README.unprivileged in the systemtap source code for information
about setting up a trusted compile server.
The restrictions enforced when --privilege=stapsys is specified
are designed to prevent unprivileged users from:
• harming the system maliciously.
The restrictions enforced when --privilege=stapusr is specified
are designed to prevent unprivileged users from:
• harming the system maliciously.
• gaining access to information which would not normally
be available to an unprivileged user.
• disrupting the performance of processes owned by other
users of the system. Some overhead to the system in
general is unavoidable since the unprivileged user's
probes will be triggered at the appropriate times. What
we would like to avoid is targeted interruption of an‐
other user's processes which would not normally be pos‐
sible by an unprivileged user.
PROBE RESTRICTIONS
A member of the groups stapusr and stapsys may use all probe
points.
A member of only the group stapusr may use only the following
probes:
• begin, begin(n)
• end, end(n)
• error(n)
• never
• process.*, where the target process is owned by the
user.
• timer.{jiffies,s,sec,ms,msec,us,usec,ns,nsec}(n)*
• timer.hz(n)
SCRIPT LANGUAGE RESTRICTIONS
The following scripting language features are unavailable to all
unprivileged users:
• any feature enabled by the Guru Mode (-g) option.
• embedded C code.
RUNTIME RESTRICTIONS
The following runtime restrictions are placed upon all unprivi‐
leged users:
• Only the default runtime code (see -R) may be used.
Additional restrictions are placed on members of only the group
stapusr:
• Probing of processes owned by other users is not per‐
mitted.
• Access of kernel memory (read and write) is not permit‐
ted.
COMMAND LINE OPTION RESTRICTIONS
Some command line options provide access to features which must
not be available to all unprivileged users:
• -g may not be specified.
• The following options may not be used by the compile-
server client:
-a, -B, -D, -I, -r, -R
ENVIRONMENT RESTRICTIONS
The following environment variables must not be set for all un‐
privileged users:
SYSTEMTAP_RUNTIME
SYSTEMTAP_TAPSET
SYSTEMTAP_DEBUGINFO_PATH
TAPSET RESTRICTIONS
In general, tapset functions are only available for members of the
group stapusr when they do not gather information that an ordinary
program running with that user's privileges would be denied access
to.
There are two categories of unprivileged tapset functions. The
first category consists of utility functions that are uncondition‐
ally available to all users; these include such things as:
cpu:long ()
exit ()
str_replace:string (prnt_str:string, srch_str:string, rplc_str:string)
The second category consists of so-called myproc-unprivileged
functions that can only gather information within their own
processes. Scripts that wish to use these functions must test the
result of the tapset function is_myproc and only call these func‐
tions if the result is 1. The script will exit immediately if any
of these functions are called by an unprivileged user within a
probe within a process which is not owned by that user. Examples
of myproc-unprivileged functions include:
print_usyms (stk:string)
user_int:long (addr:long)
usymname:string (addr:long)
A compile error is triggered when any function not in either of
the above categories is used by members of only the group stapusr.
No other built-in tapset functions may be used by members of only
the group stapusr.
As described above, systemtap's default runtime mode involves
building and loading kernel modules, with various security trade‐
offs presented. Systemtap now includes two new prototype back‐
ends: --runtime=dyninst and --runtime=bpf.
--runtime=dyninst uses Dyninst to instrument a user's own process‐
es at runtime. This backend does not use kernel modules, and does
not require root privileges, but is restricted with respect to the
kinds of probes and other constructs that a script may use.
dyninst runtime operates in target-attach mode, so it does require
a -c COMMAND or -x PID process. For example:
stap --runtime=dyninst -c 'stap -V' \
-e 'probe process.function("main")
{ println("hi from dyninst!") }'
It may be necessary to disable a conflicting selinux check with
# setsebool allow_execstack 1
--runtime=bpf compiles the user script into extended Berkeley
Packet Filter (eBPF) programs instead of a kernel module. eBPF
programs are verified by the kernel for safety and are executed by
an in-kernel virtual machine. This runtime is in an early stage
of development and currently lacks support for a number of fea‐
tures available in the default runtime. Please see the stapbpf(8)
man page for more information.
The systemtap translator generally returns with a success code of
0 if the requested script was processed and executed successfully
through the requested pass. Otherwise, errors may be printed to
stderr and a failure code is returned. Use -v or -vp N to in‐
crease (global or per-pass) verbosity to identify the source of
the trouble.
In listings mode (-l and -L), error messages are normally sup‐
pressed. A success code of 0 is returned if at least one matching
probe was found.
A script executing in pass 5 that is interrupted with ^C / SIGINT
is considered to be successful.
Over time, some features of the script language and the tapset li‐
brary may undergo incompatible changes, so that a script written
against an old version of systemtap may no longer run. In these
cases, it may help to run systemtap with the --compatible VERSION
flag, specifying the last known working version. Running system‐
tap with the --check-version flag will output a warning if any
possible incompatible elements have been parsed. Deprecation his‐
torical details may be found in the NEWS file.
The purpose of deprecation facility is to improve the experience
of scripts written for newer versions of systemtap (by adding bet‐
ter alternatives and removing conflicting or messy older alterna‐
tives), while at the same time permitting scripts written for old‐
er versions of systemtap to continue running. Deprecation is thus
intended a service to users (and an inconvenience to systemtap's
developers), rather than the other way around.
Please note that underscore-prefixed identifiers in the tapset
sometimes undergo such changes that are difficult to preserve com‐
patibility for, even with the deprecation mechanisms. Avoid rely‐
ing on these in your scripts; instead propose them for promotion
to non-underscored status.
Important files and their corresponding paths can be located in
the
stappaths (7) manual page.
stapprobes(3stap),
function::*(3stap),
probe::*(3stap),
tapset::*(3stap),
stappaths(7),
staprun(8),
stapdyn(8),
systemtap(8),
stapvars(3stap),
stapex(3stap),
stap-server(8),
stap-prep(1),
stapref(1),
awk(1),
gdb(1)
Use the Bugzilla link of the project web page or our mailing list.
http://sourceware.org/systemtap/ , <systemtap@sourceware.org>.
error::reporting(7stap),
https://sourceware.org/systemtap/wiki/HowToReportBugs
This page is part of the systemtap (a tracing and live-system
analysis tool) project. Information about the project can be
found at ⟨https://sourceware.org/systemtap/⟩. If you have a bug
report for this manual page, send it to systemtap@sourceware.org.
This page was obtained from the project's upstream Git repository
⟨git://sourceware.org/git/systemtap.git⟩ on 2025-02-02. (At that
time, the date of the most recent commit that was found in the
repository was 2025-01-28.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
STAP(1)
Pages that refer to this page: dtrace(1), stap-jupyter(1), stap-prep(1), stapref(1), stap-report(1), stapvirt(1), stapex(3stap), stapfuncs(3stap), stapprobes(3stap), stapvars(3stap), syscalls(3stap), tapset::syscalls(3stap), error::dwarf(7stap), error::fault(7stap), error::inode-uprobes(7stap), error::pass1(7stap), error::pass2(7stap), error::pass3(7stap), error::pass4(7stap), error::pass5(7stap), error::process-tracking(7stap), error::reporting(7stap), error::sdt(7stap), warning::buildid(7stap), warning::debuginfo(7stap), warning::pass5(7stap), warning::symbols(7stap), stapbpf(8), stapdyn(8), stap-exporter(8), staprun(8), stap-server(8), stapsh(8), systemtap-service(8)
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