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NAME | DESCRIPTION | ALTERNATIVES | SEE ALSO | COLOPHON |
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ERROR::DWARF(7stap) ERROR::DWARF(7stap)
error::dwarf - dwarf debuginfo quality problems
Systemtap sometimes relies on ELF/DWARF debuginfo for programs
being instrumented to locate places to probe, or context
variables to read/write, just like a symbolic debugger does.
Even though examination of the program's source code may show
variables or lines where probes may be desired, the compiler must
preserve information about them for systemtap (or a debugger such
as gdb) to get pinpoint access to the desired information. If a
script requires such data, but the compiler did not preserve
enough of it, pass-2 errors may result.
Common conditions that trigger these problems include:
debuginfo missing or mismatching
Sometimes debuginfo is installed, but does not match the
binaries being probed. See the warning::debuginfo man
page for more help for locating correct debuginfo.
compiler version
Prior to GCC version 4.5, debuginfo quality was fairly
limited. Often developers were advised to build their
programs with -O0 -g flags to disable optimization. GCC
version 4.5 introduced a facility called "variable-
tracking assignments" that allows it to generate high-
quality debuginfo under full -O2 -g optimization. It is
not perfect, but much better than before. Note that, due
to another gcc bug (PR51358) -O0 -g can actually sometimes
make debuginfo quality worse than for -O2 -g.
Another related problem involves debuginfo quality for the
prologue area of a function (PR15123), wherein a program
compiled with CFLAGS=-mfentry (especially the kernel, for
ftrace) may lack accurate debuginfo for the entry
instructions for gcc prior to version 4.8. If able,
arrange to compile your programs with -grecord-gcc-
switches CFLAGS, and/or try rerunning systemtap with
$PR15123_ASSUME_MFENTRY=1.
linux kbuild damaging debuginfo quality
An upstream kernel commit #2062afb4f804a put
-fno-var-tracking-assignments into KCFLAGS, dramatically
reducing debuginfo quality, which can cause debuginfo
failures. The simplest fix is to erase, excise, nay, erad‐
icate this line from the top level linux Makefile:
KBUILD_CFLAGS += [...] -fno-var-tracking-assignments [...]
function inlining
Even modern gcc sometimes has problems with parameters for
inlined functions. It may be necessary to change the
script to probe at a slightly different place. Try a
.statement() probe, instead of a .function() probe, some‐
where a few source lines into the body of the inlined
function. Or try putting a probe at the call site of the
inlined function. Or use the if @defined($var) { ... }
script language construct to test for the resolvability of
the context variable before using it.
instruction reordering
Heavily optimized code often smears the instructions from
multiple source statements together. This can leave sys‐
temtap with no place to choose to place a probe, especial‐
ly a statement probe specified by line number. Systemtap
may advise to try a nearby line number, but these may not
work well either. Consider placing a probe by a statement
wildcard or line number range.
elfutils configuration
It is possible that the DWARF debuginfo being sought is
available, but not in a format acceptable to the copy of
elfutils used by systemtap. For example, your copy of gcc
might produce compressed debuginfo (.zdebug_* ELF sections
or .xz files) while your copy of elfutils might lack ap‐
propriate decompression capabilities. Unfortunately,
there is no easy way to tell if this is the problem. If
you're building your own copy of elfutils, ensure all de‐
compression library headers/libraries are available at
build time.
debuginfo configuration
Some tools may generate debuginfo that is unsupported by
systemtap, such as the linux kernel CONFIG_DEBUG_IN‐
FO_SPLIT (.dwo files) option. Stick with plain ELF/DWARF
(optinally split, Fedora-style), if possible.
In order to reduce reliance on ELF/DWARF debuginfo, consider the
use of statically compiled-in instrumentation, such as kernel
tracepoints, or <sys/sdt.h> userspace markers. Such instrumenta‐
tion hook sites are relatively low cost (just one NOP instruction
for sdt.h), and nearly guarantee the availability of parameter
data and a reliable probe site, all without reliance on debugin‐
fo.
stap(1),
http://dwarfstd.org/ ,
http://sourceware.org/systemtap/wiki/TipContextVariables ,
http://gcc.gnu.org/wiki/Var_Tracking_Assignments ,
warning::debuginfo(7stap),
warning::buildid(7stap),
error::reporting(7stap)
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 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) 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
ERROR::DWARF(7stap)
Pages that refer to this page: error::pass2(7stap), error::sdt(7stap), warning::debuginfo(7stap), warning::pass5(7stap), warning::symbols(7stap)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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