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NAME | DESCRIPTION | SEE ALSO | COLOPHON |
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path_resolution(7) Miscellaneous Information Manual path_resolution(7)
path_resolution - how a pathname is resolved to a file
Some UNIX/Linux system calls have as parameter one or more
filenames. A filename (or pathname) is resolved as follows.
Step 1: start of the resolution process
If the pathname starts with the '/' character, the starting lookup
directory is the root directory of the calling process. A process
inherits its root directory from its parent. Usually this will be
the root directory of the file hierarchy. A process may get a
different root directory by use of the chroot(2) system call, or
may temporarily use a different root directory by using openat2(2)
with the RESOLVE_IN_ROOT flag set.
A process may get an entirely private mount namespace in case it—
or one of its ancestors—was started by an invocation of the
clone(2) system call that had the CLONE_NEWNS flag set. This
handles the '/' part of the pathname.
If the pathname does not start with the '/' character, the
starting lookup directory of the resolution process is the current
working directory of the process — or in the case of
openat(2)-style system calls, the dfd argument (or the current
working directory if AT_FDCWD is passed as the dfd argument). The
current working directory is inherited from the parent, and can be
changed by use of the chdir(2) system call.
Pathnames starting with a '/' character are called absolute
pathnames. Pathnames not starting with a '/' are called relative
pathnames.
Step 2: walk along the path
Set the current lookup directory to the starting lookup directory.
Now, for each nonfinal component of the pathname, where a
component is a substring delimited by '/' characters, this
component is looked up in the current lookup directory.
If the process does not have search permission on the current
lookup directory, an EACCES error is returned ("Permission
denied").
If the component is not found, an ENOENT error is returned ("No
such file or directory").
If the component is found, but is neither a directory nor a
symbolic link, an ENOTDIR error is returned ("Not a directory").
If the component is found and is a directory, we set the current
lookup directory to that directory, and go to the next component.
If the component is found and is a symbolic link, we first resolve
this symbolic link (with the current lookup directory as starting
lookup directory). Upon error, that error is returned. If the
result is not a directory, an ENOTDIR error is returned. If the
resolution of the symbolic link is successful and returns a
directory, we set the current lookup directory to that directory,
and go to the next component. Note that the resolution process
here can involve recursion if the prefix ('dirname') component of
a pathname contains a filename that is a symbolic link that
resolves to a directory (where the prefix component of that
directory may contain a symbolic link, and so on). In order to
protect the kernel against stack overflow, and also to protect
against denial of service, there are limits on the maximum
recursion depth, and on the maximum number of symbolic links
followed. An ELOOP error is returned when the maximum is exceeded
("Too many levels of symbolic links").
As currently implemented on Linux, the maximum number of symbolic
links that will be followed while resolving a pathname is 40.
Before Linux 2.6.18, the limit on the recursion depth was 5.
Starting with Linux 2.6.18, this limit was raised to 8. In Linux
4.2, the kernel's pathname-resolution code was reworked to
eliminate the use of recursion, so that the only limit that
remains is the maximum of 40 resolutions for the entire pathname.
The resolution of symbolic links during this stage can be blocked
by using openat2(2), with the RESOLVE_NO_SYMLINKS flag set.
Step 3: find the final entry
The lookup of the final component of the pathname goes just like
that of all other components, as described in the previous step,
with two differences: (i) the final component need not be a
directory (at least as far as the path resolution process is
concerned—it may have to be a directory, or a nondirectory,
because of the requirements of the specific system call), and (ii)
it is not necessarily an error if the component is not found—maybe
we are just creating it. The details on the treatment of the
final entry are described in the manual pages of the specific
system calls.
. and ..
By convention, every directory has the entries "." and "..", which
refer to the directory itself and to its parent directory,
respectively.
The path resolution process will assume that these entries have
their conventional meanings, regardless of whether they are
actually present in the physical filesystem.
One cannot walk up past the root: "/.." is the same as "/".
Mount points
After a mount dev path command, the pathname "path" refers to the
root of the filesystem hierarchy on the device "dev", and no
longer to whatever it referred to earlier.
One can walk out of a mounted filesystem: "path/.." refers to the
parent directory of "path", outside of the filesystem hierarchy on
"dev".
Traversal of mount points can be blocked by using openat2(2), with
the RESOLVE_NO_XDEV flag set (though note that this also restricts
bind mount traversal).
Trailing slashes
If a pathname ends in a '/', that forces resolution of the
preceding component as in Step 2: the component preceding the
slash either exists and resolves to a directory or it names a
directory that is to be created immediately after the pathname is
resolved. Otherwise, a trailing '/' is ignored.
Final symbolic link
If the last component of a pathname is a symbolic link, then it
depends on the system call whether the file referred to will be
the symbolic link or the result of path resolution on its
contents. For example, the system call lstat(2) will operate on
the symbolic link, while stat(2) operates on the file pointed to
by the symbolic link.
Length limit
There is a maximum length for pathnames. If the pathname (or some
intermediate pathname obtained while resolving symbolic links) is
too long, an ENAMETOOLONG error is returned ("Filename too long").
Empty pathname
In the original UNIX, the empty pathname referred to the current
directory. Nowadays POSIX decrees that an empty pathname must not
be resolved successfully. Linux returns ENOENT in this case.
Permissions
The permission bits of a file consist of three groups of three
bits; see chmod(1) and stat(2). The first group of three is used
when the effective user ID of the calling process equals the owner
ID of the file. The second group of three is used when the group
ID of the file either equals the effective group ID of the calling
process, or is one of the supplementary group IDs of the calling
process (as set by setgroups(2)). When neither holds, the third
group is used.
Of the three bits used, the first bit determines read permission,
the second write permission, and the last execute permission in
case of ordinary files, or search permission in case of
directories.
Linux uses the fsuid instead of the effective user ID in
permission checks. Ordinarily the fsuid will equal the effective
user ID, but the fsuid can be changed by the system call
setfsuid(2).
(Here "fsuid" stands for something like "filesystem user ID". The
concept was required for the implementation of a user space NFS
server at a time when processes could send a signal to a process
with the same effective user ID. It is obsolete now. Nobody
should use setfsuid(2).)
Similarly, Linux uses the fsgid ("filesystem group ID") instead of
the effective group ID. See setfsgid(2).
Bypassing permission checks: superuser and capabilities
On a traditional UNIX system, the superuser (root, user ID 0) is
all-powerful, and bypasses all permissions restrictions when
accessing files.
On Linux, superuser privileges are divided into capabilities (see
capabilities(7)). Two capabilities are relevant for file
permissions checks: CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH. (A
process has these capabilities if its fsuid is 0.)
The CAP_DAC_OVERRIDE capability overrides all permission checking,
but grants execute permission only when at least one of the file's
three execute permission bits is set.
The CAP_DAC_READ_SEARCH capability grants read and search
permission on directories, and read permission on ordinary files.
readlink(2), capabilities(7), credentials(7), symlink(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.10.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-02-02. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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man-pages@man7.org
Linux man-pages 6.10 2024-05-02 path_resolution(7)
Pages that refer to this page: access(2), acct(2), bind(2), chdir(2), chmod(2), chown(2), chroot(2), connect(2), execve(2), futimesat(2), intro(2), link(2), mkdir(2), mknod(2), mount(2), open(2), openat2(2), open_how(2type), readlink(2), rename(2), rmdir(2), send(2), stat(2), statfs(2), statx(2), symlink(2), truncate(2), umount(2), unlink(2), uselib(2), utime(2), utimensat(2), euidaccess(3), intro(3), statvfs(3), systemd.exec(5), credentials(7), symlink(7)
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HTML rendering created 2025-02-02 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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