path_resolution(7) Miscellaneous Information Manual path_resolution(7)
NAME
path_resolution - how a pathname is resolved to a file
DESCRIPTION
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 di-
rectory is the root directory of the calling process. A process inher-
its 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 di-
rectory 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 in-
herited from the parent, and can be changed by use of the chdir(2) sys-
tem 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 di-
rectory). Upon error, that error is returned. If the result is not a
directory, an ENOTDIR error is returned. If the resolution of the sym-
bolic 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 sym-
bolic 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 path-
name-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 spe-
cific system call), and (ii) it is not necessarily an error if the com-
ponent 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 re-
fer 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 what-
ever it referred to earlier.
One can walk out of a mounted filesystem: "path/.." refers to the par-
ent 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 cre-
ated 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 in-
termediate 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 direc-
tory. Nowadays POSIX decrees that an empty pathname must not be re-
solved 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 set-
groups(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 or-
dinary 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 con-
cept 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 setf-
suid(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 capa-
bilities(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.
SEE ALSO
readlink(2), capabilities(7), credentials(7), symlink(7)
Linux man-pages 6.03 2023-02-05 path_resolution(7)
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