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
system call. 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. (This is also inherited from the parent. It can be changed by use of
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
If the process does not have search permission on the current lookup directory,
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
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 (symlink), 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
symlink 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 involves recursion. 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").
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 file system.
One cannot walk down past the root: "/.." is the same as
After a "mount dev path" command, the pathname "path" refers
to the root of the file system hierarchy on the device "dev", and no
longer to whatever it referred to earlier.
One can walk out of a mounted file system: "path/.." refers to the
parent directory of "path", outside of the file system hierarchy on
If a pathname ends in a '/', that forces resolution of the preceding component
as in Step 2: it has to exist and resolve to a directory. Otherwise a trailing
'/' is ignored. (Or, equivalently, a pathname with a trailing '/' is
equivalent to the pathname obtained by appending '.' to it.)
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
will operate on the symlink, while stat(2)
the file pointed to by the symlink.
There is a maximum length for pathnames. If the pathname (or some intermediate
pathname obtained while resolving symbolic links) is too long, an
error is returned ("Filename too long").
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.
The permission bits of a file consist of three groups of three bits, cf.
. 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 "file system 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 ("file system 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
). Two capabilities are relevant for file permissions
. (A process has
these capabilities if its fsuid is 0.)
capability overrides all permission checking, but
only grants execute permission when at least one of the file's three execute
permission bits is set.
capability grants read and search permission on
directories, and read permission on ordinary files.
This page is part of release 3.44 of the Linux man-pages
description of the project, and information about reporting bugs, can be found