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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 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. 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 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 (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 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. In kernels before 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.
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 down 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".
Trailing slashes
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.)
Final symlink
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 symlink, while
stat(2) operates on the file pointed to by the symlink.
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, cf. 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.
SEE ALSO
readlink(2), capabilities(7), credentials(7), symlink(7)
COLOPHON
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