PATH_RESOLUTION(7)          Linux Programmer's 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

       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 (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
       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

       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.

       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

   . 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

   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

   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.

       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 release 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2021-08-27                 PATH_RESOLUTION(7)