path_resolution(7) — Linux manual page


PATH_RESOLUTION(7)        Linux Programmer's Manual       PATH_RESOLUTION(7)

NAME         top

       path_resolution - how a pathname is resolved to a file

DESCRIPTION         top

       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

   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 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.

       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,

       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

       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: 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.

       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

       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

       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         top

       readlink(2), capabilities(7), credentials(7), symlink(7)

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
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       latest version of this page, can be found at

Linux                            2020-04-11               PATH_RESOLUTION(7)

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