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.  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 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

   . 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 "/".

   Mount points
       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 "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

   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, 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 "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
       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
       only grants execute permission 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 3.41 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can be
       found at

Linux                              2009-12-05                 PATH_RESOLUTION(7)