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

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 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 "/".

   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 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 sym-
       bolic 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 ("File name 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 "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 setf-
       suid(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.

SEE ALSO

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

COLOPHON

       This  page is part of release 3.25 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2009-12-05							PATH_RESOLUTION(7)