path_resolution

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 ances-
       tors -- was started by an invocation of the clone(2) system  call  that
       had  the CLONE_NEWNS flag set.)  This handles the '/' part of the path-
       name.

       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 non-final 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 start-
       ing  lookup  directory).   Upon  error, that error is returned.  If the
       result is not a directory, an ENOTDIR error is returned.  If the  reso-
       lution of the symlink is successful and returns a directory, we set the
       current lookup directory to that directory, and go to the  next  compo-
       nent.   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").

   . and ..
       By convention, every directory has the  entries  "."  and  "..",  which
       refer  to  the  directory  itself  and to its parent directory, respec-
       tively.

       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 par-
       ent 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  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 ("File name too long").

   Empty pathname
       In the original Unix, the empty pathname referred to the current direc-
       tory.  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  set-
       groups(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
       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 capa-
       bilities(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
       capabilities(7), credentials(7), symlink(7)

COLOPHON
       This page is part of release 3.23 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                             2008-11-20                PATH_RESOLUTION(7)