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NAME

       Unix/Linux path resolution - find the file referred to by a filename

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 current 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  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 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
       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 symlinks 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  non-directory, 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 filesystem.

       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 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".

   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
       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 current 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 current process, or is one
       of the supplementary group IDs  of  the  current  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 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

       capabilities(7)