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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 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 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 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 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  com-
       ponent  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	recur-
       sion,  so that the only limit that remains is the maximum of 40 resolu-
       tions for the entire pathname.

   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  spe-
       cific system call), and (ii) it is not necessarily an error if the com-
       ponent 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, respec-

       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  par-
       ent 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 ("Filename 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.

       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
       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 con-
       cept 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-

       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 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
       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.

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

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Linux				  2015-12-05		    PATH_RESOLUTION(7)