path_resolution

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  di-
       rectory  is  the root directory of the calling process.  (A process in-
       herits 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  di-
       rectory  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 re-
       sult  is not a directory, an ENOTDIR error is returned.  If the resolu-
       tion 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 be-
       fore 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  re-
       fer 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 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) op-
       erates on the file pointed to by the symlink.

   Length limit
       There is a maximum length for pathnames.  If the pathname (or some  in-
       termediate  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 re-
       solved successfully.  Linux returns ENOENT in this case.

   Permissions
       The permission bits of a file consist of three groups  of  three  bits;
       see  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 or-
       dinary 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-
       suid(2).)

       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
       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
       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.

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

COLOPHON
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-11-26                PATH_RESOLUTION(7)