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 rel-
       ative 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 direc-
       tory.

       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 direc-
       tory").

       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 ENOT-
       DIR 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 previ-
       ous 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 direc-
       tory, 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 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 sym-
       bolic 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  path-
       name 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 supple-
       mentary 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  per-
       mission 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 effec-
       tive 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 obso-
       lete now.  Nobody should use setfsuid(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)