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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  dif‐
       ferent  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 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 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  resolu‐
       tion  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
       component  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 fol‐
       lowed while resolving a pathname is 40.  In kernels before 2.6.18, the limit on the recur‐
       sion  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  recursion,  so
       that the only limit that remains is the maximum of 40 resolutions 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 compo‐
       nents, 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 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  mean‐
       ings, 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
       ("Filename 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  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 setgroups(2)).  When neither
       holds, the third group is used.

       Of the three bits used, the first bit determines read permission, the second write permis‐
       sion,  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  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 ("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 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 grants execute per‐
       mission 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 4.04 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2015-12-05                         PATH_RESOLUTION(7)

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