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PATH_RESOLUTION(7)         Linux Programmer's Manual        PATH_RESOLUTION(7)



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 call-
       ing  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 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 cur-
       rent 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 cur-
       rent 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 direc-
       tory, we set the current lookup directory to that directory, and go to the next component.  Note that the reso-
       lution  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 sym-
       bolic 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 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  trail-
       ing '/' 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  sys-
       tem 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 resolv-
       ing 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 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 pro-
       cess, 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 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 ("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 rele-
       vant  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), credentials(7), symlink(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-11-20                PATH_RESOLUTION(7)