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 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  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 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  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 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  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  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  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 3.54 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/.