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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, or may temporarily use a
       different root directory by using openat2(2) with the RESOLVE_IN_ROOT flag set.

       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 — or in the case of
       openat(2)-style system calls, the dfd  argument  (or  the  current  working  directory  if
       AT_FDCWD  is passed as the dfd argument).  The current working directory is inherited from
       the parent, and 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  symbolic  link 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  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
       followed while resolving a pathname is 40.  In kernels before 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 recursion,
       so  that  the  only  limit  that  remains  is the maximum of 40 resolutions for the entire
       pathname.

       The resolution of symbolic links during this stage can be  blocked  by  using  openat2(2),
       with the RESOLVE_NO_SYMLINKS flag set.

   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 up 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".

       Traversal  of  mount  points  can be blocked by using openat2(2), with the RESOLVE_NO_XDEV
       flag set (though note that this also restricts bind mount traversal).

   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;  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 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

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