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

       path_resolution - how a pathname is resolved to a file

       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  di-
       rectory is the root directory of the calling process.  A process inher-
       its 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) sys-
       tem 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  di-
       rectory  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 in-
       herited from the parent, and can be changed by use of the chdir(2) sys-
       tem 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 start-
       ing lookup directory).  Upon error, that error is returned.  If the re-
       sult is not a directory, an ENOTDIR error is returned.  If the  resolu-
       tion of the symbolic link is successful and returns a directory, we set
       the current lookup directory to that directory, and go to the next com-
       ponent.  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 com-
       ponent 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 be-
       fore  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 recur-
       sion, so that the only limit that remains is the maximum of 40  resolu-
       tions 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  spe-
       cific system call), and (ii) it is not necessarily an error if the com-
       ponent 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  re-
       fer 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 par-
       ent 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) op-
       erates on the file pointed to by the symlink.

   Length limit
       There is a maximum length for pathnames.  If the pathname (or some  in-
       termediate  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 direc-
       tory.   Nowadays  POSIX  decrees that an empty pathname must not be re-
       solved successfully.  Linux returns ENOENT in this case.

       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 set-
       groups(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 or-
       dinary 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 con-
       cept  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 setf-

       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

       On Linux, superuser privileges are divided into capabilities (see capa-
       bilities(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.

       readlink(2), capabilities(7), credentials(7), symlink(7)

       This  page  is  part of release 5.07 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

Linux                             2020-04-11                PATH_RESOLUTION(7)

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