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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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