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       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
       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 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.

   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,

       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

   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.

       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

       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
       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.


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


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