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NAME

       Unix/Linux path resolution - find the file referred to by a filename

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 non-final 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 non-directory, 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 file system.

       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 file system hierarchy on the device "dev", and no longer to
       whatever it referred to earlier.

       One can walk out of a mounted file  system:  "path/.."  refers  to  the
       parent  directory  of  "path",  outside of the file system 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 ("File name 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 "file  system  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 ("file system 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
       only  grants  execute  permission 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

       capabilities(7), credentials(7), symlink(7)

COLOPHON

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