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path_resolution


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 current 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  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 start-
       ing lookup directory). Upon error, that  error  is  returned.   If  the
       result  is not a directory, an ENOTDIR error is returned.  If the reso-
       lution of the symlink is successful and returns a directory, we set the
       current  lookup  directory to that directory, and go to the next compo-
       nent.  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 symlinks followed. An ELOOP error

   . and ..
       By  convention,  every  directory  has  the entries "." and "..", which
       refer to the directory itself and  to  its  parent  directory,  respec-
       tively.

       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 par-
       ent directory of "path", outside of the filesystem 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 direc-
       tory.  Nowadays POSIX decrees  that  an  empty  pathname  must  not  be
       resolved successfully. Linux returns ENOENT in this case.

   Permissions
       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 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
       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)


Linux 2.6.7 2004-06-21 PATH_RESOLUTION(2)



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