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           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 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 symbolic links followed.  An ELOOP
           error is returned when the maximum is exceeded  ("Too  many  levels  of
           symbolic links").
           refer to the directory itself and  to  its  parent  directory,  respec-
           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 ("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
           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  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
       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)

    Linux 2009-12-05 PATH_RESOLUTION(7)


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