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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

   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 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 "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  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;  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.


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


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