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PATH_RESOLUTION(7)         Linux Programmer's Manual        PATH_RESOLUTION(7)



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 pro-
       cess  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  direc-
       tory  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 direc-
       tory").

       If the component is found, but is neither a directory nor a symbolic link, an ENOT-
       DIR 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 max-
       imum number of symbolic links followed.  An ELOOP error is returned when the  maxi-
       mum 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 sys-
       tem 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 sys-
       tem.

       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 sys-
       tem 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 oper-
       ate 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.   Nowa-
       days 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.  Ordi-
       narily 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  directo-
       ries, and read permission on ordinary files.

SEE ALSO
       capabilities(7), credentials(7), symlink(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2008-11-20                PATH_RESOLUTION(7)

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