NAME | DESCRIPTION | SEE ALSO | COLOPHON

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

NAME         top

       path_resolution - how a pathname is resolved to a file

DESCRIPTION         top

       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 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 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.

   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 "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.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 3.73 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

Linux                            2009-12-05               PATH_RESOLUTION(7)