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     NAME
          path_resolution - how a pathname is resolved to a file

     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
          If the pathname starts with the aq/aq 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, or may temporarily use a
          different root directory by using openat2(2) with the
          RESOLVE_IN_ROOT flag set.

          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 aq/aq part of the pathname.

          If the pathname does not start with the aq/aq character, the
          starting lookup directory of the resolution process is the
          current working directory of the process - or in the case of
          openat(2)-style system calls, the dfd argument (or the cur-
          rent working directory if AT_FDCWD is passed as the dfd
          argument).  The current working directory is inherited from
          the parent, and can be changed by use of the chdir(2) system
          call.)

          Pathnames starting with a aq/aq character are called absolute
          pathnames.  Pathnames not starting with a aq/aq 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 path-
          name, where a component is a substring delimited by aq/aq
          characters, this component is looked up in the current
          lookup directory.

          If the process does not have search permission on the cur-
          rent lookup directory, an EACCES error is returned ("Permis-
          sion 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

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          symbolic link, an ENOTDIR error is returned ("Not a direc-
          tory").

          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 sym-
          bolic link 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 sym-
          bolic 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 path-
          name 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 pathname.

          The resolution of symbolic links during this stage can be
          blocked by using openat2(2), with the RESOLVE_NO_SYMLINKS
          flag set.

        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 pre-
          vious step, with two differences: (i) the final component
          need not be a directory (at least as far as the path resolu-
          tion 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 "..",

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          which refer to the directory itself and to its parent direc-
          tory, 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 up 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".

          Traversal of mount points can be blocked by using
          openat2(2), with the RESOLVE_NO_XDEV flag set (though note
          that this also restricts bind mount traversal).

        Trailing slashes
          If a pathname ends in a aq/aq, that forces resolution of the
          preceding component as in Step 2: it has to exist and
          resolve to a directory.  Otherwise, a trailing aq/aq is
          ignored.  (Or, equivalently, a pathname with a trailing aq/aq
          is equivalent to the pathname obtained by appending aq.aq 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 sym-
          bolic 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

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          three bits; see chmod(1) and stat(2).  The first group of
          three is used when the effective user ID of the calling pro-
          cess 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 per-
          mission, 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 sys-
          tem 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 restric-
          tions 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
          readlink(2), capabilities(7), credentials(7), symlink(7)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages

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          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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