doc-rst: add kernel-include directive

The kernel-include directive is needed to include the auto generated rst
content from a build (pre-) process. E.g. the linux_tv Makefile
generates intermediate reST-files from header files. Since there is a O=
option:

  make O=dir [targets] Locate all output files in "dir"

We need to include intermediate reST files from arbitrary (O=/tmp/foo)
locations:

The 'kernel-include' reST-directive is a replacement for the 'include'
directive. The 'kernel-include' directive expand environment variables
in the path name and allows to include files from arbitrary locations.

.. hint::

  Including files from arbitrary locations (e.g. from '/etc') is a
  security risk for builders. This is why the 'include' directive from
  docutils *prohibit* pathnames pointing to locations *above* the
  filesystem tree where the reST document with the include directive is
  placed.

Substrings of the form $name or ${name} are replaced by the value of
environment variable name. Malformed variable names and references to
non-existing variables are left unchanged.
Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/sphinx/kernel_include.py

+#!/usr/bin/env python3
+# -*- coding: utf-8; mode: python -*-
+# pylint: disable=R0903, C0330, R0914, R0912, E0401
+
+u"""
+    kernel-include
+    ~~~~~~~~~~~~~~
+
+    Implementation of the ``kernel-include`` reST-directive.
+
+    :copyright:  Copyright (C) 2016  Markus Heiser
+    :license:    GPL Version 2, June 1991 see linux/COPYING for details.
+
+    The ``kernel-include`` reST-directive is a replacement for the ``include``
+    directive. The ``kernel-include`` directive expand environment variables in
+    the path name and allows to include files from arbitrary locations.
+
+    .. hint::
+
+      Including files from arbitrary locations (e.g. from ``/etc``) is a
+      security risk for builders. This is why the ``include`` directive from
+      docutils *prohibit* pathnames pointing to locations *above* the filesystem
+      tree where the reST document with the include directive is placed.
+
+    Substrings of the form $name or ${name} are replaced by the value of
+    environment variable name. Malformed variable names and references to
+    non-existing variables are left unchanged.
+"""
+
+# ==============================================================================
+# imports
+# ==============================================================================
+
+import os.path
+
+from docutils import io, nodes, statemachine
+from docutils.utils.error_reporting import SafeString, ErrorString
+from docutils.parsers.rst import directives
+from docutils.parsers.rst.directives.body import CodeBlock, NumberLines
+from docutils.parsers.rst.directives.misc import Include
+
+# ==============================================================================
+def setup(app):
+# ==============================================================================
+
+    app.add_directive("kernel-include", KernelInclude)
+
+# ==============================================================================
+class KernelInclude(Include):
+# ==============================================================================
+
+    u"""KernelInclude (``kernel-include``) directive"""
+
+    def run(self):
+        path = os.path.realpath(
+            os.path.expandvars(self.arguments[0]))
+
+        # to get a bit security back, prohibit /etc:
+        if path.startswith(os.sep + "etc"):
+            raise self.severe(
+                'Problems with "%s" directive, prohibited path: %s'
+                % (self.name, path))
+
+        self.arguments[0] = path
+
+        #return super(KernelInclude, self).run() # won't work, see HINTs in _run()
+        return self._run()
+
+    def _run(self):
+        """Include a file as part of the content of this reST file."""
+
+        # HINT: I had to copy&paste the whole Include.run method. I'am not happy
+        # with this, but due to security reasons, the Include.run method does
+        # not allow absolute or relative pathnames pointing to locations *above*
+        # the filesystem tree where the reST document is placed.
+
+        if not self.state.document.settings.file_insertion_enabled:
+            raise self.warning('"%s" directive disabled.' % self.name)
+        source = self.state_machine.input_lines.source(
+            self.lineno - self.state_machine.input_offset - 1)
+        source_dir = os.path.dirname(os.path.abspath(source))
+        path = directives.path(self.arguments[0])
+        if path.startswith('<') and path.endswith('>'):
+            path = os.path.join(self.standard_include_path, path[1:-1])
+        path = os.path.normpath(os.path.join(source_dir, path))
+
+        # HINT: this is the only line I had to change / commented out:
+        #path = utils.relative_path(None, path)
+
+        path = nodes.reprunicode(path)
+        encoding = self.options.get(
+            'encoding', self.state.document.settings.input_encoding)
+        e_handler=self.state.document.settings.input_encoding_error_handler
+        tab_width = self.options.get(
+            'tab-width', self.state.document.settings.tab_width)
+        try:
+            self.state.document.settings.record_dependencies.add(path)
+            include_file = io.FileInput(source_path=path,
+                                        encoding=encoding,
+                                        error_handler=e_handler)
+        except UnicodeEncodeError as error:
+            raise self.severe('Problems with "%s" directive path:\n'
+                              'Cannot encode input file path "%s" '
+                              '(wrong locale?).' %
+                              (self.name, SafeString(path)))
+        except IOError as error:
+            raise self.severe('Problems with "%s" directive path:\n%s.' %
+                      (self.name, ErrorString(error)))
+        startline = self.options.get('start-line', None)
+        endline = self.options.get('end-line', None)
+        try:
+            if startline or (endline is not None):
+                lines = include_file.readlines()
+                rawtext = ''.join(lines[startline:endline])
+            else:
+                rawtext = include_file.read()
+        except UnicodeError as error:
+            raise self.severe('Problem with "%s" directive:\n%s' %
+                              (self.name, ErrorString(error)))
+        # start-after/end-before: no restrictions on newlines in match-text,
+        # and no restrictions on matching inside lines vs. line boundaries
+        after_text = self.options.get('start-after', None)
+        if after_text:
+            # skip content in rawtext before *and incl.* a matching text
+            after_index = rawtext.find(after_text)
+            if after_index < 0:
+                raise self.severe('Problem with "start-after" option of "%s" '
+                                  'directive:\nText not found.' % self.name)
+            rawtext = rawtext[after_index + len(after_text):]
+        before_text = self.options.get('end-before', None)
+        if before_text:
+            # skip content in rawtext after *and incl.* a matching text
+            before_index = rawtext.find(before_text)
+            if before_index < 0:
+                raise self.severe('Problem with "end-before" option of "%s" '
+                                  'directive:\nText not found.' % self.name)
+            rawtext = rawtext[:before_index]
+
+        include_lines = statemachine.string2lines(rawtext, tab_width,
+                                                  convert_whitespace=True)
+        if 'literal' in self.options:
+            # Convert tabs to spaces, if `tab_width` is positive.
+            if tab_width >= 0:
+                text = rawtext.expandtabs(tab_width)
+            else:
+                text = rawtext
+            literal_block = nodes.literal_block(rawtext, source=path,
+                                    classes=self.options.get('class', []))
+            literal_block.line = 1
+            self.add_name(literal_block)
+            if 'number-lines' in self.options:
+                try:
+                    startline = int(self.options['number-lines'] or 1)
+                except ValueError:
+                    raise self.error(':number-lines: with non-integer '
+                                     'start value')
+                endline = startline + len(include_lines)
+                if text.endswith('\n'):
+                    text = text[:-1]
+                tokens = NumberLines([([], text)], startline, endline)
+                for classes, value in tokens:
+                    if classes:
+                        literal_block += nodes.inline(value, value,
+                                                      classes=classes)
+                    else:
+                        literal_block += nodes.Text(value, value)
+            else:
+                literal_block += nodes.Text(text, text)
+            return [literal_block]
+        if 'code' in self.options:
+            self.options['source'] = path
+            codeblock = CodeBlock(self.name,
+                                  [self.options.pop('code')], # arguments
+                                  self.options,
+                                  include_lines, # content
+                                  self.lineno,
+                                  self.content_offset,
+                                  self.block_text,
+                                  self.state,
+                                  self.state_machine)
+            return codeblock.run()
+        self.state_machine.insert_input(include_lines, path)
+        return []