docs: kernel_abi.py: fix command injection

The kernel-abi directive passes its argument straight to the shell.
This is unfortunate and unnecessary.

Let's always use paths relative to $srctree/Documentation/ and use
subprocess.check_call() instead of subprocess.Popen(shell=True).

This also makes the code shorter.

Link: https://fosstodon.org/@jani/111676532203641247
Reported-by: Jani Nikula <[e-mail]>
Cc: [e-mail]
Signed-off-by: Vegard Nossum <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/admin-guide/abi-obsolete.rst

···
 The description of the interface will document the reason why it is
 obsolete and when it can be expected to be removed.
 
-.. kernel-abi:: $srctree/Documentation/ABI/obsolete
+.. kernel-abi:: ABI/obsolete
    :rst:

Documentation/admin-guide/abi-removed.rst

···
 ABI removed symbols
 ===================
 
-.. kernel-abi:: $srctree/Documentation/ABI/removed
+.. kernel-abi:: ABI/removed
    :rst:

Documentation/admin-guide/abi-stable.rst

···
 Most interfaces (like syscalls) are expected to never change and always
 be available.
 
-.. kernel-abi:: $srctree/Documentation/ABI/stable
+.. kernel-abi:: ABI/stable
    :rst:

Documentation/admin-guide/abi-testing.rst

···
 name to the description of these interfaces, so that the kernel
 developers can easily notify them if any changes occur.
 
-.. kernel-abi:: $srctree/Documentation/ABI/testing
+.. kernel-abi:: ABI/testing
    :rst:

Documentation/sphinx/kernel_abi.py

···
 import re
 import kernellog
 
-from os import path
-
 from docutils import nodes, statemachine
 from docutils.statemachine import ViewList
 from docutils.parsers.rst import directives, Directive
···
     }
 
     def run(self):
-
         doc = self.state.document
         if not doc.settings.file_insertion_enabled:
             raise self.warning("docutils: file insertion disabled")
 
-        env = doc.settings.env
-        cwd = path.dirname(doc.current_source)
-        cmd = "get_abi.pl rest --enable-lineno --dir "
-        cmd += self.arguments[0]
+        srctree = os.path.abspath(os.environ["srctree"])
+
+        args = [
+            os.path.join(srctree, 'scripts/get_abi.pl'),
+            'rest',
+            '--enable-lineno',
+            '--dir', os.path.join(srctree, 'Documentation', self.arguments[0]),
+        ]
 
         if 'rst' in self.options:
-            cmd += " --rst-source"
+            args.append('--rst-source')
 
-        srctree = path.abspath(os.environ["srctree"])
-
-        fname = cmd
-
-        # extend PATH with $(srctree)/scripts
-        path_env = os.pathsep.join([
-            srctree + os.sep + "scripts",
-            os.environ["PATH"]
-        ])
-        shell_env = os.environ.copy()
-        shell_env["PATH"]    = path_env
-        shell_env["srctree"] = srctree
-
-        lines = self.runCmd(cmd, shell=True, cwd=cwd, env=shell_env)
+        lines = subprocess.check_output(args, cwd=os.path.dirname(doc.current_source)).decode('utf-8')
         nodeList = self.nestedParse(lines, self.arguments[0])
         return nodeList
-
-    def runCmd(self, cmd, **kwargs):
-        u"""Run command ``cmd`` and return its stdout as unicode."""
-
-        try:
-            proc = subprocess.Popen(
-                cmd
-                , stdout = subprocess.PIPE
-                , stderr = subprocess.PIPE
-                , **kwargs
-            )
-            out, err = proc.communicate()
-
-            out, err = codecs.decode(out, 'utf-8'), codecs.decode(err, 'utf-8')
-
-            if proc.returncode != 0:
-                raise self.severe(
-                    u"command '%s' failed with return code %d"
-                    % (cmd, proc.returncode)
-                )
-        except OSError as exc:
-            raise self.severe(u"problems with '%s' directive: %s."
-                              % (self.name, ErrorString(exc)))
-        return out
 
     def nestedParse(self, lines, fname):
         env = self.state.document.settings.env