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 <jani.nikula@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231231235959.3342928-2-vegard.nossum@oracle.com

Documentation/admin-guide/abi-obsolete.rst

@@ -7,5 +7,5 @@ marked to be removed at some later point in time.
 The description of the interface will document the reason why it is
 obsolete and when it can be expected to be removed.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

@@ -1,5 +1,5 @@
 ABI removed symbols
 ===================
 
-.. kernel-abi:: $srctree/Documentation/ABI/removed
+.. kernel-abi:: ABI/removed
    :rst:

Documentation/admin-guide/abi-stable.rst

@@ -10,5 +10,5 @@ for at least 2 years.
 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

@@ -16,5 +16,5 @@ Programs that use these interfaces are strongly encouraged to add their
 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

@@ -39,8 +39,6 @@ import sys
 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
@@ -73,60 +71,26 @@ class KernelCmd(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
         content = ViewList()