Issue #22636: avoid using a shell in the ctypes.util module

Replace os.popen() with subprocess.Popen. Based on patch by Victor Stinner.

If the "gcc", "cc" or "objdump" command is not available, the code was
supposed to raise an OSError exception. But there was a bug in the code. The
shell code returns the exit code 10 if the required command is missing, and the
code tries to check for the status 10. The problem is that os.popen() doesn't
return the exit code directly, but a status which should be processed by
os.WIFEXITED() and os.WEXITSTATUS(). In practice, the exception was never
raised. The OSError exception was not documented and ctypes.util.find_library()
is expected to return None if the library is not found.

Lib/ctypes/test/test_find.py

 import unittest
-import os
+import os, os.path
 import sys
 import test.support
 from ctypes import *
@@ -64,6 +64,11 @@ class Test_OpenGL_libs(unittest.TestCase):
             self.skipTest('lib_gle not available')
         self.gle.gleGetJoinStyle
 
+    def test_shell_injection(self):
+        result = find_library('; echo Hello shell > ' + test.support.TESTFN)
+        self.assertFalse(os.path.lexists(test.support.TESTFN))
+        self.assertIsNone(result)
+
 # On platforms where the default shared library suffix is '.so',
 # at least some libraries can be loaded as attributes of the cdll
 # object, since ctypes now tries loading the lib again

Lib/ctypes/util.py

-import sys, os
-import contextlib
+import os
+import shutil
 import subprocess
+import sys
 
 # find_library(name) returns the pathname of a library, or None.
 if os.name == "nt":
@@ -94,28 +95,43 @@ elif os.name == "posix":
     import re, tempfile
 
     def _findLib_gcc(name):
-        expr = r'[^\(\)\s]*lib%s\.[^\(\)\s]*' % re.escape(name)
-        fdout, ccout = tempfile.mkstemp()
-        os.close(fdout)
-        cmd = 'if type gcc >/dev/null 2>&1; then CC=gcc; elif type cc >/dev/null 2>&1; then CC=cc;else exit 10; fi;' \
-              'LANG=C LC_ALL=C $CC -Wl,-t -o ' + ccout + ' 2>&1 -l' + name
-        try:
-            f = os.popen(cmd)
+        # Run GCC's linker with the -t (aka --trace) option and examine the
+        # library name it prints out. The GCC command will fail because we
+        # haven't supplied a proper program with main(), but that does not
+        # matter.
+        expr = os.fsencode(r'[^\(\)\s]*lib%s\.[^\(\)\s]*' % re.escape(name))
+
+        c_compiler = shutil.which('gcc')
+        if not c_compiler:
+            c_compiler = shutil.which('cc')
+        if not c_compiler:
+            # No C compiler available, give up
+            return None
+
+        temp = tempfile.NamedTemporaryFile()
         try:
-                trace = f.read()
-            finally:
-                rv = f.close()
+            args = [c_compiler, '-Wl,-t', '-o', temp.name, '-l' + name]
+
+            env = dict(os.environ)
+            env['LC_ALL'] = 'C'
+            env['LANG'] = 'C'
+            proc = subprocess.Popen(args,
+                                    stdout=subprocess.PIPE,
+                                    stderr=subprocess.STDOUT,
+                                    env=env)
+            with proc:
+                trace = proc.stdout.read()
         finally:
             try:
-                os.unlink(ccout)
+                temp.close()
             except FileNotFoundError:
+                # Raised if the file was already removed, which is the normal
+                # behaviour of GCC if linking fails
                 pass
-        if rv == 10:
-            raise OSError('gcc or cc command not found')
         res = re.search(expr, trace)
         if not res:
             return None
-        return res.group(0)
+        return os.fsdecode(res.group(0))
 
 
     if sys.platform == "sunos5":
@@ -123,55 +139,65 @@ elif os.name == "posix":
         def _get_soname(f):
             if not f:
                 return None
-            cmd = "/usr/ccs/bin/dump -Lpv 2>/dev/null " + f
-            with contextlib.closing(os.popen(cmd)) as f:
-                data = f.read()
-            res = re.search(r'\[.*\]\sSONAME\s+([^\s]+)', data)
+
+            proc = subprocess.Popen(("/usr/ccs/bin/dump", "-Lpv", f),
+                                    stdout=subprocess.PIPE,
+                                    stderr=subprocess.DEVNULL)
+            with proc:
+                data = proc.stdout.read()
+            res = re.search(br'\[.*\]\sSONAME\s+([^\s]+)', data)
             if not res:
                 return None
-            return res.group(1)
+            return os.fsdecode(res.group(1))
     else:
         def _get_soname(f):
             # assuming GNU binutils / ELF
             if not f:
                 return None
-            cmd = 'if ! type objdump >/dev/null 2>&1; then exit 10; fi;' \
-                  "objdump -p -j .dynamic 2>/dev/null " + f
-            f = os.popen(cmd)
-            try:
-                dump = f.read()
-            finally:
-                rv = f.close()
-            if rv == 10:
-                raise OSError('objdump command not found')
-            res = re.search(r'\sSONAME\s+([^\s]+)', dump)
+            objdump = shutil.which('objdump')
+            if not objdump:
+                # objdump is not available, give up
+                return None
+
+            proc = subprocess.Popen((objdump, '-p', '-j', '.dynamic', f),
+                                    stdout=subprocess.PIPE,
+                                    stderr=subprocess.DEVNULL)
+            with proc:
+                dump = proc.stdout.read()
+            res = re.search(br'\sSONAME\s+([^\s]+)', dump)
             if not res:
                 return None
-            return res.group(1)
+            return os.fsdecode(res.group(1))
 
     if sys.platform.startswith(("freebsd", "openbsd", "dragonfly")):
 
         def _num_version(libname):
             # "libxyz.so.MAJOR.MINOR" => [ MAJOR, MINOR ]
-            parts = libname.split(".")
+            parts = libname.split(b".")
             nums = []
             try:
                 while parts:
                     nums.insert(0, int(parts.pop()))
             except ValueError:
                 pass
-            return nums or [ sys.maxsize ]
+            return nums or [sys.maxsize]
 
         def find_library(name):
             ename = re.escape(name)
             expr = r':-l%s\.\S+ => \S*/(lib%s\.\S+)' % (ename, ename)
-            with contextlib.closing(os.popen('/sbin/ldconfig -r 2>/dev/null')) as f:
-                data = f.read()
+            expr = os.fsencode(expr)
+
+            proc = subprocess.Popen(('/sbin/ldconfig', '-r'),
+                                    stdout=subprocess.PIPE,
+                                    stderr=subprocess.DEVNULL)
+            with proc:
+                data = proc.stdout.read()
+
             res = re.findall(expr, data)
             if not res:
                 return _get_soname(_findLib_gcc(name))
             res.sort(key=_num_version)
-            return res[-1]
+            return os.fsdecode(res[-1])
 
     elif sys.platform == "sunos5":
 
@@ -179,17 +205,24 @@ elif os.name == "posix":
             if not os.path.exists('/usr/bin/crle'):
                 return None
 
+            env = dict(os.environ)
+            env['LC_ALL'] = 'C'
+
             if is64:
-                cmd = 'env LC_ALL=C /usr/bin/crle -64 2>/dev/null'
+                args = ('/usr/bin/crle', '-64')
             else:
-                cmd = 'env LC_ALL=C /usr/bin/crle 2>/dev/null'
+                args = ('/usr/bin/crle',)
 
             paths = None
-            with contextlib.closing(os.popen(cmd)) as f:
-                for line in f.readlines():
+            proc = subprocess.Popen(args,
+                                    stdout=subprocess.PIPE,
+                                    stderr=subprocess.DEVNULL,
+                                    env=env)
+            with proc:
+                for line in proc.stdout:
                     line = line.strip()
-                    if line.startswith('Default Library Path (ELF):'):
-                        paths = line.split()[4]
+                    if line.startswith(b'Default Library Path (ELF):'):
+                        paths = os.fsdecode(line).split()[4]
 
             if not paths:
                 return None

Misc/NEWS

@@ -13,6 +13,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #22636: Avoid shell injection problems with
+  ctypes.util.find_library().
+
 - Issue #16182: Fix various functions in the "readline" module to use the
   locale encoding, and fix get_begidx() and get_endidx() to return code point
   indexes.