gh-126554: ctypes: Correctly handle NULL dlsym values (GH-126555)

For dlsym(), a return value of NULL does not necessarily indicate
an error [1].

Therefore, to avoid using stale (or NULL) dlerror() values, we must:

 1. clear the previous error state by calling dlerror()
 2. call dlsym()
 3. call dlerror()

If the return value of dlerror() is not NULL, an error occured.

In ctypes we choose to treat a NULL return value from dlsym()
as a "not found" error. This is the same as the fallback
message we use on Windows, Cygwin or when getting/formatting
the error reason fails.

[1]: https://man7.org/linux/man-pages/man3/dlsym.3.html

Signed-off-by: Georgios Alexopoulos <grgalex42@gmail.com>
Signed-off-by: Georgios Alexopoulos <grgalex@ba.uoa.gr>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>

Lib/test/test_ctypes/test_dlerror.py

@@ -0,0 +1,123 @@
+import os
+import sys
+import unittest
+import platform
+
+FOO_C = r"""
+#include <unistd.h>
+
+/* This is a 'GNU indirect function' (IFUNC) that will be called by
+   dlsym() to resolve the symbol "foo" to an address. Typically, such
+   a function would return the address of an actual function, but it
+   can also just return NULL.  For some background on IFUNCs, see
+   https://willnewton.name/uncategorized/using-gnu-indirect-functions.
+
+   Adapted from Michael Kerrisk's answer: https://stackoverflow.com/a/53590014.
+*/
+
+asm (".type foo STT_GNU_IFUNC");
+
+void *foo(void)
+{
+    write($DESCRIPTOR, "OK", 2);
+    return NULL;
+}
+"""
+
+
+@unittest.skipUnless(sys.platform.startswith('linux'),
+                     'Test only valid for Linux')
+class TestNullDlsym(unittest.TestCase):
+    """GH-126554: Ensure that we catch NULL dlsym return values
+
+    In rare cases, such as when using GNU IFUNCs, dlsym(),
+    the C function that ctypes' CDLL uses to get the address
+    of symbols, can return NULL.
+
+    The objective way of telling if an error during symbol
+    lookup happened is to call glibc's dlerror() and check
+    for a non-NULL return value.
+
+    However, there can be cases where dlsym() returns NULL
+    and dlerror() is also NULL, meaning that glibc did not
+    encounter any error.
+
+    In the case of ctypes, we subjectively treat that as
+    an error, and throw a relevant exception.
+
+    This test case ensures that we correctly enforce
+    this 'dlsym returned NULL -> throw Error' rule.
+    """
+
+    def test_null_dlsym(self):
+        import subprocess
+        import tempfile
+
+        # To avoid ImportErrors on Windows, where _ctypes does not have
+        # dlopen and dlsym,
+        # import here, i.e., inside the test function.
+        # The skipUnless('linux') decorator ensures that we're on linux
+        # if we're executing these statements.
+        from ctypes import CDLL, c_int
+        from _ctypes import dlopen, dlsym
+
+        retcode = subprocess.call(["gcc", "--version"],
+                                  stdout=subprocess.DEVNULL,
+                                  stderr=subprocess.DEVNULL)
+        if retcode != 0:
+            self.skipTest("gcc is missing")
+
+        pipe_r, pipe_w = os.pipe()
+        self.addCleanup(os.close, pipe_r)
+        self.addCleanup(os.close, pipe_w)
+
+        with tempfile.TemporaryDirectory() as d:
+            # Create a C file with a GNU Indirect Function (FOO_C)
+            # and compile it into a shared library.
+            srcname = os.path.join(d, 'foo.c')
+            dstname = os.path.join(d, 'libfoo.so')
+            with open(srcname, 'w') as f:
+                f.write(FOO_C.replace('$DESCRIPTOR', str(pipe_w)))
+            args = ['gcc', '-fPIC', '-shared', '-o', dstname, srcname]
+            p = subprocess.run(args, capture_output=True)
+
+            if p.returncode != 0:
+                # IFUNC is not supported on all architectures.
+                if platform.machine() == 'x86_64':
+                    # It should be supported here. Something else went wrong.
+                    p.check_returncode()
+                else:
+                    # IFUNC might not be supported on this machine.
+                    self.skipTest(f"could not compile indirect function: {p}")
+
+            # Case #1: Test 'PyCFuncPtr_FromDll' from Modules/_ctypes/_ctypes.c
+            L = CDLL(dstname)
+            with self.assertRaisesRegex(AttributeError, "function 'foo' not found"):
+                # Try accessing the 'foo' symbol.
+                # It should resolve via dlsym() to NULL,
+                # and since we subjectively treat NULL
+                # addresses as errors, we should get
+                # an error.
+                L.foo
+
+            # Assert that the IFUNC was called
+            self.assertEqual(os.read(pipe_r, 2), b'OK')
+
+            # Case #2: Test 'CDataType_in_dll_impl' from Modules/_ctypes/_ctypes.c
+            with self.assertRaisesRegex(ValueError, "symbol 'foo' not found"):
+                c_int.in_dll(L, "foo")
+
+            # Assert that the IFUNC was called
+            self.assertEqual(os.read(pipe_r, 2), b'OK')
+
+            # Case #3: Test 'py_dl_sym' from Modules/_ctypes/callproc.c
+            L = dlopen(dstname)
+            with self.assertRaisesRegex(OSError, "symbol 'foo' not found"):
+                dlsym(L, "foo")
+
+            # Assert that the IFUNC was called
+            self.assertEqual(os.read(pipe_r, 2), b'OK')
+
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/C_API/2024-11-07-20-24-58.gh-issue-126554.ri12eb.rst

@@ -0,0 +1,2 @@
+Fix error handling in :class:`ctypes.CDLL` objects
+which could result in a crash in rare situations.

Modules/_ctypes/_ctypes.c

@@ -956,34 +956,50 @@ CDataType_in_dll_impl(PyObject *type, PyTypeObject *cls, PyObject *dll,
         return NULL;
     }
 
+#undef USE_DLERROR
 #ifdef MS_WIN32
     Py_BEGIN_ALLOW_THREADS
     address = (void *)GetProcAddress(handle, name);
     Py_END_ALLOW_THREADS
-    if (!address) {
-        PyErr_Format(PyExc_ValueError,
-                     "symbol '%s' not found",
-                     name);
-        return NULL;
-    }
 #else
-    address = (void *)dlsym(handle, name);
-    if (!address) {
     #ifdef __CYGWIN__
-/* dlerror() isn't very helpful on cygwin */
-        PyErr_Format(PyExc_ValueError,
-                     "symbol '%s' not found",
-                     name);
+        // dlerror() isn't very helpful on cygwin
     #else
-        PyErr_SetString(PyExc_ValueError, dlerror());
+        #define USE_DLERROR
+        /* dlerror() always returns the latest error.
+         *
+         * Clear the previous value before calling dlsym(),
+         * to ensure we can tell if our call resulted in an error.
+         */
+        (void)dlerror();
     #endif
-        return NULL;
-    }
+    address = (void *)dlsym(handle, name);
 #endif
+
+    if (address) {
         ctypes_state *st = get_module_state_by_def(Py_TYPE(type));
         return PyCData_AtAddress(st, type, address);
     }
 
+    #ifdef USE_DLERROR
+    const char *dlerr = dlerror();
+    if (dlerr) {
+        PyObject *message = PyUnicode_DecodeLocale(dlerr, "surrogateescape");
+        if (message) {
+            PyErr_SetObject(PyExc_ValueError, message);
+            Py_DECREF(message);
+            return NULL;
+        }
+        // Ignore errors from PyUnicode_DecodeLocale,
+        // fall back to the generic error below.
+        PyErr_Clear();
+    }
+    #endif
+#undef USE_DLERROR
+    PyErr_Format(PyExc_ValueError, "symbol '%s' not found", name);
+    return NULL;
+}
+
 /*[clinic input]
 _ctypes.CDataType.from_param as CDataType_from_param
 
@@ -3759,6 +3775,7 @@ PyCFuncPtr_FromDll(PyTypeObject *type, PyObject *args, PyObject *kwds)
         return NULL;
     }
 
+#undef USE_DLERROR
 #ifdef MS_WIN32
     address = FindAddress(handle, name, (PyObject *)type);
     if (!address) {
@@ -3774,20 +3791,41 @@ PyCFuncPtr_FromDll(PyTypeObject *type, PyObject *args, PyObject *kwds)
         return NULL;
     }
 #else
-    address = (PPROC)dlsym(handle, name);
-    if (!address) {
     #ifdef __CYGWIN__
-/* dlerror() isn't very helpful on cygwin */
-        PyErr_Format(PyExc_AttributeError,
-                     "function '%s' not found",
-                     name);
+        //dlerror() isn't very helpful on cygwin */
     #else
-        PyErr_SetString(PyExc_AttributeError, dlerror());
+        #define USE_DLERROR
+        /* dlerror() always returns the latest error.
+         *
+         * Clear the previous value before calling dlsym(),
+         * to ensure we can tell if our call resulted in an error.
+         */
+        (void)dlerror();
     #endif
+    address = (PPROC)dlsym(handle, name);
+
+    if (!address) {
+	#ifdef USE_DLERROR
+        const char *dlerr = dlerror();
+        if (dlerr) {
+            PyObject *message = PyUnicode_DecodeLocale(dlerr, "surrogateescape");
+            if (message) {
+                PyErr_SetObject(PyExc_AttributeError, message);
+                Py_DECREF(ftuple);
+                Py_DECREF(message);
+                return NULL;
+            }
+            // Ignore errors from PyUnicode_DecodeLocale,
+            // fall back to the generic error below.
+            PyErr_Clear();
+        }
+	#endif
+        PyErr_Format(PyExc_AttributeError, "function '%s' not found", name);
         Py_DECREF(ftuple);
         return NULL;
     }
 #endif
+#undef USE_DLERROR
     ctypes_state *st = get_module_state_by_def(Py_TYPE(type));
     if (!_validate_paramflags(st, type, paramflags)) {
         Py_DECREF(ftuple);

Modules/_ctypes/callproc.c

@@ -1623,13 +1623,39 @@ static PyObject *py_dl_sym(PyObject *self, PyObject *args)
     if (PySys_Audit("ctypes.dlsym/handle", "O", args) < 0) {
         return NULL;
     }
+#undef USE_DLERROR
+    #ifdef __CYGWIN__
+        // dlerror() isn't very helpful on cygwin
+    #else
+        #define USE_DLERROR
+        /* dlerror() always returns the latest error.
+         *
+         * Clear the previous value before calling dlsym(),
+         * to ensure we can tell if our call resulted in an error.
+         */
+        (void)dlerror();
+    #endif
     ptr = dlsym((void*)handle, name);
-    if (!ptr) {
-        PyErr_SetString(PyExc_OSError,
-                               dlerror());
+    if (ptr) {
+        return PyLong_FromVoidPtr(ptr);
+    }
+	#ifdef USE_DLERROR
+    const char *dlerr = dlerror();
+    if (dlerr) {
+        PyObject *message = PyUnicode_DecodeLocale(dlerr, "surrogateescape");
+        if (message) {
+            PyErr_SetObject(PyExc_OSError, message);
+            Py_DECREF(message);
             return NULL;
         }
-    return PyLong_FromVoidPtr(ptr);
+        // Ignore errors from PyUnicode_DecodeLocale,
+        // fall back to the generic error below.
+        PyErr_Clear();
+    }
+	#endif
+	#undef USE_DLERROR
+    PyErr_Format(PyExc_OSError, "symbol '%s' not found", name);
+    return NULL;
 }
 #endif