selftests/bpf: Test PTR_MAYBE_NULL arguments of struct_ops operators.

Test if the verifier verifies nullable pointer arguments correctly for BPF
struct_ops programs.

"test_maybe_null" in struct bpf_testmod_ops is the operator defined for the
test cases here.

A BPF program should check a pointer for NULL beforehand to access the
value pointed by the nullable pointer arguments, or the verifier should
reject the programs. The test here includes two parts; the programs
checking pointers properly and the programs not checking pointers
beforehand. The test checks if the verifier accepts the programs checking
properly and rejects the programs not checking at all.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20240209023750.1153905-5-thinker.li@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -555,7 +555,11 @@ static int bpf_dummy_reg(void *kdata)
 {
 	struct bpf_testmod_ops *ops = kdata;
 
-	ops->test_2(4, 3);
+	/* Some test cases (ex. struct_ops_maybe_null) may not have test_2
+	 * initialized, so we need to check for NULL.
+	 */
+	if (ops->test_2)
+		ops->test_2(4, 3);
 
 	return 0;
 }
@@ -573,9 +577,16 @@ static void bpf_testmod_test_2(int a, int b)
 {
 }
 
+static int bpf_testmod_ops__test_maybe_null(int dummy,
+					    struct task_struct *task__nullable)
+{
+	return 0;
+}
+
 static struct bpf_testmod_ops __bpf_testmod_ops = {
 	.test_1 = bpf_testmod_test_1,
 	.test_2 = bpf_testmod_test_2,
+	.test_maybe_null = bpf_testmod_ops__test_maybe_null,
 };
 
 struct bpf_struct_ops bpf_bpf_testmod_ops = {

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.h

@@ -5,6 +5,8 @@
 
 #include <linux/types.h>
 
+struct task_struct;
+
 struct bpf_testmod_test_read_ctx {
 	char *buf;
 	loff_t off;
@@ -31,6 +33,8 @@ struct bpf_iter_testmod_seq {
 struct bpf_testmod_ops {
 	int (*test_1)(void);
 	void (*test_2)(int a, int b);
+	/* Used to test nullable arguments. */
+	int (*test_maybe_null)(int dummy, struct task_struct *task);
 };
 
 #endif /* _BPF_TESTMOD_H */

tools/testing/selftests/bpf/prog_tests/test_struct_ops_maybe_null.c

@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Meta Platforms, Inc. and affiliates. */
+#include <test_progs.h>
+
+#include "struct_ops_maybe_null.skel.h"
+#include "struct_ops_maybe_null_fail.skel.h"
+
+/* Test that the verifier accepts a program that access a nullable pointer
+ * with a proper check.
+ */
+static void maybe_null(void)
+{
+	struct struct_ops_maybe_null *skel;
+
+	skel = struct_ops_maybe_null__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "struct_ops_module_open_and_load"))
+		return;
+
+	struct_ops_maybe_null__destroy(skel);
+}
+
+/* Test that the verifier rejects a program that access a nullable pointer
+ * without a check beforehand.
+ */
+static void maybe_null_fail(void)
+{
+	struct struct_ops_maybe_null_fail *skel;
+
+	skel = struct_ops_maybe_null_fail__open_and_load();
+	if (ASSERT_ERR_PTR(skel, "struct_ops_module_fail__open_and_load"))
+		return;
+
+	struct_ops_maybe_null_fail__destroy(skel);
+}
+
+void test_struct_ops_maybe_null(void)
+{
+	/* The verifier verifies the programs at load time, so testing both
+	 * programs in the same compile-unit is complicated. We run them in
+	 * separate objects to simplify the testing.
+	 */
+	if (test__start_subtest("maybe_null"))
+		maybe_null();
+	if (test__start_subtest("maybe_null_fail"))
+		maybe_null_fail();
+}

tools/testing/selftests/bpf/progs/struct_ops_maybe_null.c

@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Meta Platforms, Inc. and affiliates. */
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include "../bpf_testmod/bpf_testmod.h"
+
+char _license[] SEC("license") = "GPL";
+
+pid_t tgid = 0;
+
+/* This is a test BPF program that uses struct_ops to access an argument
+ * that may be NULL. This is a test for the verifier to ensure that it can
+ * rip PTR_MAYBE_NULL correctly.
+ */
+SEC("struct_ops/test_maybe_null")
+int BPF_PROG(test_maybe_null, int dummy,
+	     struct task_struct *task)
+{
+	if (task)
+		tgid = task->tgid;
+
+	return 0;
+}
+
+SEC(".struct_ops.link")
+struct bpf_testmod_ops testmod_1 = {
+	.test_maybe_null = (void *)test_maybe_null,
+};
+

tools/testing/selftests/bpf/progs/struct_ops_maybe_null_fail.c

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Meta Platforms, Inc. and affiliates. */
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include "../bpf_testmod/bpf_testmod.h"
+
+char _license[] SEC("license") = "GPL";
+
+pid_t tgid = 0;
+
+SEC("struct_ops/test_maybe_null_struct_ptr")
+int BPF_PROG(test_maybe_null_struct_ptr, int dummy,
+	     struct task_struct *task)
+{
+	tgid = task->tgid;
+
+	return 0;
+}
+
+SEC(".struct_ops.link")
+struct bpf_testmod_ops testmod_struct_ptr = {
+	.test_maybe_null = (void *)test_maybe_null_struct_ptr,
+};
+