bpf: Add tests for PTR_TO_BTF_ID vs. null comparison

Add two tests for PTR_TO_BTF_ID vs. null ptr comparison,
one for PTR_TO_BTF_ID in the ctx structure and the
other for PTR_TO_BTF_ID after one level pointer chasing.
In both cases, the test ensures condition is not
removed.

For example, for this test
 struct bpf_fentry_test_t {
     struct bpf_fentry_test_t *a;
 };
 int BPF_PROG(test7, struct bpf_fentry_test_t *arg)
 {
     if (arg == 0)
         test7_result = 1;
     return 0;
 }
Before the previous verifier change, we have xlated codes:
  int test7(long long unsigned int * ctx):
  ; int BPF_PROG(test7, struct bpf_fentry_test_t *arg)
     0: (79) r1 = *(u64 *)(r1 +0)
  ; int BPF_PROG(test7, struct bpf_fentry_test_t *arg)
     1: (b4) w0 = 0
     2: (95) exit
After the previous verifier change, we have:
  int test7(long long unsigned int * ctx):
  ; int BPF_PROG(test7, struct bpf_fentry_test_t *arg)
     0: (79) r1 = *(u64 *)(r1 +0)
  ; if (arg == 0)
     1: (55) if r1 != 0x0 goto pc+4
  ; test7_result = 1;
     2: (18) r1 = map[id:6][0]+48
     4: (b7) r2 = 1
     5: (7b) *(u64 *)(r1 +0) = r2
  ; int BPF_PROG(test7, struct bpf_fentry_test_t *arg)
     6: (b4) w0 = 0
     7: (95) exit
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200630171241.2523875-1-yhs@fb.com

net/bpf/test_run.c

@@ -147,6 +147,20 @@ int noinline bpf_fentry_test6(u64 a, void *b, short c, int d, void *e, u64 f)
 	return a + (long)b + c + d + (long)e + f;
 }
 
+struct bpf_fentry_test_t {
+	struct bpf_fentry_test_t *a;
+};
+
+int noinline bpf_fentry_test7(struct bpf_fentry_test_t *arg)
+{
+	return (long)arg;
+}
+
+int noinline bpf_fentry_test8(struct bpf_fentry_test_t *arg)
+{
+	return (long)arg->a;
+}
+
 int noinline bpf_modify_return_test(int a, int *b)
 {
 	*b += 1;
@@ -185,6 +199,7 @@ int bpf_prog_test_run_tracing(struct bpf_prog *prog,
 			      const union bpf_attr *kattr,
 			      union bpf_attr __user *uattr)
 {
+	struct bpf_fentry_test_t arg = {};
 	u16 side_effect = 0, ret = 0;
 	int b = 2, err = -EFAULT;
 	u32 retval = 0;
@@ -197,7 +212,9 @@ int bpf_prog_test_run_tracing(struct bpf_prog *prog,
 		    bpf_fentry_test3(4, 5, 6) != 15 ||
 		    bpf_fentry_test4((void *)7, 8, 9, 10) != 34 ||
 		    bpf_fentry_test5(11, (void *)12, 13, 14, 15) != 65 ||
-		    bpf_fentry_test6(16, (void *)17, 18, 19, (void *)20, 21) != 111)
+		    bpf_fentry_test6(16, (void *)17, 18, 19, (void *)20, 21) != 111 ||
+		    bpf_fentry_test7((struct bpf_fentry_test_t *)0) != 0 ||
+		    bpf_fentry_test8(&arg) != 0)
 			goto out;
 		break;
 	case BPF_MODIFY_RETURN:

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

@@ -36,7 +36,7 @@ void test_fentry_fexit(void)
 	fentry_res = (__u64 *)fentry_skel->bss;
 	fexit_res = (__u64 *)fexit_skel->bss;
 	printf("%lld\n", fentry_skel->bss->test1_result);
-	for (i = 0; i < 6; i++) {
+	for (i = 0; i < 8; i++) {
 		CHECK(fentry_res[i] != 1, "result",
 		      "fentry_test%d failed err %lld\n", i + 1, fentry_res[i]);
 		CHECK(fexit_res[i] != 1, "result",

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

@@ -55,3 +55,25 @@ int BPF_PROG(test6, __u64 a, void *b, short c, int d, void * e, __u64 f)
 		e == (void *)20 && f == 21;
 	return 0;
 }
+
+struct bpf_fentry_test_t {
+	struct bpf_fentry_test_t *a;
+};
+
+__u64 test7_result = 0;
+SEC("fentry/bpf_fentry_test7")
+int BPF_PROG(test7, struct bpf_fentry_test_t *arg)
+{
+	if (arg == 0)
+		test7_result = 1;
+	return 0;
+}
+
+__u64 test8_result = 0;
+SEC("fentry/bpf_fentry_test8")
+int BPF_PROG(test8, struct bpf_fentry_test_t *arg)
+{
+	if (arg->a == 0)
+		test8_result = 1;
+	return 0;
+}

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

@@ -56,3 +56,25 @@ int BPF_PROG(test6, __u64 a, void *b, short c, int d, void *e, __u64 f, int ret)
 		e == (void *)20 && f == 21 && ret == 111;
 	return 0;
 }
+
+struct bpf_fentry_test_t {
+	struct bpf_fentry_test *a;
+};
+
+__u64 test7_result = 0;
+SEC("fexit/bpf_fentry_test7")
+int BPF_PROG(test7, struct bpf_fentry_test_t *arg)
+{
+	if (arg == 0)
+		test7_result = 1;
+	return 0;
+}
+
+__u64 test8_result = 0;
+SEC("fexit/bpf_fentry_test8")
+int BPF_PROG(test8, struct bpf_fentry_test_t *arg)
+{
+	if (arg->a == 0)
+		test8_result = 1;
+	return 0;
+}