bpf: btf: Ensure the member->offset is in the right order

This patch ensures the member->offset of a struct
is in the correct order (i.e the later member's offset cannot
go backward).

The current "pahole -J" BTF encoder does not generate something
like this.  However, checking this can ensure future encoder
will not violate this.

Fixes: 69b693f0 ("bpf: btf: Introduce BPF Type Format (BTF)")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/btf.c

@@ -1519,9 +1519,9 @@ static s32 btf_struct_check_meta(struct btf_verifier_env *env,
 {
 	bool is_union = BTF_INFO_KIND(t->info) == BTF_KIND_UNION;
 	const struct btf_member *member;
+	u32 meta_needed, last_offset;
 	struct btf *btf = env->btf;
 	u32 struct_size = t->size;
-	u32 meta_needed;
 	u16 i;
 
 	meta_needed = btf_type_vlen(t) * sizeof(*member);
@@ -1534,6 +1534,7 @@ static s32 btf_struct_check_meta(struct btf_verifier_env *env,
 
 	btf_verifier_log_type(env, t, NULL);
 
+	last_offset = 0;
 	for_each_member(i, t, member) {
 		if (!btf_name_offset_valid(btf, member->name_off)) {
 			btf_verifier_log_member(env, t, member,
@@ -1555,6 +1556,16 @@ static s32 btf_struct_check_meta(struct btf_verifier_env *env,
 			return -EINVAL;
 		}
 
+		/*
+		 * ">" instead of ">=" because the last member could be
+		 * "char a[0];"
+		 */
+		if (last_offset > member->offset) {
+			btf_verifier_log_member(env, t, member,
+						"Invalid member bits_offset");
+			return -EINVAL;
+		}
+
 		if (BITS_ROUNDUP_BYTES(member->offset) > struct_size) {
 			btf_verifier_log_member(env, t, member,
 						"Memmber bits_offset exceeds its struct size");
@@ -1562,6 +1573,7 @@ static s32 btf_struct_check_meta(struct btf_verifier_env *env,
 		}
 
 		btf_verifier_log_member(env, t, member, NULL);
+		last_offset = member->offset;
 	}
 
 	return meta_needed;

tools/testing/selftests/bpf/test_btf.c

@@ -247,6 +247,34 @@ static struct btf_raw_test raw_tests[] = {
 	.max_entries = 4,
 },
 
+{
+	.descr = "struct test #3 Invalid member offset",
+	.raw_types = {
+		/* int */					/* [1] */
+		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),
+		/* int64 */					/* [2] */
+		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 64, 8),
+
+		/* struct A { */				/* [3] */
+		BTF_TYPE_ENC(NAME_TBD, BTF_INFO_ENC(BTF_KIND_STRUCT, 0, 2), 16),
+		BTF_MEMBER_ENC(NAME_TBD, 1, 64),	/* int m;		*/
+		BTF_MEMBER_ENC(NAME_TBD, 2, 0),		/* int64 n; */
+		/* } */
+		BTF_END_RAW,
+	},
+	.str_sec = "\0A\0m\0n\0",
+	.str_sec_size = sizeof("\0A\0m\0n\0"),
+	.map_type = BPF_MAP_TYPE_ARRAY,
+	.map_name = "struct_test3_map",
+	.key_size = sizeof(int),
+	.value_size = 16,
+	.key_type_id = 1,
+	.value_type_id = 3,
+	.max_entries = 4,
+	.btf_load_err = true,
+	.err_str = "Invalid member bits_offset",
+},
+
 /* Test member exceeds the size of struct.
  *
  * struct A {