bpf: Add btf_struct_ids_match function

Adding btf_struct_ids_match function to check if given address provided
by BTF object + offset is also address of another nested BTF object.

This allows to pass an argument to helper, which is defined via parent
BTF object + offset, like for bpf_d_path (added in following changes):

  SEC("fentry/filp_close")
  int BPF_PROG(prog_close, struct file *file, void *id)
  {
    ...
    ret = bpf_d_path(&file->f_path, ...

The first bpf_d_path argument is hold by verifier as BTF file object
plus offset of f_path member.

The btf_struct_ids_match function will walk the struct file object and
check if there's nested struct path object on the given offset.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200825192124.710397-9-jolsa@kernel.org

include/linux/bpf.h

@@ -1358,6 +1358,8 @@ int btf_struct_access(struct bpf_verifier_log *log,
 		      const struct btf_type *t, int off, int size,
 		      enum bpf_access_type atype,
 		      u32 *next_btf_id);
+bool btf_struct_ids_match(struct bpf_verifier_log *log,
+			  int off, u32 id, u32 need_type_id);
 int btf_resolve_helper_id(struct bpf_verifier_log *log,
 			  const struct bpf_func_proto *fn, int);
 

kernel/bpf/btf.c

@@ -4160,6 +4160,37 @@ int btf_struct_access(struct bpf_verifier_log *log,
 	return -EINVAL;
 }
 
+bool btf_struct_ids_match(struct bpf_verifier_log *log,
+			  int off, u32 id, u32 need_type_id)
+{
+	const struct btf_type *type;
+	int err;
+
+	/* Are we already done? */
+	if (need_type_id == id && off == 0)
+		return true;
+
+again:
+	type = btf_type_by_id(btf_vmlinux, id);
+	if (!type)
+		return false;
+	err = btf_struct_walk(log, type, off, 1, &id);
+	if (err != WALK_STRUCT)
+		return false;
+
+	/* We found nested struct object. If it matches
+	 * the requested ID, we're done. Otherwise let's
+	 * continue the search with offset 0 in the new
+	 * type.
+	 */
+	if (need_type_id != id) {
+		off = 0;
+		goto again;
+	}
+
+	return true;
+}
+
 int btf_resolve_helper_id(struct bpf_verifier_log *log,
 			  const struct bpf_func_proto *fn, int arg)
 {

kernel/bpf/verifier.c

@@ -3995,16 +3995,21 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 				goto err_type;
 		}
 	} else if (arg_type == ARG_PTR_TO_BTF_ID) {
+		bool ids_match = false;
+
 		expected_type = PTR_TO_BTF_ID;
 		if (type != expected_type)
 			goto err_type;
 		if (!fn->check_btf_id) {
 			if (reg->btf_id != meta->btf_id) {
-				verbose(env, "Helper has type %s got %s in R%d\n",
-					kernel_type_name(meta->btf_id),
-					kernel_type_name(reg->btf_id), regno);
-
-				return -EACCES;
+				ids_match = btf_struct_ids_match(&env->log, reg->off, reg->btf_id,
+								 meta->btf_id);
+				if (!ids_match) {
+					verbose(env, "Helper has type %s got %s in R%d\n",
+						kernel_type_name(meta->btf_id),
+						kernel_type_name(reg->btf_id), regno);
+					return -EACCES;
+				}
 			}
 		} else if (!fn->check_btf_id(reg->btf_id, arg)) {
 			verbose(env, "Helper does not support %s in R%d\n",
@@ -4012,7 +4017,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 
 			return -EACCES;
 		}
-		if (!tnum_is_const(reg->var_off) || reg->var_off.value || reg->off) {
+		if ((reg->off && !ids_match) || !tnum_is_const(reg->var_off) || reg->var_off.value) {
 			verbose(env, "R%d is a pointer to in-kernel struct with non-zero offset\n",
 				regno);
 			return -EACCES;