Subject: Re: [PATCH bpf-next v1 03/13] bpf: Allow struct_ops prog to return referenced kptr

From: Martin KaFai Lau <[email-masked]>
Date: Wed, 18 Dec 2024 14:29:41 -0800

On 12/13/24 3:29 PM, Amery Hung wrote:
> @@ -15993,13 +16001,15 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char
>   	const char *exit_ctx = "At program exit";
>   	struct tnum enforce_attach_type_range = tnum_unknown;
>   	const struct bpf_prog *prog = env->prog;
> -	struct bpf_reg_state *reg;
> +	struct bpf_reg_state *reg = reg_state(env, regno);
>   	struct bpf_retval_range range = retval_range(0, 1);
>   	enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
>   	int err;
>   	struct bpf_func_state *frame = env->cur_state->frame[0];
>   	const bool is_subprog = frame->subprogno;
>   	bool return_32bit = false;
> +	struct btf *btf = bpf_prog_get_target_btf(prog);
> +	const struct btf_type *ret_type = NULL;
>   
>   	/* LSM and struct_ops func-ptr's return type could be "void" */
>   	if (!is_subprog || frame->in_exception_callback_fn) {
> @@ -16008,10 +16018,31 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char
>   			if (prog->expected_attach_type == BPF_LSM_CGROUP)
>   				/* See below, can be 0 or 0-1 depending on hook. */
>   				break;
> -			fallthrough;
> +			if (!prog->aux->attach_func_proto->type)
> +				return 0;
> +			break;
>   		case BPF_PROG_TYPE_STRUCT_OPS:
>   			if (!prog->aux->attach_func_proto->type)
>   				return 0;
> +
> +			if (frame->in_exception_callback_fn)
> +				break;
> +
> +			/* Allow a struct_ops program to return a referenced kptr if it
> +			 * matches the operator's return type and is in its unmodified
> +			 * form. A scalar zero (i.e., a null pointer) is also allowed.
> +			 */
> +			ret_type = btf_type_by_id(btf, prog->aux->attach_func_proto->type);
> +			if (btf_type_is_ptr(ret_type) && reg->type & PTR_TO_BTF_ID &&

The "reg->type & PTR_TO_BTF_ID" does not look right. It should be 
"base_type(reg->type) == PTR_TO_BTF_ID".

> +			    reg->ref_obj_id) {
> +				if (reg->btf_id != ret_type->type) {

reg->btf could be a bpf prog's btf (i.e. prog->aux->btf) instead of the kernel 
btf, so only comparing btf_id here is not very correct.

One way could be to first compare the reg->btf == prog->aux->attach_btf.
prog->aux->attach_btf here must be a kernel btf.

Another way is, btf_type_resolve_ptr() should be a better helper than 
btf_type_by_id() here. It only returns non NULL if the type is a pointer and 
also skips the modifiers like "const" before returning. Then it can directly
compare the "struct btf_type *" returned by 
'btf_type_resolve_ptr(prog->aux->attach_btf, prog->aux->attach_func_proto->type, 
NULL)' and 'btf_type_resolve_ptr(reg->btf, reg->btf_id, NULL)'

May as well enforce the pointer returned by an "ops" must be a struct (i.e. 
__btf_type_is_struct(t) == true). This enforcement can be done in 
bpf_struct_ops_desc_init().

> +					verbose(env, "Return kptr type, struct %s, doesn't match function prototype, struct %s\n",
> +						btf_type_name(reg->btf, reg->btf_id),
> +						btf_type_name(btf, ret_type->type));
> +					return -EINVAL;
> +				}
> +				return __check_ptr_off_reg(env, reg, regno, false);
> +			}
>   			break;
>   		default:
>   			break;
> @@ -16033,8 +16064,6 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char
>   		return -EACCES;
>   	}
>   
> -	reg = cur_regs(env) + regno;
> -
>   	if (frame->in_async_callback_fn) {
>   		/* enforce return zero from async callbacks like timer */
>   		exit_ctx = "At async callback return";
> @@ -16133,6 +16162,11 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char
>   	case BPF_PROG_TYPE_NETFILTER:
>   		range = retval_range(NF_DROP, NF_ACCEPT);
>   		break;
> +	case BPF_PROG_TYPE_STRUCT_OPS:
> +		if (!ret_type || !btf_type_is_ptr(ret_type))
> +			return 0;
> +		range = retval_range(0, 0);
> +		break;
>   	case BPF_PROG_TYPE_EXT:
>   		/* freplace program can return anything as its return value
>   		 * depends on the to-be-replaced kernel func or bpf program.