Re: [PATCH bpf-next v8 3/4] bpf: Add kfunc bpf_rcu_read_lock/unlock()

From: Martin KaFai Lau <martin.lau@linux.dev>
To: Yonghong Song <yhs@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	kernel-team@fb.com, Martin KaFai Lau <martin.lau@kernel.org>,
	bpf@vger.kernel.org
Subject: Re: [PATCH bpf-next v8 3/4] bpf: Add kfunc bpf_rcu_read_lock/unlock()
Date: Tue, 22 Nov 2022 16:24:52 -0800	[thread overview]
Message-ID: <3ee3af12-0542-e33c-2e9b-c6838de6ba64@linux.dev> (raw)
In-Reply-To: <20221122195335.1782147-1-yhs@fb.com>

On 11/22/22 11:53 AM, Yonghong Song wrote:
> +	if (flag & MEM_RCU) {
> +		/* Mark value register as MEM_RCU only if it is protected by
> +		 * bpf_rcu_read_lock() and the ptr reg is trusted (PTR_TRUSTED or
> +		 * ref_obj_id != 0). MEM_RCU itself can already indicate
> +		 * trustedness inside the rcu read lock region. But Mark it
> +		 * as PTR_TRUSTED as well similar to MEM_ALLOC.
> +		 */
> +		if (!env->cur_state->active_rcu_lock ||
> +		    (!(reg->type & PTR_TRUSTED) && !reg->ref_obj_id))

Can is_trusted_reg() be reused or MEM_ALLOC is not applicable here?

> +			flag &= ~MEM_RCU;
> +		else
> +			flag |= PTR_TRUSTED;
> +	} else if (reg->type & MEM_RCU) {
> +		/* ptr (reg) is marked as MEM_RCU, but value reg is not marked as MEM_RCU.
> +		 * Mark the value reg as PTR_UNTRUSTED conservatively.
> +		 */
> +		flag |= PTR_UNTRUSTED;

Should PTR_UNTRUSTED tagging be limited to ret == PTR_TO_BTF_ID instead of 
tagging SCALAR also?

[ ... ]

> @@ -11754,6 +11840,11 @@ static int check_ld_abs(struct bpf_verifier_env *env, struct bpf_insn *insn)
>   		return -EINVAL;
>   	}
>   
> +	if (env->prog->aux->sleepable && env->cur_state->active_rcu_lock) {

I don't know the details about ld_abs :).  Why sleepable check is needed here?

Others lgtm.