netdev - Re: [PATCH bpf-next v3 01/10] bpf: Add dynptr type for skb metadata

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Message-ID: <87ecu7xoss.fsf@cloudflare.com>
Date: Wed, 23 Jul 2025 11:02:43 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: Alexei Starovoitov <ast@...nel.org>,  Andrii Nakryiko
 <andrii@...nel.org>,  Arthur Fabre <arthur@...hurfabre.com>,  Daniel
 Borkmann <daniel@...earbox.net>,  Eric Dumazet <edumazet@...gle.com>,
  Jakub Kicinski <kuba@...nel.org>,  Jesper Dangaard Brouer
 <hawk@...nel.org>,  Jesse Brandeburg <jbrandeburg@...udflare.com>,  Joanne
 Koong <joannelkoong@...il.com>,  Lorenzo Bianconi <lorenzo@...nel.org>,
  Toke Høiland-Jørgensen <thoiland@...hat.com>,  Yan
 Zhai
 <yan@...udflare.com>,  kernel-team@...udflare.com,
  netdev@...r.kernel.org,  Stanislav Fomichev <sdf@...ichev.me>,
  bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 01/10] bpf: Add dynptr type for skb metadata

On Tue, Jul 22, 2025 at 05:37 PM -07, Martin KaFai Lau wrote:
> On 7/21/25 3:52 AM, Jakub Sitnicki wrote:
>> @@ -21788,12 +21798,17 @@ static void specialize_kfunc(struct bpf_verifier_env *env,
>>   	if (offset)
>>   		return;
>>   -	if (func_id == special_kfunc_list[KF_bpf_dynptr_from_skb]) {
>> +	if (func_id == special_kfunc_list[KF_bpf_dynptr_from_skb] ||
>> +	    func_id == special_kfunc_list[KF_bpf_dynptr_from_skb_meta]) {
>
> I don't think this check is needed. The skb_meta is writable to tc.
>
>>   		seen_direct_write = env->seen_direct_write;
>>   		is_rdonly = !may_access_direct_pkt_data(env, NULL, BPF_WRITE);
>
> is_rdonly is always false here.
>
>>   -		if (is_rdonly)
>> -			*addr = (unsigned long)bpf_dynptr_from_skb_rdonly;
>> +		if (is_rdonly) {
>> +			if (func_id == special_kfunc_list[KF_bpf_dynptr_from_skb])
>> +				*addr = (unsigned long)bpf_dynptr_from_skb_rdonly;
>> +			else if (func_id == special_kfunc_list[KF_bpf_dynptr_from_skb_meta])
>> +				*addr = (unsigned long)bpf_dynptr_from_skb_meta_rdonly;
>> +		}
>
> [ ... ]
>
>> +int bpf_dynptr_from_skb_meta_rdonly(struct __sk_buff *skb, u64 flags,
>
> so I suspect this is never used and not needed now. Please check.
> It can be revisited in the future when other hooks are supported. It will be a
> useful comment in the commit message.

You're right. This is dead code ATM. I missed that. Will remove.

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