netdev - Re: [PATCH net-next v5 03/15] bpf: introduce timestamp_used to allow UDP socket fetched in bpf prog

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Message-ID: <060c5a50-85b6-4f1c-b458-33084858db12@linux.dev>
Date: Fri, 17 Jan 2025 18:16:59 -0800
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, dsahern@...nel.org, willemdebruijn.kernel@...il.com,
 willemb@...gle.com, ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 eddyz87@...il.com, song@...nel.org, yonghong.song@...ux.dev,
 john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, jolsa@...nel.org, horms@...nel.org, bpf@...r.kernel.org,
 netdev@...r.kernel.org
Subject: Re: [PATCH net-next v5 03/15] bpf: introduce timestamp_used to allow
 UDP socket fetched in bpf prog

On 1/17/25 5:58 PM, Jason Xing wrote:
>> On 1/15/25 5:12 PM, Jason Xing wrote:
>>>>> Also, I need to set allow_direct_access to one as long as there is
>>>>> "sock_ops.is_fullsock = 1;" in the existing callbacks.
>>>> Only set allow_direct_access when the sk is fullsock in the "existing" sockops
>>>> callback.
>>> Only "existing"? Then how can the bpf program access those members of
>>> the tcp socket structure in the current/new timestamping callbacks?
>> There is at least one sk write:
>>
>>          case offsetof(struct bpf_sock_ops, sk_txhash):
>>                  SOCK_OPS_GET_OR_SET_FIELD(sk_txhash, sk_txhash,
>>                                           struct sock, type);
>>
>> afaict, the kernel always writes sk->sk_txhash with the sk lock held. The new
>> timestamping callbacks cannot write because it does not hold the lock.
> Surely, I will handle the sk_txhash case as you suggested 🙂

to be clear, not setting the allow_tcp_access in the new timestamping cb should do.