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: <CAL+tcoBLDvTDQSH-509xDBHXkmutCOZzqX-N0HkPSVzkPjB62Q@mail.gmail.com>
Date: Sat, 18 Jan 2025 10:37:16 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
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 Sat, Jan 18, 2025 at 10:17 AM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
> 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.

Right, I will only apply to the existing callbacks. I think your last
email is pretty clear to me and dispelled my concern. Prior to this, I
was worried about not being allowed to access struct tcp_sock in
timestamping cb. Thanks for your guidance.

Thanks,
Jason