linux-kernel - Re: [PATCH bpf-next v9 2/3] bpf, sockmap: Fix FIONREAD for sockmap

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Message-ID: <87zf60sl0y.fsf@cloudflare.com>
Date: Mon, 26 Jan 2026 11:53:01 +0100
From: Jakub Sitnicki <jakub@...udflare.com>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>
Cc: bpf@...r.kernel.org,  John Fastabend <john.fastabend@...il.com>,  "David
 S. Miller" <davem@...emloft.net>,  Eric Dumazet <edumazet@...gle.com>,
  Jakub Kicinski <kuba@...nel.org>,  Paolo Abeni <pabeni@...hat.com>,
  Simon Horman <horms@...nel.org>,  Neal Cardwell <ncardwell@...gle.com>,
  Kuniyuki Iwashima <kuniyu@...gle.com>,  David Ahern <dsahern@...nel.org>,
  Andrii Nakryiko <andrii@...nel.org>,  Eduard Zingerman
 <eddyz87@...il.com>,  Alexei Starovoitov <ast@...nel.org>,  Daniel
 Borkmann <daniel@...earbox.net>,  Martin KaFai Lau <martin.lau@...ux.dev>,
  Song Liu <song@...nel.org>,  Yonghong Song <yonghong.song@...ux.dev>,  KP
 Singh <kpsingh@...nel.org>,  Stanislav Fomichev <sdf@...ichev.me>,  Hao
 Luo <haoluo@...gle.com>,  Jiri Olsa <jolsa@...nel.org>,  Shuah Khan
 <shuah@...nel.org>,  Michal Luczaj <mhal@...x.co>,  Cong Wang
 <cong.wang@...edance.com>,  netdev@...r.kernel.org,
  linux-kernel@...r.kernel.org,  linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next v9 2/3] bpf, sockmap: Fix FIONREAD for sockmap

On Sat, Jan 24, 2026 at 07:32 PM +08, Jiayuan Chen wrote:
> A socket using sockmap has its own independent receive queue: ingress_msg.
> This queue may contain data from its own protocol stack or from other
> sockets.
>
> Therefore, for sockmap, relying solely on copied_seq and rcv_nxt to
> calculate FIONREAD is not enough.
>
> This patch adds a new msg_tot_len field in the psock structure to record
> the data length in ingress_msg. Additionally, we implement new ioctl
> interfaces for TCP and UDP to intercept FIONREAD operations.
>
> Note that we intentionally do not include sk_receive_queue data in the
> FIONREAD result. Data in sk_receive_queue has not yet been processed by
> the BPF verdict program, and may be redirected to other sockets or
> dropped. Including it would create semantic ambiguity since this data
> may never be readable by the user.
>
> Unix and VSOCK sockets have similar issues, but fixing them is outside
> the scope of this patch as it would require more intrusive changes.
>
> Previous work by John Fastabend made some efforts towards FIONREAD support:
> commit e5c6de5fa025 ("bpf, sockmap: Incorrectly handling copied_seq")
> Although the current patch is based on the previous work by John Fastabend,
> it is acceptable for our Fixes tag to point to the same commit.
>
>                                                       FD1:read()
>                                                       --  FD1->copied_seq++
>                                                           |  [read data]
>                                                           |
>                                    [enqueue data]         v
>                   [sockmap]     -> ingress to self ->  ingress_msg queue
> FD1 native stack  ------>                                 ^
> -- FD1->rcv_nxt++               -> redirect to other      | [enqueue data]
>                                        |                  |
>                                        |             ingress to FD1
>                                        v                  ^
>                                       ...                 |  [sockmap]
>                                                      FD2 native stack
>
> Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@...ux.dev>
> ---

Thanks, Jiayuan!

Reviewed-by: Jakub Sitnicki <jakub@...udflare.com>