Re: [PATCH net-next] tcp: tighten the FIN exception in tcp_sequence()

[Jakub Kicinski <kuba@kernel.org>]

On Wed, 10 Jun 2026 00:09:24 +0200 Simon Baatz via B4 Relay wrote:
> From: Simon Baatz <gmbnomis@gmail.com>
> 
> Commit 1e3bb184e941 ("tcp: re-enable acceptance of FIN packets when
> RWIN is 0") added a special case in tcp_sequence() to mirror the FIN
> exception in tcp_data_queue(), which accepts bare in-order FINs even
> when the advertised window is zero. That behavior is not
> RFC-compliant, but was introduced in commit 2bd99aef1b19 ("tcp: accept
> bare FIN packets under memory pressure") to break tight FIN/ACK loops
> caused by broken clients.
> 
> However, the condition added by commit 1e3bb184e941 ("tcp: re-enable
> acceptance of FIN packets when RWIN is 0") is broader than required
> and allows other non-compliant packets as well.
> 
> Tighten the tcp_sequence() FIN exception to only allow packets where
> the packet is a bare in-order FIN and only the FIN flag extends beyond
> tcp_max_receive_window(). In particular, this exception is only
> reachable if tcp_max_receive_window() is zero. Otherwise the packet is
> already accepted by the normal sequence check.
> 
> The existing packetdrill test tcp_rcv_zero_wnd_fin.pkt exercises this
> behavior already and does not need to be changed.
> 
> Signed-off-by: Simon Baatz <gmbnomis@gmail.com>

This is odd. You are sending this patch which shares a lot of
similarities with Eric's patch:
https://lore.kernel.org/all/20260608151452.706822-1-edumazet@google.com/

Why are you submitting your own patch instead of discussing it further
with Eric and letting him send v2?

Eric, how would you like to proceed?