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

[Simon Baatz]

Hi Jakub,

On Fri, Jun 12, 2026 at 03:43:55PM -0700, Jakub Kicinski wrote:
> On Wed, 10 Jun 2026 00:09:24 +0200 Simon Baatz via B4 Relay wrote:
> > From: Simon Baatz <gmbnomis@xxxxxxxxx>
> > 
> > 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@xxxxxxxxx>
> 
> 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@xxxxxxxxxx/
> 
> Why are you submitting your own patch instead of discussing it further
> with Eric and letting him send v2?

That's what I understood from Eric's reply to my comments.  He asked
for an alternative, so I sent this as a concrete sugggestion.

Sorry if that was not the right way to proceed.  I'm happy to
continue the discussion in Eric's thread instead and treat this as
part of that discussion.

> 
> Eric, how would you like to proceed?

-- 
Simon Baatz <gmbnomis@xxxxxxxxx>