Re: [PATCH net-next v3 6/6] selftests/net: packetdrill: add tcp_rcv_neg_window.pkt

[Simon Baatz]

On Sat, Mar 14, 2026 at 06:07:05PM +0100, Simon Baatz wrote:
> Hi Eric,
> 
> On Sat, Mar 14, 2026 at 04:58:28AM +0100, Eric Dumazet wrote:
> > On Wed, Mar 11, 2026 at 12:09???AM Simon Baatz <gmbnomis@xxxxxxxxx> wrote:
> > >
> > > Hi Eric,
> > >
> > > On Tue, Mar 10, 2026 at 09:54:58AM +0100, Eric Dumazet wrote:
> > > > On Mon, Mar 9, 2026 at 9:03???AM Simon Baatz via B4 Relay
> > > > <devnull+gmbnomis.gmail.com@xxxxxxxxxx> wrote:
> > > > >
> > > > > From: Simon Baatz <gmbnomis@xxxxxxxxx>
> > > > >
> > > > > The test ensures we correctly apply the maximum advertised window limit
> > > > > when rcv_nxt advances past rcv_mwnd_seq, so that the "usable window"
> > > > > is properly clamped to zero rather than becoming negative.
> > > > >
> > > > > Signed-off-by: Simon Baatz <gmbnomis@xxxxxxxxx>
> > > > > ---
> > > > >  .../net/packetdrill/tcp_rcv_neg_window.pkt         | 26 ++++++++++++++++++++++
> > > > >  1 file changed, 26 insertions(+)
> > > > >
> > > > > diff --git a/tools/testing/selftests/net/packetdrill/tcp_rcv_neg_window.pkt b/tools/testing/selftests/net/packetdrill/tcp_rcv_neg_window.pkt
> > > > > new file mode 100644
> > > > > index 0000000000000000000000000000000000000000..15a9b4938f16d175ac54f3fd192ed2b59b0a4399
> > > > > --- /dev/null
> > > > > +++ b/tools/testing/selftests/net/packetdrill/tcp_rcv_neg_window.pkt
> > > > > @@ -0,0 +1,26 @@
> > > > > +// SPDX-License-Identifier: GPL-2.0
> > > > > +
> > > > > +--mss=1000
> > > > > +
> > > > > +`./defaults.sh`
> > > > > +
> > > > > +// Establish a connection.
> > > > > +   +0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
> > > > > +   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
> > > > > +   +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [20000], 4) = 0
> > > > > +   +0 bind(3, ..., ...) = 0
> > > > > +   +0 listen(3, 1) = 0
> > > > > +
> > > > > +   +0 < S 0:0(0) win 32792 <mss 1000,nop,wscale 7>
> > > > > +   +0 > S. 0:0(0) ack 1 win 18980 <mss 1460,nop,wscale 0>
> > > > > +  +.1 < . 1:1(0) ack 1 win 257
> > > > > +
> > > > > +   +0 accept(3, ..., ...) = 4
> > > > > +
> > > > > +// A too big packet is accepted if the receive queue is empty
> > > > > +   +0 < P. 1:20001(20000) ack 1 win 257
> > > >
> > > > We do not see the answer, it seems this test is not complete ?
> > >
> > > Actually we do not want to see an answer.  The packet won't trigger
> > > an immediate ACK (it is larger than the advertised window, but does
> > > not cause immediate memory pressure).
> > >
> > > When we then send a RST before the delayed ACK would be generated:
> > >
> > > > > +// Send a RST immediately so that there is no rcv_wup/rcv_mwnd_seq update yet
> > > > > +   +0 < R. 20001:20001(0) ack 1 win 257
> > >
> > > We are in a state where rcv_wup, rcv_wnd, and rcv_mwnd_seq have not
> > > been updated yet, but we must still accept the RST
> > > (rcv_nxt == 20001 > rcv_mwnd_seq, tcp_max_receive_window() == 0)
> > >
> > > > > +
> > > > > +  +.1 %{ assert tcpi_state == TCP_CLOSE, tcpi_state }%
> > >
> > > And we verify that we accepted the RST here.
> > >
> > > Given how subtle this sequence is, and considering the limited value
> > > of this test, I am also fine with dropping it if it is too fragile or
> > > confusing.
> > 
> > Sorry I missed your answer.
> > 
> > Ok then please use :
> > 
> > // A too big packet is accepted if the receive queue is empty
> >    +0 < P. 1:20001(20000) ack 1 win 257
> >    +0 %{ assert tcpi_bytes_received == 20000, tcpi_bytes_received;
> > assert tcpi_bytes_acked == 0, tcpi_bytes_acked }%
> 
> Unfortunately, tcpi_bytes_acked is the TX direction, it will always
> be 0 here.
> 
> Instead, we can still test that the oversized packet is accepted and
> indirectly verify that no immediate ACK is sent by eliciting and
> checking a RST:
> 
> // A too big packet is accepted if the receive queue is empty, but does not trigger
> // an immediate ACK.
>    +0 < P. 1:20001(20000) ack 1 win 257
>    +0 %{ assert tcpi_bytes_received == 20000, tcpi_bytes_received; }%
> 
> // Send a RST immediately so that there is no rcv_wup/rcv_mwnd_seq update yet
>    +0 < R. 20001:20001(0) ack 1 win 257
> 
> // Verify that the RST was accepted. Indirectly this also verifies that no immediate
> // ACK was sent for the data packet above.
>    +0 < . 20001:20001(0) ack 1 win 257
>     * > R 1:1(0)
> 
> As the series is merged now (thank you!), I will send this
> separately, as suggested.

Patch is at: https://lore.kernel.org/netdev/20260316-improve_tcp_neg_usable_wnd_test-v1-1-f16d5e365107@xxxxxxxxx/

-- 
Simon Baatz <gmbnomis@xxxxxxxxx>