Re: [PATCH net v2 2/2] selftests: net: check timestamp echo after an old ACK

[Jeff Jo <jeffjo@openai.com>]

On Fri, Sep 25, 2026 at 10:47:14PM +0000, netdev-bot+sashiko@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential
> issue(s) to consider.
> 
> Critical: 0 · High: 0 · Medium: 1 · Low: 0
> 
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> 
> - [Medium] The new selftest
>   tools/testing/selftests/net/packetdrill/tcp_old_ack_ts.pkt checks for...
> 
> --
> 
> selftests: net: check timestamp echo after an old ACK
> 
> This adds a packetdrill regression test for a gap-filling segment that
> carries an old ACK. Two peer data segments arrive out of order. The test
> then requires the ACK that closes the gap to echo the delayed segment's
> timestamp (301000) and not the stale saved value (1000).
> 
> Link: https://github.com/google/packetdrill/commit/83f72d3f9085d0e26eb4d206fe4d7cfab5b6d872
> 
> > diff --git a/tools/testing/selftests/net/packetdrill/tcp_old_ack_ts.pkt b/tools/testing/selftests/net/packetdrill/tcp_old_ack_ts.pkt
> > new file mode 100644
> > index 0000000000000..f072e5e4c4494
> > --- /dev/null
> > +++ b/tools/testing/selftests/net/packetdrill/tcp_old_ack_ts.pkt
> > @@ -0,0 +1,22 @@
> 
> [ ... ]
> 
> > +0.300 write(4, ..., 17) = 17
> > ++0 > P. 1:18(17) ack 1 <nop,nop,TS val 300 ecr 1000>
> > +0.310 < P. 18:35(17) ack 18 win 20000 <nop,nop,TS val 301001 ecr 300>
> > ++0 > . 18:18(0) ack 1 <nop,nop,TS val 310 ecr 1000,nop,nop,sack 18:35>
> > +0.320 < P. 1:18(17) ack 1 win 20000 <nop,nop,TS val 301000 ecr 100>
> > ++0 > . 18:18(0) ack 35 <nop,nop,TS val 320 ecr 301000>
>                                                   ^^^^^^^^^^^
> 
> [Severity: Medium]
> Can this test report PASS on a kernel that doesn't have the fix?
> 
> The commit message says packetdrill versions older than 83f72d3f9085 do
> not check the outgoing TSecr:
> 
>   older tools incorrectly pass on an unfixed kernel
> 
> This ecr 301000 check is the only line in the script that depends on the
> fix. The seq, ack and SACK checks give the same result with or without
> it, and so does the read() of 34 bytes below.
> 
> The runner only checks that some packetdrill is in PATH. It then reports
> PASS whenever packetdrill exits 0:
> 
> tools/testing/selftests/net/packetdrill/ksft_runner.sh:
>     if [ -z "$(which packetdrill)" ]; then
>     	ktap_skip_all "packetdrill not found in PATH"
>     	exit "$KSFT_SKIP"
>     fi
>     ...
>     	unshare -n packetdrill ${ip_args[$ip_version]} ${optargs[@]} $script > /dev/null \
>     		&& ktap_test_pass $ip_version || $failfunc $ip_version
> 
> On a kernel without the fix, tcp_ack() sends the gap filler down the
> old_ack path, since ack 1 is before snd_una 18. That kernel does not have
> this update:
> 
> net/ipv4/tcp_input.c:tcp_ack() {
>     ...
> old_ack:
> 	...
> 	if (flag & FLAG_UPDATE_TS_RECENT)
> 		tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);
>     ...
> }
> 
> As a result, ts_recent stays at 1000. With an older packetdrill, such as
> a distro package, wouldn't ipv4, ipv6 and ipv4-mapped-ipv6 all still pass?
> 
> Could the script add a check that doesn't rely on packetdrill's TSecr
> handling? For example, after the gap-closing ACK, inject:
> 
>   < P. 35:36(1) ack 18 win 20000 <nop,nop,TS val 2000 ecr 320>
> 
> With the fix, ts_recent is 301000, so the PAWS check in
> tcp_validate_incoming() drops the segment. The segment carries data, so
> tcp_oow_rate_limited() does not suppress the dup-ACK, and the reply is
> ack 35.
> 
> Without the fix, ts_recent is 1000. The segment is accepted and the reply
> is ack 36. That difference would show up whichever packetdrill version is
> used.
> 
> > ++0 read(4, ..., 34) = 34
> 
> -- 
> Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260924224456.55690-4-jeffjo%40openai.com


Yes, affected older packetdrill versions can
falsely pass this test, as noted in the commit message.

We previously tested a PAWS-based assertion, but Eric requested a
direct timestamp-echo check once packetdrill was fixed and the
runners updated [1]. Neal has since merged the packetdrill fix [2]
and requested the revised test.

With updated packetdrill, we verified that this test fails on the
unfixed kernel and passes with the kernel fix. V2 follows that
approach and documents the required packetdrill commit.

[1] https://lore.kernel.org/netdev/CANn89i+nY499TfuGMvMq3P=Pxw6vBxe+uYH1zioK=bhU--GXYw@mail.gmail.com/
[2] https://github.com/google/packetdrill/commit/83f72d3f9085d0e26eb4d206fe4d7cfab5b6d872