Re: [PATCH net v2] selftests/net: Fix icmp.sh race between ping and tcpdump startup

On Fri Sep 11, 2026 at 9:40 PM -03, Jakub Kicinski wrote:

On Fri, 11 Sep 2026 15:41:33 -0300 Ricardo B. Marlière (SUSE) wrote:

quoted

icmp.sh backgrounds ping and then starts tcpdump, with no synchronization
between the two. If tcpdump is not capturing yet by the time the ICMP
unreachable replies come back, the test fails even though the kernel
behaved correctly:

  FAIL - got ICMP response from , should be 192.0.0.8

The empty address is misleading: it means the capture matched nothing
before its 10s timeout, not that the reply had a wrong source address.
NS1's Icmp InDestUnreachs counter still increments across such a failure,
so the replies were generated and did reach NS1. Ping sends several probes
over its 3s deadline, so losing every one of them takes a multi-second
stall in tcpdump startup, which does happen on loaded CI hosts [1].

Fix it by starting tcpdump first and waiting for its "listening" banner
via slowwait() before sending traffic. Run ping synchronously, then kill
and reap tcpdump if it did not already exit on -c 1.

It used to pass in NIPA CI and now it fails:
https://netdev-ctrl.bots.linux.dev/logs/vmksft/net-dbg/results/819341/86-icmp-sh/stdout

Thanks for the pointer, I'll prepare v3

quoted

Fixes: 7e9838b7915e ("selftests/net: Add icmp.sh for testing ICMP dummy address responses")

Please don't add Fixes tags to selftest improvements.

Wouldn't this mean that the commit would not be picked up by stable,
creating a gap there?

quoted

Link: https://openqa.opensuse.org/tests/5907626/logfile?filename=icmp_sh.tap.txt#line-2 [1]
Signed-off-by: Ricardo B. Marlière (SUSE) <redacted>