Re: [PATCH net] selftests: net: fix race condition in icmp.sh causing CKI sporadic failures

[Antoine Tenart <atenart@kernel.org>]

On Mon, Oct 12, 2026 at 06:23:32PM +0530, Prasenjit Mukherjee wrote:
> CKI reports sporadic failures in icmp.sh, Davide suggests this might be
> caused by a race condition where tcpdump starts capturing after the last of
> expected ICMP unreachable packets have been sent.
> Fix this replacing tcpdump with a tc flower filter that
> matches ICMP Destination Unreachable (type 3, code 0),
> and match on act_gact counters instead.
> This will remove any race condition to appear.
> 
> Fixes: 7e9838b7915e2 ("selftests/net: Add icmp.sh for testing ICMP dummy address responses")
> CC: Toke Høiland-Jørgensen <toke@redhat.com>
> Suggested-by: Davide Caratti <dcaratti@redhat.com>
> Reviewed-by: Davide Caratti <dcaratti@redhat.com>
> Reported-by: CKI Project <cki-project@redhat.com>
> Closes: https://datawarehouse.cki-project.org/issue/3658
> Signed-off-by: Prasenjit Mukherjee <pmukherj@redhat.com>

Reviewed-by: Antoine Tenart <atenart@kernel.org>

> ---
>  tools/testing/selftests/net/icmp.sh | 32 ++++++++++++++---------------
>  1 file changed, 16 insertions(+), 16 deletions(-)
> 
> diff --git a/tools/testing/selftests/net/icmp.sh b/tools/testing/selftests/net/icmp.sh
> index 824cb0e35eff..0714e8b34d12 100755
> --- a/tools/testing/selftests/net/icmp.sh
> +++ b/tools/testing/selftests/net/icmp.sh
> @@ -27,11 +27,8 @@ PINGADDR=172.16.1.1
>  RT2=172.16.0.0/24
>  H2_IP6=2001:db8:1::2
>  
> -TMPFILE=$(mktemp)
> -
>  cleanup()
>  {
> -    rm -f "$TMPFILE"
>      cleanup_ns $NS1 $NS2
>  }
>  
> @@ -52,21 +49,24 @@ ip -netns $NS2 route add $RT2 via inet6 $H1_IP6
>  
>  # Make sure ns2 will respond with ICMP unreachable
>  ip netns exec $NS2 sysctl -qw net.ipv4.icmp_ratelimit=0 net.ipv4.ip_forward=1
> +# Install tc flower filter to count ICMP unreachable from 192.0.0.8
> +ip netns exec $NS1 tc qdisc add dev veth0 clsact
> +ip netns exec $NS1 tc filter add dev veth0 ingress \
> +	protocol ip pref 1 handle 1 flower \
> +	src_ip 192.0.0.8 ip_proto icmp type 3 code 0 \
> +	action pass
>  
> -# Run the test - a ping runs in the background, and we capture ICMP responses
> -# with tcpdump; -c 1 means it should exit on the first ping, but add a timeout
> -# in case something goes wrong
> -ip netns exec $NS1 ping -w 3 -i 0.5 $PINGADDR >/dev/null &
> -ip netns exec $NS1 timeout 10 tcpdump -tpni veth0 -c 1 'icmp and icmp[icmptype] != icmp-echo' > $TMPFILE 2>/dev/null
> +# Send ping to trigger ICMP unreachable from ns2
> +ip netns exec $NS1 ping -c 1 $PINGADDR >/dev/null 2>&1
>  
> -# Parse response and check for dummy address
> -# tcpdump output looks like:
> -# IP 192.0.0.8 > 172.16.0.1: ICMP net 172.16.1.1 unreachable, length 92
> -RESP_IP=$(awk '{print $2}' < $TMPFILE)
> -if [[ "$RESP_IP" != "192.0.0.8" ]]; then
> -    echo "FAIL - got ICMP response from $RESP_IP, should be 192.0.0.8"
> -    exit 1
> -else
> +# Verify ICMP unreachable was received from dummy address 192.0.0.8 per RFC7600
> +COUNT=$(ip netns exec $NS1 tc -j -s filter show dev veth0 ingress \
> +	| jq '.[1].options.actions[].stats.packets')
> +
> +if [[ "$COUNT" -eq 1 ]]; then
>      echo "OK"
>      exit 0
> +else
> +    echo "FAIL - expected 1 ICMP unreachable from 192.0.0.8, got $COUNT"
> +    exit 1
>  fi
> -- 
> 2.55.0
>