netdev - Re: [PATCH net v4] selftests: net: add test for destination in broadcast packets

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Message-ID: <20250829111921.GI31759@horms.kernel.org>
Date: Fri, 29 Aug 2025 12:19:21 +0100
From: Simon Horman <horms@...nel.org>
To: Oscar Maes <oscmaes92@...il.com>
Cc: netdev@...r.kernel.org, bacs@...recast.net, brett@...recast.net,
	kuba@...nel.org, pabeni@...hat.com, davem@...emloft.net,
	dsahern@...nel.org, stable@...r.kernel.org
Subject: Re: [PATCH net v4] selftests: net: add test for destination in
 broadcast packets

On Thu, Aug 28, 2025 at 01:42:42PM +0200, Oscar Maes wrote:
> Add test to check the broadcast ethernet destination field is set
> correctly.
> 
> This test sends a broadcast ping, captures it using tcpdump and
> ensures that all bits of the 6 octet ethernet destination address
> are correctly set by examining the output capture file.
> 
> Signed-off-by: Oscar Maes <oscmaes92@...il.com>
> Co-authored-by: Brett A C Sheffield <bacs@...recast.net>

...

> +test_broadcast_ether_dst() {
> +	local rc=0
> +	CAPFILE=$(mktemp -u cap.XXXXXXXXXX)
> +	OUTPUT=$(mktemp -u out.XXXXXXXXXX)
> +
> +	echo "Testing ethernet broadcast destination"
> +
> +	# start tcpdump listening for icmp
> +	# tcpdump will exit after receiving a single packet
> +	# timeout will kill tcpdump if it is still running after 2s
> +	timeout 2s ip netns exec "${CLIENT_NS}" \
> +		tcpdump -i link0 -c 1 -w "${CAPFILE}" icmp &> "${OUTPUT}" &
> +	pid=$!
> +	slowwait 1 grep -qs "listening" "${OUTPUT}"
> +
> +	# send broadcast ping
> +	ip netns exec "${CLIENT_NS}" \
> +		ping -W0.01 -c1 -b 255.255.255.255 &> /dev/null
> +
> +	# wait for tcpdump for exit after receiving packet
> +	wait "${pid}"

Hi Oscar and Brett,

I am concerned that if something goes wrong this may block forever.
Also, I'm wondering if this test could make use of the tcpdump helpers
provided in tools/testing/selftests/net/forwarding/lib.sh

> +
> +	# compare ethernet destination field to ff:ff:ff:ff:ff:ff
> +	ether_dst=$(tcpdump -r "${CAPFILE}" -tnne 2>/dev/null | \
> +			awk '{sub(/,/,"",$3); print $3}')
> +	if [[ "${ether_dst}" == "ff:ff:ff:ff:ff:ff" ]]; then
> +		echo "[ OK ]"
> +		rc="${ksft_pass}"
> +	else
> +		echo "[FAIL] expected dst ether addr to be ff:ff:ff:ff:ff:ff," \
> +			"got ${ether_dst}"
> +		rc="${ksft_fail}"
> +	fi
> +
> +	return "${rc}"
> +}

...