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

On 2025-08-29 12:19, Simon Horman wrote:

On Thu, Aug 28, 2025 at 01:42:42PM +0200, Oscar Maes wrote:

quoted

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 <redacted>
Co-authored-by: Brett A C Sheffield [off-list ref]

...

quoted

+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

tcpdump is started with `timeout 2s`, which will kill it if the 2s timeout is
exceeded.  Is that not sufficient here?

quoted

+
+	# 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}"
+}

...

bacs
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