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

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Message-ID: <aLGSplLd9LjVzZOk@karahi.gladserv.com>
Date: Fri, 29 Aug 2025 13:44:38 +0200
From: Brett Sheffield <bacs@...recast.net>
To: Simon Horman <horms@...nel.org>
Cc: Oscar Maes <oscmaes92@...il.com>, netdev@...r.kernel.org,
	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 2025-08-29 12:19, Simon Horman wrote:
> 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

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

> > +
> > +	# 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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