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

[Oscar Maes <oscmaes92@gmail.com>]

On Fri, Aug 29, 2025 at 03:11:20PM +0200, Brett Sheffield wrote:
> 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@gmail.com>
> > > Co-authored-by: Brett A C Sheffield <bacs@librecast.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
> 
> Thanks for the review Simon.  Further to previous email after some more thought
> and poking at lib.sh
> 
> We're starting tcpdump with -c1 so that it exits immediately when the packet is
> received, and we catch this with the wait() so that, in the best case, we
> continue immediately, and in the worse case the `timeout 2s` kills tcpdump and
> we move on to cleanup. I *think* this is pretty safe.
> 
> Taking a look at the forwarding/lib.sh it looks like we could use
> tcpdump_start() and pass in $TCPDUMP_EXTRA_FLAGS but I don't think this buys us
> much here, as we'd still need to wait() or a sleep() or otherwise detect that
> tcpdump is finished so we can continue. I don't see anything in lib.sh to aid us
> with that?
> 
> That said, it might be good to use the helper function anyway and keep the
> wait() for consistency. There don't seem to be many tests using the tcpdump
> helper functions yet, but it's probably the right way to move.
> 
> What do you think, Oscar?  It looks like lib.sh tcpdump_start() takes all the
> arguments, including for your namespaces.  Up to you if you want to call that
> instead.
> 
> Now I know it's there, I'll try to use that for future tests.
> 
> I don't *think* there's anything here that needs a v4, unless the timeout() call
> is thought to be insufficient to kill tcpdump.  There's a -k switch if we want
> to SIGKILL it :-)
> 

I agree with Brett here.
I tried using forwarding/lib.sh but it made the test unnecessarily complex
and difficult to read/debug. I suggest we keep it as-is.

Simon - what do you think?

> > > +
> > > +	# 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}"
> > > +}
> > 
> > ...
> 
> -- 
> Brett Sheffield (he/him)
> Librecast - Decentralising the Internet with Multicast
> https://librecast.net/
> https://blog.brettsheffield.com/