[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <20250901122524-oscmaes92@gmail.com>
Date: Mon, 1 Sep 2025 14:25:24 +0200
From: Oscar Maes <oscmaes92@...il.com>
To: Brett Sheffield <bacs@...recast.net>, Simon Horman <horms@...nel.org>
Cc: 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 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@...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
>
> 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/
Powered by blists - more mailing lists