Re: [PATCH net v2 2/2] selftests/bpf: cover AF_XDP multi-buffer TX software checksum

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- [Medium] generate_packet() in
  tools/testing/selftests/bpf/prog_tests/xdp_metadata.c fills the…

Pre-existing issues:
- [Medium] The freplace check in test_xdp_metadata() never retries.

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selftests/bpf: cover AF_XDP multi-buffer TX software checksum

This commit extends test_xdp_metadata to send a UDP packet once as a
single TX descriptor and once split across two descriptors with
XDP_PKT_CONTD. The existing metadata and checksum checks run for both.
It adds a bind_flags argument to open_xsk(), a split_len argument to
generate_packet() and a num argument to complete_tx().

> diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
> index 5c31054ad4a4c..1cf70fb800fba 100644
> --- a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
> +++ b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c

[ ... ]

> @@ -133,36 +135,36 @@ static void close_xsk(struct xsk *xsk)

[ ... ]

> -	eth = data;
> +	memset(pkt, 0, sizeof(pkt));
> +	eth = (void *)pkt;
>  	iph = (void *)(eth + 1);
>  	udph = (void *)(iph + 1);
>  
> -	memcpy(eth->h_dest, "\x00\x00\x00\x00\x00\x02", ETH_ALEN);
> -	memcpy(eth->h_source, "\x00\x00\x00\x00\x00\x01", ETH_ALEN);
> +	memcpy(eth->h_dest, RX_MAC, ETH_ALEN);
> +	memcpy(eth->h_source, TX_MAC, ETH_ALEN);

[Severity: Medium]
Should RX_MAC and TX_MAC be copied here? In this file they are text
strings meant for the ip link commands:

#define TX_MAC "00:00:00:00:00:01"
#define RX_MAC "00:00:00:00:00:02"

With ETH_ALEN as the length, memcpy() copies the first six ASCII
characters, "00:00:". Both h_dest and h_source then become
30:30:3a:30:30:3a.

The old binary literals matched the addresses set on veTX and veRX. Now
every AF_XDP frame that generate_packet() builds has a destination MAC
that doesn't match veRX. That covers the single-descriptor packet, the
new multi-buffer packet and the freplace packet. Won't eth_type_trans()
on the receive side mark these frames PACKET_OTHERHOST?

The test still passes because rx() in progs/xdp_metadata.c only filters
on the UDP destination port:

	if (udp->dest != bpf_htons(8080))
		return XDP_PASS;

Was this change meant to be part of a patch that only adds the
multi-buffer case? Keeping the "\x00\x00\x00\x00\x00\x02" and
"\x00\x00\x00\x00\x00\x01" literals would avoid the problem.

>  	eth->h_proto = htons(ETH_P_IP);

[ ... ]

> @@ -518,7 +564,7 @@ void test_xdp_metadata(void)
>  		goto out;
>  
>  	/* Send packet to trigger . */
> -	if (!ASSERT_GE(generate_packet(&tx_xsk, AF_XDP_CONSUMER_PORT), 0,
> +	if (!ASSERT_GE(generate_packet(&tx_xsk, AF_XDP_CONSUMER_PORT, 0), 0,
>  		       "generate freplace packet"))
>  		goto out;

[Severity: Medium]
This isn't a bug introduced by this patch, but the polling loop after
this send in test_xdp_metadata() never seems to run:

	int retries = 10;
	...
	while (!retries--) {
		if (bpf_obj2->bss->called)
			break;
		usleep(10);
	}
	ASSERT_GT(bpf_obj2->bss->called, 0, "not called");

On the first check retries is 10, so !retries-- is false and the loop
body is skipped. The assertion then runs right after sendto() and the
netns switch.

freplace_rx() increments called from the veth NAPI receive path, which
runs in NET_RX softirq. If that softirq is deferred to ksoftirqd, can
this assertion fail without a single retry?

Should the condition be retries-- > 0 instead?

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261024141157.59301-1-testwrzwrz%40gmail.com