Re: [PATCH net-next V6 2/5] selftests: drv-net: Test XDP_PASS/DROP support

[Nimrod Oren <noren@nvidia.com>]

On 19/07/2025 11:30, Mohsin Bashir wrote:
> Test XDP_PASS/DROP in single buffer and multi buffer mode when
> XDP native support is available.
> 
> ./drivers/net/xdp.py
> TAP version 13
> 1..4
> ok 1 xdp.test_xdp_native_pass_sb
> ok 2 xdp.test_xdp_native_pass_mb
> ok 3 xdp.test_xdp_native_drop_sb
> ok 4 xdp.test_xdp_native_drop_mb
> \# Totals: pass:4 fail:0 xfail:0 xpass:0 skip:0 error:0
> 
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
> ---
>  tools/testing/selftests/drivers/net/Makefile  |   1 +
>  tools/testing/selftests/drivers/net/xdp.py    | 303 ++++++++++++++++++
>  .../selftests/net/lib/xdp_native.bpf.c        | 158 +++++++++
>  3 files changed, 462 insertions(+)
>  create mode 100755 tools/testing/selftests/drivers/net/xdp.py
>  create mode 100644 tools/testing/selftests/net/lib/xdp_native.bpf.c
> 

...

> +
> +static struct udphdr *filter_udphdr(struct xdp_md *ctx, __u16 port)
> +{
> +	void *data_end = (void *)(long)ctx->data_end;
> +	void *data = (void *)(long)ctx->data;
> +	struct udphdr *udph = NULL;
> +	struct ethhdr *eth = data;
> +
> +	if (data + sizeof(*eth) > data_end)
> +		return NULL;
> +

This check assumes that the packet headers reside in the linear part of
the xdp_buff. However, this assumption does not hold across all drivers.
For example, in mlx5, the linear part is empty when using multi-buffer
mode with striding rq configuration. This causes all multi-buffer test
cases to fail over mlx5.

To ensure correctness across all drivers, all direct accesses to packet
data should use these safer helper functions instead:
bpf_xdp_load_bytes() and bpf_xdp_store_bytes().

Related discussion and context can be found here:
https://github.com/xdp-project/xdp-tools/pull/409

> +	if (eth->h_proto == bpf_htons(ETH_P_IP)) {
> +		struct iphdr *iph = data + sizeof(*eth);
> +
> +		if (iph + 1 > (struct iphdr *)data_end ||
> +		    iph->protocol != IPPROTO_UDP)
> +			return NULL;
> +
> +		udph = (void *)eth + sizeof(*iph) + sizeof(*eth);
> +	} else if (eth->h_proto  == bpf_htons(ETH_P_IPV6)) {
> +		struct ipv6hdr *ipv6h = data + sizeof(*eth);
> +
> +		if (ipv6h + 1 > (struct ipv6hdr *)data_end ||
> +		    ipv6h->nexthdr != IPPROTO_UDP)
> +			return NULL;
> +
> +		udph = (void *)eth + sizeof(*ipv6h) + sizeof(*eth);
> +	} else {
> +		return NULL;
> +	}
> +
> +	if (udph + 1 > (struct udphdr *)data_end)
> +		return NULL;
> +
> +	if (udph->dest != bpf_htons(port))
> +		return NULL;
> +
> +	record_stats(ctx, STATS_RX);
> +
> +	return udph;
> +}