Re: [RFC PATCH bpf-next v1 6/7] veth: support the rx_csum XDP metadata hint

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Incorrectly mapping CHECKSUM_PARTIAL to XDP_CSUM_VERIFIED violates the metadata kfunc UAPI and leads to corrupted checksums on forwarded packets.
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commit 43f8c99f073896fcdca62b5034e71d604e446b2b
Author: Vladimir Vdovin <deliran@verdict.gg>

veth: support the rx_csum XDP metadata hint

This commit implements xmo_rx_csum from skb->ip_summed. It surfaces whatever
checksum verdict the skb already carries, making the metadata kfunc testable
without a NIC since veth has no real hardware.

> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index 1c51421491753..b7bc5a3b07e50 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c

[ ... ]

> @@ -1700,6 +1700,28 @@ static int veth_xdp_rx_vlan_tag(const struct xdp_md *ctx, __be16 *vlan_proto,

[ ... ]

> +static int veth_xdp_rx_csum(const struct xdp_md *ctx,
> +			    enum xdp_csum_status *csum_status)
> +{
> +	const struct veth_xdp_buff *_ctx = (void *)ctx;
> +	const struct sk_buff *skb = _ctx->skb;
> +
> +	if (!skb)
> +		return -ENODATA;
> +
> +	/* veth has no real hardware; surface whatever checksum verdict the
> +	 * skb already carries (e.g. CHECKSUM_PARTIAL/UNNECESSARY from a local
> +	 * sender or a previous validation).
> +	 */
> +	if (skb->ip_summed == CHECKSUM_UNNECESSARY ||
> +	    skb->ip_summed == CHECKSUM_PARTIAL)
> +		*csum_status = XDP_CSUM_VERIFIED;

[Severity: High]
Does mapping CHECKSUM_PARTIAL to XDP_CSUM_VERIFIED introduce a regression by
violating the metadata kfunc UAPI? CHECKSUM_PARTIAL indicates that the
payload checksum is incomplete.

If a packet is generated locally and sent over veth, it can carry
skb->ip_summed == CHECKSUM_PARTIAL. If an XDP program reads this metadata
via bpf_xdp_metadata_rx_csum(), it will see XDP_CSUM_VERIFIED. If the
program then forwards the packet out a physical NIC via bpf_redirect(), the
transmitted packet would have an incomplete, invalid checksum on the wire
since only the pseudo-header checksum was computed.

> +	else
> +		*csum_status = XDP_CSUM_NONE;
> +
> +	return 0;
> +}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260630191510.81402-1-deliran@verdict.gg?part=6