Re: [RFC PATCH bpf-next v1 1/7] xdp: let XDP programs assert the RX checksum over redirect

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The XDP_FLAGS_RX_CSUM_UNNECESSARY flag is lost when an AF_XDP zero-copy buffer is converted to an xdp_frame during XDP_REDIRECT.
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commit ba787c7855bcf7d5c0ecdc2fe7e6a11cde6a4c00
Author: Vladimir Vdovin <deliran@verdict.gg>

xdp: let XDP programs assert the RX checksum over redirect

This patch adds a non-dev-bound kfunc, bpf_xdp_assert_rx_csum(), allowing XDP
programs to assert that an L4 checksum is correct. It sets a new flag on the
buffer, XDP_FLAGS_RX_CSUM_UNNECESSARY, which is translated to 
CHECKSUM_UNNECESSARY when an skb is built out of the frame.

> diff --git a/net/core/xdp.c b/net/core/xdp.c
> index 9890a30584ba7..63ee36ec93de2 100644
> --- a/net/core/xdp.c
> +++ b/net/core/xdp.c

[ ... ]

> @@ -961,6 +964,31 @@ __bpf_kfunc int bpf_xdp_metadata_rx_vlan_tag(const struct xdp_md *ctx,
>  	return -EOPNOTSUPP;
>  }
>  
> +/**
> + * bpf_xdp_assert_rx_csum - Assert the packet's L4 checksum is correct.
> + * @ctx: XDP context pointer.
> + *
> + * Mark the frame so that an skb later built out of it (e.g. on the cpumap
> + * redirect path, see __xdp_build_skb_from_frame()) is set to
> + * CHECKSUM_UNNECESSARY instead of being validated in software when it enters
> + * the stack.
> + *
> + * This is an assertion made by the XDP program: the kernel cannot verify it.
> + * The program takes responsibility for the checksum being correct, the same
> + * way it is already trusted to rewrite arbitrary packet contents. If the
> + * program modifies L4 data after calling this kfunc the assertion may no
> + * longer hold.
> + *
> + * Return: 0.
> + */
> +__bpf_kfunc int bpf_xdp_assert_rx_csum(struct xdp_md *ctx)
> +{
> +	struct xdp_buff *xdp = (struct xdp_buff *)ctx;
> +
> +	xdp->flags |= XDP_FLAGS_RX_CSUM_UNNECESSARY;

[Severity: Medium]
Does this flag get lost when an AF_XDP zero-copy buffer is redirected?

Looking at xdp_convert_zc_to_xdp_frame() in net/core/xdp.c, it initializes
the new frame with memset() but does not copy the flags from the original
xdp_buff:

net/core/xdp.c:xdp_convert_zc_to_xdp_frame() {
	...
	memset(xdpf, 0, sizeof(*xdpf));
	...
	xdpf->frame_sz = PAGE_SIZE;
	xdpf->mem_type = MEM_TYPE_PAGE_ORDER0;

	xsk_buff_free(xdp);
	return xdpf;
}

Should xdp_convert_zc_to_xdp_frame() be updated to copy xdp->flags to
xdpf->flags so the RX checksum assertion survives redirect for zero-copy
frames?

> +	return 0;
> +}
> +
>  __bpf_kfunc_end_defs();

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