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Date: Mon, 25 Aug 2025 15:39:23 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Amery Hung <ameryhung@gmail.com>
Cc: bpf@vger.kernel.org, netdev@vger.kernel.org,
 alexei.starovoitov@gmail.com, andrii@kernel.org, daniel@iogearbox.net,
 martin.lau@kernel.org, mohsin.bashr@gmail.com, saeedm@nvidia.com,
 tariqt@nvidia.com, mbloch@nvidia.com, maciej.fijalkowski@intel.com,
 kernel-team@meta.com
Subject: Re: [RFC bpf-next v1 3/7] bpf: Support pulling non-linear xdp data
Message-ID: <20250825153923.0d98c69d@kernel.org>
In-Reply-To: <20250825193918.3445531-4-ameryhung@gmail.com>
References: <20250825193918.3445531-1-ameryhung@gmail.com>
	<20250825193918.3445531-4-ameryhung@gmail.com>
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On Mon, 25 Aug 2025 12:39:14 -0700 Amery Hung wrote:
> +__bpf_kfunc int bpf_xdp_pull_data(struct xdp_md *x, u32 len, u64 flags)
> +{
> +	struct xdp_buff *xdp = (struct xdp_buff *)x;
> +	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> +	void *data_end, *data_hard_end = xdp_data_hard_end(xdp);
> +	int i, delta, buff_len, n_frags_free = 0, len_free = 0;
> +
> +	buff_len = xdp_get_buff_len(xdp);
> +
> +	if (unlikely(len > buff_len))
> +		return -EINVAL;
> +
> +	if (!len)
> +		len = xdp_get_buff_len(xdp);
> +
> +	data_end = xdp->data + len;
> +	delta = data_end - xdp->data_end;
> +
> +	if (delta <= 0)
> +		return 0;
> +
> +	if (unlikely(data_end > data_hard_end))
> +		return -EINVAL;

Is this safe against pointers wrapping on 32b systems?

Maybe it's better to do:

	 if (unlikely(data_hard_end - xdp->data_end < delta))

?

> +	for (i = 0; i < sinfo->nr_frags && delta; i++) {
> +		skb_frag_t *frag = &sinfo->frags[i];
> +		u32 shrink = min_t(u32, delta, skb_frag_size(frag));
> +
> +		memcpy(xdp->data_end + len_free, skb_frag_address(frag), shrink);
> +
> +		len_free += shrink;
> +		delta -= shrink;
> +		if (bpf_xdp_shrink_data(xdp, frag, shrink, false))
> +			n_frags_free++;

possibly

		else
			break;

and then you don't have to check delta in the for loop condition?

> +	}
> +
> +	for (i = 0; i < sinfo->nr_frags - n_frags_free; i++) {
> +		memcpy(&sinfo->frags[i], &sinfo->frags[i + n_frags_free],
> +		       sizeof(skb_frag_t));

This feels like it'd really want to be a memmove(), no?

> +	}
> +
> +	sinfo->nr_frags -= n_frags_free;
> +	sinfo->xdp_frags_size -= len_free;
> +	xdp->data_end = data_end;
> +
> +	if (unlikely(!sinfo->nr_frags))
> +		xdp_buff_clear_frags_flag(xdp);
> +
> +	return 0;
> +}