netdev - Re: [PATCH v18 bpf-next 20/23] net: xdp: introduce bpf_xdp

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Date:   Tue, 16 Nov 2021 07:13:57 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        lorenzo.bianconi@...hat.com, davem@...emloft.net, ast@...nel.org,
        daniel@...earbox.net, shayagr@...zon.com, john.fastabend@...il.com,
        dsahern@...nel.org, brouer@...hat.com, echaudro@...hat.com,
        jasowang@...hat.com, alexander.duyck@...il.com, saeed@...nel.org,
        maciej.fijalkowski@...el.com, magnus.karlsson@...el.com,
        tirthendu.sarkar@...el.com, toke@...hat.com
Subject: Re: [PATCH v18 bpf-next 20/23] net: xdp: introduce bpf_xdp_pointer
 utility routine

On Mon, 15 Nov 2021 23:33:14 +0100 Lorenzo Bianconi wrote:
> +	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> +	u32 headsize = xdp->data_end - xdp->data;
> +	u32 count = 0, frame_offset = headsize;
> +	int i;
> +
> +	if (offset < headsize) {
> +		int size = min_t(int, headsize - offset, len);
> +		void *src = flush ? buf : xdp->data + offset;
> +		void *dst = flush ? xdp->data + offset : buf;
> +
> +		memcpy(dst, src, size);
> +		count = size;
> +		offset = 0;
> +	}

is this missing
	else
		offset -= headsize;
?

I'm struggling to understand this. Say
	headsize = 400
	frag[0].size = 200

	offset = 500
	len = 50

we enter the loop having missed the previous if...

> +	for (i = 0; i < sinfo->nr_frags; i++) {
> +		skb_frag_t *frag = &sinfo->frags[i];
> +		u32 frag_size = skb_frag_size(frag);
> +
> +		if (count >= len)
> +			break;
> +
> +		if (offset < frame_offset + frag_size) {

		500 < 400 + 200 => true

> +			int size = min_t(int, frag_size - offset, len - count);

			size = min(200 - 500, 50 - 0)
			size = -300 ??

> +			void *addr = skb_frag_address(frag);
> +			void *src = flush ? buf + count : addr + offset;
> +			void *dst = flush ? addr + offset : buf + count;
> +
> +			memcpy(dst, src, size);
> +			count += size;
> +			offset = 0;
> +		}
> +		frame_offset += frag_size;