Subject: Re: [PATCH v11 bpf-next 17/18] net: xdp: introduce bpf_xdp_adjust_data helper

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Wed, 18 Aug 2021 14:58:47 +0200

Lorenzo Bianconi <[email-masked]> writes:

>> Lorenzo Bianconi <[email-masked]> writes:
>> 
> [...]
>> > + *	Description
>> > + *		For XDP frames split over multiple buffers, the
>> > + *		*xdp_md*\ **->data** and*xdp_md *\ **->data_end** pointers
>> > + *		will point to the start and end of the first fragment only.
>> > + *		This helper can be used to access subsequent fragments by
>> > + *		moving the data pointers. To use, an XDP program can call
>> > + *		this helper with the byte offset of the packet payload that
>> > + *		it wants to access; the helper will move *xdp_md*\ **->data**
>> > + *		and *xdp_md *\ **->data_end** so they point to the requested
>> > + *		payload offset and to the end of the fragment containing this
>> > + *		byte offset, and return the byte offset of the start of the
>> > + *		fragment.
>> 
>> This comment is wrong now :)
>
> actually we are still returning the byte offset of the start of the fragment
> (base_offset).

Hmm, right, I was looking at the 'return 0':

> +BPF_CALL_2(bpf_xdp_adjust_data, struct xdp_buff *, xdp, u32, offset)
> +{
> +	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> +	u32 base_offset = xdp->mb.headlen;
> +	int i;
> +
> +	if (!xdp_buff_is_mb(xdp) || offset > sinfo->xdp_frags_size)
> +		return -EINVAL;
> +
> +	if (offset < xdp->mb.headlen) {
> +		/* linear area */
> +		xdp->data = xdp->data_hard_start + xdp->mb.headroom;
> +		xdp->data_end = xdp->data + xdp->mb.headlen;
> +		return 0;
> +	}

But I guess that's an offset; but that means the helper is not doing
what it says it's doing if it's within the first fragment. That should
probably be made consistent... :)

-Toke