Re: [PATCH v8 bpf-next 08/14] bpf: add multi-buff support to the bpf_xdp_adjust_tail() API

[Vladimir Oltean <olteanv@gmail.com>]

On Thu, Apr 08, 2021 at 02:51:00PM +0200, Lorenzo Bianconi wrote:
> From: Eelco Chaudron <echaudro@redhat.com>
> 
> This change adds support for tail growing and shrinking for XDP multi-buff.
> 
> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
>  include/net/xdp.h |  5 ++++
>  net/core/filter.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 68 insertions(+)
> 
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index c8eb7cf4ebed..55751cf2badf 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -159,6 +159,11 @@ static inline void xdp_set_frag_size(skb_frag_t *frag, u32 size)
>  	frag->bv_len = size;
>  }
>  
> +static inline unsigned int xdp_get_frag_tailroom(const skb_frag_t *frag)
> +{
> +	return PAGE_SIZE - xdp_get_frag_size(frag) - xdp_get_frag_offset(frag);
> +}
> +

This is an interesting requirement. Must an XDP frame fragment be a full
PAGE_SIZE? enetc does not fulfill it, and I suspect that none of the
drivers with a "shared page" memory model will.

>  struct xdp_frame {
>  	void *data;
>  	u16 len;
> diff --git a/net/core/filter.c b/net/core/filter.c
> index cae56d08a670..c4eb1392f88e 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3855,11 +3855,74 @@ static const struct bpf_func_proto bpf_xdp_adjust_head_proto = {
>  	.arg2_type	= ARG_ANYTHING,
>  };
>  
> +static int bpf_xdp_mb_adjust_tail(struct xdp_buff *xdp, int offset)
> +{
> +	struct xdp_shared_info *xdp_sinfo = xdp_get_shared_info_from_buff(xdp);
> +
> +	if (unlikely(xdp_sinfo->nr_frags == 0))
> +		return -EINVAL;

This function is called if xdp->mb is true, but we check whether
nr_frags != 0? Is this condition possible?

> +	if (offset >= 0) {
> +		skb_frag_t *frag = &xdp_sinfo->frags[xdp_sinfo->nr_frags - 1];
> +		int size;
> +
> +		if (unlikely(offset > xdp_get_frag_tailroom(frag)))
> +			return -EINVAL;
> +
> +		size = xdp_get_frag_size(frag);
> +		memset(xdp_get_frag_address(frag) + size, 0, offset);
> +		xdp_set_frag_size(frag, size + offset);
> +		xdp_sinfo->data_length += offset;
> +	} else {
> +		int i, frags_to_free = 0;
> +
> +		offset = abs(offset);
> +
> +		if (unlikely(offset > ((int)(xdp->data_end - xdp->data) +
> +				       xdp_sinfo->data_length -
> +				       ETH_HLEN)))

I think code alignment should be to xdp->data_end, not to (int).

Also: should we have some sort of helper for calculating the total
length of an xdp_frame (head + frags)? Maybe it's just me, but I find it
slightly confusing that xdp_sinfo->data_length does not account for
everything.

> +			return -EINVAL;
> +
> +		for (i = xdp_sinfo->nr_frags - 1; i >= 0 && offset > 0; i--) {
> +			skb_frag_t *frag = &xdp_sinfo->frags[i];
> +			int size = xdp_get_frag_size(frag);
> +			int shrink = min_t(int, offset, size);
> +
> +			offset -= shrink;
> +			if (likely(size - shrink > 0)) {
> +				/* When updating the final fragment we have
> +				 * to adjust the data_length in line.
> +				 */
> +				xdp_sinfo->data_length -= shrink;
> +				xdp_set_frag_size(frag, size - shrink);
> +				break;
> +			}
> +
> +			/* When we free the fragments,
> +			 * xdp_return_frags_from_buff() will take care
> +			 * of updating the xdp share info data_length.

s/xdp share info data_length/data_length from xdp_shared_info/

> +			 */
> +			frags_to_free++;
> +		}
> +
> +		if (unlikely(frags_to_free))
> +			xdp_return_num_frags_from_buff(xdp, frags_to_free);
> +
> +		if (unlikely(offset > 0))
> +			xdp->data_end -= offset;
> +	}
> +
> +	return 0;
> +}
> +
>  BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
>  {
>  	void *data_hard_end = xdp_data_hard_end(xdp); /* use xdp->frame_sz */
>  	void *data_end = xdp->data_end + offset;
>  
> +	if (unlikely(xdp->mb))
> +		return bpf_xdp_mb_adjust_tail(xdp, offset);
> +
>  	/* Notice that xdp_data_hard_end have reserved some tailroom */
>  	if (unlikely(data_end > data_hard_end))
>  		return -EINVAL;
> -- 
> 2.30.2
>