Subject: Re: [PATCH net-next 1/4] bpf: xdp: Allow head adjustment in XDP prog

From: Daniel Borkmann <[email-masked]>
Date: Sat, 03 Dec 2016 01:22:05 +0100

On 12/03/2016 12:23 AM, Martin KaFai Lau wrote:
> This patch allows XDP prog to extend/remove the packet
> data at the head (like adding or removing header).  It is
> done by adding a new XDP helper bpf_xdp_adjust_head().
>
> It also renames bpf_helper_changes_skb_data() to
> bpf_helper_changes_pkt_data() to better reflect
> that XDP prog does not work on skb.
>
> Signed-off-by: Martin KaFai Lau <[email-masked]>
[...]
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 56b43587d200..6902e2f73e38 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2234,7 +2234,34 @@ static const struct bpf_func_proto bpf_skb_change_head_proto = {
>   	.arg3_type	= ARG_ANYTHING,
>   };
>
> -bool bpf_helper_changes_skb_data(void *func)
> +BPF_CALL_2(bpf_xdp_adjust_head, struct xdp_buff *, xdp, int, offset)
> +{
> +	/* Both mlx4 and mlx5 driver align each packet to PAGE_SIZE when
> +	 * XDP prog is set.
> +	 * If the above is not true for the other drivers to support
> +	 * bpf_xdp_adjust_head, struct xdp_buff can be extended.
> +	 */
> +	void *head = (void *)((unsigned long)xdp->data & PAGE_MASK);
> +	void *new_data = xdp->data + offset;
> +
> +	if (new_data < head || new_data >= xdp->data_end)
> +		/* The packet length must be >=1 */

Patch looks generally good to me. Should the min pkt len here be
limited to ETH_HLEN instead of 1?

> +		return -EINVAL;
> +
> +	xdp->data = new_data;
> +
> +	return 0;
> +}
> +
> +static const struct bpf_func_proto bpf_xdp_adjust_head_proto = {
> +	.func		= bpf_xdp_adjust_head,
> +	.gpl_only	= false,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type	= ARG_PTR_TO_CTX,
> +	.arg2_type	= ARG_ANYTHING,
> +};
> +
> +bool bpf_helper_changes_pkt_data(void *func)
>   {
>   	if (func == bpf_skb_vlan_push ||
>   	    func == bpf_skb_vlan_pop ||
[...]