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

From: Jesper Dangaard Brouer <[email-masked]>
Date: Sat, 3 Dec 2016 16:24:13 +0100

On Fri, 2 Dec 2016 15:23:30 -0800
Martin KaFai Lau <[email-masked]> wrote:

> -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 */
> +		return -EINVAL;
> +
> +	xdp->data = new_data;
> +
> +	return 0;
> +}

First time I read this code, I was about to complain about you didn't
use XDP_PACKET_HEADROOM in your boundary check.  But then I noticed the
PAGE_MASK.  If you rename "head" to "page_boundary" or "page_start"
then IMHO the code would be more readable.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer