Subject: Re: [PATCH bpf-next] xdp: introduce convert_to_xdp_buff utility routine

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 26 May 2020 16:20:08 +0200

On Tue, 26 May 2020 15:48:13 +0200
Lorenzo Bianconi <[email-masked]> wrote:

> Introduce convert_to_xdp_buff utility routine to initialize xdp_buff
> fields from xdp_frames ones. Rely on convert_to_xdp_buff in veth xdp
> code
> 
> Suggested-by: Jesper Dangaard Brouer <[email-masked]>
> Signed-off-by: Lorenzo Bianconi <[email-masked]>
> ---
>  drivers/net/veth.c | 12 ++----------
>  include/net/xdp.h  | 10 ++++++++++
>  2 files changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index b586d2fa5551..dfbe553f967e 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -559,27 +559,19 @@ static struct sk_buff *veth_xdp_rcv_one(struct veth_rq *rq,
>  					struct veth_xdp_tx_bq *bq,
>  					struct veth_stats *stats)
>  {
> -	void *hard_start = frame->data - frame->headroom;
>  	int len = frame->len, delta = 0;
>  	struct xdp_frame orig_frame;
>  	struct bpf_prog *xdp_prog;
>  	unsigned int headroom;
>  	struct sk_buff *skb;
>  
> -	/* bpf_xdp_adjust_head() assures BPF cannot access xdp_frame area */
> -	hard_start -= sizeof(struct xdp_frame);
> -
>  	rcu_read_lock();
>  	xdp_prog = rcu_dereference(rq->xdp_prog);
>  	if (likely(xdp_prog)) {
>  		struct xdp_buff xdp;
>  		u32 act;
>  
> -		xdp.data_hard_start = hard_start;
> -		xdp.data = frame->data;
> -		xdp.data_end = frame->data + frame->len;
> -		xdp.data_meta = frame->data - frame->metasize;
> -		xdp.frame_sz = frame->frame_sz;
> +		convert_to_xdp_buff(frame, &xdp);
>  		xdp.rxq = &rq->xdp_rxq;
>  
>  		act = bpf_prog_run_xdp(xdp_prog, &xdp);
> @@ -626,7 +618,7 @@ static struct sk_buff *veth_xdp_rcv_one(struct veth_rq *rq,
>  	rcu_read_unlock();
>  
>  	headroom = sizeof(struct xdp_frame) + frame->headroom - delta;
> -	skb = veth_build_skb(hard_start, headroom, len, frame->frame_sz);
> +	skb = veth_build_skb(frame, headroom, len, frame->frame_sz);
>  	if (!skb) {
>  		xdp_return_frame(frame);
>  		stats->rx_drops++;
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 90f11760bd12..5dbdd65866a9 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -106,6 +106,16 @@ void xdp_warn(const char *msg, const char *func, const int line);
>  
>  struct xdp_frame *xdp_convert_zc_to_xdp_frame(struct xdp_buff *xdp);
>  
> +static inline
> +void convert_to_xdp_buff(struct xdp_frame *frame, struct xdp_buff *xdp)
> +{
> +	xdp->data_hard_start = (void *)frame;

This assumption is problematic.  You are suppose to deduct this from
frame->data pointer.

Currently the xdp_frame is designed and access such that is is possible
to use another memory area for xdp_frame.  That would break after this
change.

This should instead be:

 xdp->data_hard_start = frame->data - (frame->headroom + sizeof(struct xdp_frame));

> +	xdp->data = frame->data;
> +	xdp->data_end = frame->data + frame->len;
> +	xdp->data_meta = frame->data - frame->metasize;
> +	xdp->frame_sz = frame->frame_sz;
> +}
> +

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