netdev - Re: [PATCH net-next] xdp: add multi-buff support for xdp running in generic mode

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Date: Tue, 28 Nov 2023 18:29:20 +0100
From: Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To: kuba@...nel.org
Cc: netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	lorenzo@...nel.org, pabeni@...hat.com, bpf@...r.kernel.org,
	hawk@...nel.org, toke@...hat.com
Subject: Re: [PATCH net-next] xdp: add multi-buff support for xdp running in
 generic mode

> Similar to native xdp, do not always linearize the skb in
> netif_receive_generic_xdp routine but create a non-linear xdp_buff to be
> processed by the eBPF program. This allow to add multi-buffer support
> for xdp running in generic mode.
> 
> Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> ---
>  net/core/dev.c | 28 +++++++++++++++++++++++-----
>  1 file changed, 23 insertions(+), 5 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 3950ced396b5..5a58f3e28657 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4853,6 +4853,12 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
>  	xdp_init_buff(xdp, frame_sz, &rxqueue->xdp_rxq);
>  	xdp_prepare_buff(xdp, hard_start, skb_headroom(skb) - mac_len,
>  			 skb_headlen(skb) + mac_len, true);
> +	if (skb_is_nonlinear(skb)) {
> +		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
> +		xdp_buff_set_frags_flag(xdp);
> +	} else {
> +		xdp_buff_clear_frags_flag(xdp);
> +	}
>  
>  	orig_data_end = xdp->data_end;
>  	orig_data = xdp->data;
> @@ -4882,6 +4888,14 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
>  		skb->len += off; /* positive on grow, negative on shrink */
>  	}
>  
> +	/* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
> +	 * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
> +	 */
> +	if (xdp_buff_has_frags(xdp))
> +		skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> +	else
> +		skb->data_len = 0;
> +
>  	/* check if XDP changed eth hdr such SKB needs update */
>  	eth = (struct ethhdr *)xdp->data;
>  	if ((orig_eth_type != eth->h_proto) ||
> @@ -4927,9 +4941,9 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
>  	if (skb_is_redirected(skb))
>  		return XDP_PASS;
>  
> -	/* XDP packets must be linear and must have sufficient headroom
> -	 * of XDP_PACKET_HEADROOM bytes. This is the guarantee that also
> -	 * native XDP provides, thus we need to do it here as well.
> +	/* XDP packets must have sufficient headroom of XDP_PACKET_HEADROOM
> +	 * bytes. This is the guarantee that also native XDP provides,
> +	 * thus we need to do it here as well.
>  	 */
>  	if (skb_cloned(skb) || skb_is_nonlinear(skb) ||
>  	    skb_headroom(skb) < XDP_PACKET_HEADROOM) {
> @@ -4943,8 +4957,12 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
>  				     hroom > 0 ? ALIGN(hroom, NET_SKB_PAD) : 0,
>  				     troom > 0 ? troom + 128 : 0, GFP_ATOMIC))
>  			goto do_drop;
> -		if (skb_linearize(skb))
> -			goto do_drop;
> +
> +		/* XDP does not support fraglist */
> +		if (skb_has_frag_list(skb) || !xdp_prog->aux->xdp_has_frags) {
> +			if (skb_linearize(skb))
> +				goto do_drop;

@Jakub: iirc we were discussing something similar for veth [0].
Here pskb_expand_head() reallocates skb paged data (skb_shinfo()->frags[])
just if the skb is cloned and if it is zero-copied [1] while in skb_cow_data()
we always reallocate the paged area if skb_shinfo()->nr_frags is set [2].
Since the eBPF program can theoretically modify paged data, I would say we
should do the same we did for veth even here, right?

Regards,
Lorenzo

[0] https://lore.kernel.org/netdev/20220312131806.1c2919ba@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com/
[1] https://elixir.bootlin.com/linux/v6.6.2/source/net/core/skbuff.c#L2113
[2] https://elixir.bootlin.com/linux/v6.6.2/source/net/core/skbuff.c#L5016

> +		}
>  	}
>  
>  	act = bpf_prog_run_generic_xdp(skb, xdp, xdp_prog);
> -- 
> 2.43.0
> 

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