netdev - Re: [bpf-next V4 PATCH 13/14] bpf: finally expose xdp_rxq

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Date:   Sat, 6 Jan 2018 01:54:48 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        dsahern@...il.com, gospo@...adcom.com, bjorn.topel@...el.com,
        michael.chan@...adcom.com
Subject: Re: [bpf-next V4 PATCH 13/14] bpf: finally expose xdp_rxq_info to XDP
 bpf-programs

On 01/03/2018 11:26 AM, Jesper Dangaard Brouer wrote:
> Now all XDP driver have been updated to setup xdp_rxq_info and assign
> this to xdp_buff->rxq.  Thus, it is now safe to enable access to some
> of the xdp_rxq_info struct members.
> 
> This patch extend xdp_md and expose UAPI to userspace for
> ingress_ifindex and rx_queue_index.  Access happens via bpf
> instruction rewrite, that load data directly from struct xdp_rxq_info.
> 
> * ingress_ifindex map to xdp_rxq_info->dev->ifindex
> * rx_queue_index  map to xdp_rxq_info->queue_index
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> Acked-by: Alexei Starovoitov <ast@...nel.org>
> ---
>  include/uapi/linux/bpf.h |    3 +++
>  net/core/filter.c        |   19 +++++++++++++++++++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 69eabfcb9bdb..a6000a95d40e 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -899,6 +899,9 @@ struct xdp_md {
>  	__u32 data;
>  	__u32 data_end;
>  	__u32 data_meta;
> +	/* Below access go though struct xdp_rxq_info */
> +	__u32 ingress_ifindex; /* rxq->dev->ifindex */
> +	__u32 rx_queue_index;  /* rxq->queue_index  */
>  };
>  
>  enum sk_action {
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 130b842c3a15..acdb94c0e97f 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4304,6 +4304,25 @@ static u32 xdp_convert_ctx_access(enum bpf_access_type type,
>  				      si->dst_reg, si->src_reg,
>  				      offsetof(struct xdp_buff, data_end));
>  		break;
> +	case offsetof(struct xdp_md, ingress_ifindex):
> +		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct xdp_buff, rxq),
> +				      si->dst_reg, si->src_reg,
> +				      offsetof(struct xdp_buff, rxq));
> +		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct xdp_rxq_info, dev),
> +				      si->dst_reg, si->dst_reg,
> +				      offsetof(struct xdp_rxq_info, dev));
> +		*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->dst_reg,
> +				      bpf_target_off(struct net_device,
> +						     ifindex, 4, target_size));

The bpf_target_off() is actually only used in the context of narrow ctx access.

This should just be:

*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->dst_reg,
		      offsetof(struct net_device, ifindex));

> +		break;
> +	case offsetof(struct xdp_md, rx_queue_index):
> +		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct xdp_buff, rxq),
> +				      si->dst_reg, si->src_reg,
> +				      offsetof(struct xdp_buff, rxq));
> +		*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->dst_reg,
> +				      bpf_target_off(struct xdp_rxq_info,
> +						queue_index, 4, target_size));

And here:

*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->dst_reg,
		      offsetof(struct xdp_rxq_info, queue_index));

> +		break;
>  	}
>  
>  	return insn - insn_buf;
>