netdev - Re: __sk_buff.data

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Date:   Thu, 20 Apr 2017 16:32:32 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: __sk_buff.data_end

On Thu, 2017-04-20 at 16:28 +0200, Daniel Borkmann wrote:
> 
> I see what you mean now. Yes, that's fine. We already do something
> similar essentially with skb->ifindex access already (skb->dev +
> dev->ifindex), f.e.:
> 
> [...]
> 	case offsetof(struct __sk_buff, ifindex):
> 		BUILD_BUG_ON(FIELD_SIZEOF(struct net_device, ifindex)
> != 4);
> 
> 		*insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff,
> dev),
> 				      si->dst_reg, si->src_reg,
> 				      offsetof(struct sk_buff, dev));
> 		*insn++ = BPF_JMP_IMM(BPF_JEQ, si->dst_reg, 0, 1);
> 		*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->dst_reg,
> 				      offsetof(struct net_device,
> ifindex));
> 		break;
> [...]

Oh, right, good point.

> Which is not too different from the above. You'd probably need to
> populate the struct wifi_data each time if you place it onto the
> stack, but perhaps could be optimized by storing that somewhere
> else (e.g. somewhere via netdev, etc) and walking the pointer from
> there, which would also spare you the cb[] save/restore.

Hmm. I don't see what "somewhere else" I could possibly have though,
given that I want the (kernel-side) context to be "struct sk_buff *"
to allow the skb helpers?

johannes