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Date:   Tue, 18 Sep 2018 04:58:37 +0200
From:   Dominique Martinet <asmadeus@codewreck.org>
To:     David Miller <davem@davemloft.net>
Cc:     doronrk@fb.com, tom@quantonium.net, davejwatson@fb.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] kcm: remove any offset before parsing messages
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David Miller wrote on Mon, Sep 17, 2018:
> > No, they can see it, so it's possible to make a KCM program that works
> > right now if you are careful (I'm not sure why the offset within bpf is
> > different from the offset in the kernel though, it looks like the bpf
> > program skips the qos part of the control buffer)
> 
> What helper is used in the BPF program to get this offset value?
> 
> (also good info to add to the commit message)

Dave defined one himself ; for a simple protocol where the offset is in
the first four bytes of the message.

The whole bpf program could look like this:

------
struct kcm_rx_msg { int full_len; int offset; };
static inline struct kcm_rx_msg *kcm_rx_msg(struct __sk_buff *skb) {
	return (struct kcm_rx_msg *)skb->cb;
}
int decode_framing(struct __sk_buff *skb) {
	return load_word(skb, kcm_rx_msg(skb)->offset);
}
------

If we go towards documenting it, adding a helper would be useful yes;
buf if we pull that becomes unnecessary.
(I'll add the example program in the commit message anyway at your
suggestion)

-- 
Dominique Martinet