netdev - Re: [PATCH v2 1/3] bpf: Use 1<<16 as ceiling for immediate alignment in verifier.

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Date:   Wed, 17 May 2017 18:00:03 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     David Miller <davem@...emloft.net>
CC:     <ast@...com>, <daniel@...earbox.net>,
        <alexei.starovoitov@...il.com>, <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] bpf: Use 1<<16 as ceiling for immediate alignment
 in verifier.

On 17/05/17 17:13, David Miller wrote:
> Both cases are common in real BPF programs.  The offsets really are
> necessary.  It's funny because initially I tried to implement this
> without the auxiliary offset and it simply doesn't work. :-)
> 
> We always have to track when you've seen the offset that cancels out
> the NET_IP_ALIGN.  And as stated it can occur both before and after
> variable offsets have been introduced.
> 
> You have to catch both:
> 
> 	ptr += variable;
> 	ptr += 14;
> 
> and:
> 
> 	ptr += 14;
> 	ptr += variable; /* align = 4 */
> 
> And always see at the end that "NET_IP_ALIGN + offsets" will
> be properly 4 byte aligned.

Did you see the algorithms I posted with two masks?  Effectively they
 are tracking the 'quotient' and 'remainder', but in a was that might
 be easier to manipulate (bit-twiddly etc.) than the aux offset.  I
 think they handle both the above cases, in a nicely general way.

-Ed