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

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Message-Id: <20170516.155211.2029345941344452552.davem@davemloft.net>
Date:   Tue, 16 May 2017 15:52:11 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     ecree@...arflare.com
Cc:     daniel@...earbox.net, ast@...com, 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.

From: Edward Cree <ecree@...arflare.com>
Date: Tue, 16 May 2017 13:37:42 +0100

> On 15/05/17 17:04, David Miller wrote:
>> If we use 1<<31, then sequences like:
>>
>> 	R1 = 0
>> 	R1 <<= 2
>>
>> do silly things.
> Hmm.  It might be a bit late for this, but I wonder if, instead of handling
>  alignments as (1 << align), you could store them as -(1 << align), i.e.
>  leading 1s followed by 'align' 0s.
> Now the alignment of 0 is 0 (really 1 << 32), which doesn't change when
>  left-shifted some more.  Shifts of other numbers' alignments also do the
>  right thing, e.g. align(6) << 2 = (-2) << 2 = -8 = align(6 << 2).  Of
>  course you do all this in unsigned, to make sure right shifts work.
> This also makes other arithmetic simple to track; for instance, align(a + b)
>  is at worst align(a) | align(b).  (Of course, this bound isn't tight.)
> A number is 2^(n+1)-aligned if the 2^n bit of its alignment is cleared.
> Considered as unsigned numbers, smaller values are stricter alignments.

Thanks for the bit twiddling suggestion, I'll take a look!