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

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Message-ID: <754f2c39-fdb0-2407-c2f2-aa36d506d202@solarflare.com>
Date:   Tue, 16 May 2017 13:37:42 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     David Miller <davem@...emloft.net>, <daniel@...earbox.net>
CC:     <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.

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.

-Ed