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

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Date:   Fri, 19 May 2017 15:21:49 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Alexei Starovoitov <ast@...com>,
        David Miller <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>
CC:     <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 19/05/17 02:22, Alexei Starovoitov wrote:
> In your .py I'd only change __str__(self) to print them in mask,value
> as the order they're passed into constructor to make it easier to read. 
Actually I was going to go the other way and change the ctor to take
 value,mask.  But I agree they're inconsistent right now.

> this mul algo I don't completely understand. It feels correct,
> but I'm not sure we really need it for the kernel. 
You're probably right; I was just driven by a completionist desire to
 cover everything I could.

> What I love about the whole thing that it works for access into
> packet, access into map values and in the future for any other
> variable length access.
Sure, but don't start thinking it subsumes all the other checks.  We
 will still need e.g. max/min tracking, because packet length isn't
 always a power of 2.

> Are you planning to work on the kernel patch for this algo?
> Once we have it the verifier will be smarter regarding
> alignment tracking than any compiler i know :) 
I'm currently translating the algos to C.  But for the kernel patch,
 I'll need to read & understand the existing verifier code, so it
 might take a while :)  (I don't suppose there's any design document
 or hacking-notes you could point me at?)
But I'll give it a go for sure.

-Ed