netdev - Re: Alignment in BPF verifier

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Date:   Wed, 24 May 2017 14:46:19 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...com>,
        David Miller <davem@...emloft.net>
CC:     <alexei.starovoitov@...il.com>, <netdev@...r.kernel.org>
Subject: Re: Alignment in BPF verifier

On 23/05/17 22:27, Daniel Borkmann wrote:
> On 05/23/2017 09:45 PM, Alexei Starovoitov wrote:
>> On 5/23/17 7:41 AM, Edward Cree wrote:
>>> Hmm, that means that we can't do arithmetic on a
>>>  PTR_TO_MAP_VALUE_OR_NULL, we have to convert it to a PTR_TO_MAP_VALUE
>>>  first by NULL-checking it.  That's probably fine, but I can just about
>>>  imagine some compiler optimisation reordering them.  Any reason not to
>>>  split this out into a different reg->field, rather than overloading id?
>>
>> 'id' is sort of like 'version' of a pointer and has the same meaning in
>> both cases. How exactly do you see this split?
I was thinking there would be reg->id and reg->map_id.  Both could share the
 env->id_gen, since that's not likely to run out, but they'd be separate
 fields so that a PTR_TO_MAP_VALUE_OR_NULL could say "this is either map_value
 plus a 4-byte-aligned offset less than 24, or NULL plus that same offset",
 and then if another pointer with the same map_id and no variable-offset part
 was NULL-checked, we could convert both pointers to PTR_TO_MAP_VALUE.  (I'm
 getting rid of PTR_TO_MAP_VALUE_ADJ in my patch, along with several other
 types, by taking the 'we have an offset' part out of the bpf_reg_type.)
> So far we haven't run into this kind of optimization
> from llvm side yet[...] Out of curiosity, did you run into it with llvm?
No, purely theoretical.  I haven't even built/installed llvm yet, I'm just
 working with the bytecode in test_verifier.c for now.  I'm merely trying to
 not have restrictions that are unnecessary; but since allowing this kind of
 construct would take a non-zero amount of work, I'll file it for later.

-Ed