Re: [PATCH] filter: added BPF random opcode

[Chema Gonzalez <chema@google.com>]

On Tue, Apr 15, 2014 at 8:04 AM, Daniel Borkmann <dborkman@redhat.com> wrote:
> On 04/15/2014 04:41 PM, Eric Dumazet wrote:
>>
>> On Tue, 2014-04-15 at 09:24 +0200, Daniel Borkmann wrote:
>>
>>>> @@ -773,6 +779,7 @@ static bool convert_bpf_extensions(struct
>>>> sock_filter *fp,
>>>>         case SKF_AD_OFF + SKF_AD_NLATTR:
>>>>         case SKF_AD_OFF + SKF_AD_NLATTR_NEST:
>>>>         case SKF_AD_OFF + SKF_AD_CPU:
>>>> +       case SKF_AD_OFF + SKF_AD_RANDOM:
>>>
>>>
>>> I think instead of a function call, this sould rather be modelled
>>> directly into the internal insn set and thus converted differently,
>>> so we can spare us the call.
>>
>>
>> Hmmm... this would need percpu storage, thus preempt disable/enable
>> calls, and prandom_u32_state() is about 40 instructions.
>>
>> This is really not worth the pain.
>
>
> Absolutely, that was not what I meant actually. Calling to
> prandom_u32_state() is fine, no need to have another prng just
> for that. I was just wondering if it makes sense to model that
> directly as an instruction into a jump-table target that calls
> prandom_u32() from there instead 'indirectly'. Need to think
> about this a bit more ...
I thought about that. In fact, the original patch (written for
FreeBSD) worked by extending the ISA with a new load mode that did "A
= random();". After seeing your eBPF work, I got convinced that we
want to get a generic ISA when possible, where random is typically not
an insn. Also, the bpf_call approach makes it much easier to integrate
with the different JITs.

-Chema