Subject: Re: [PATCH] bpf: optimize constant blinding

From: Jiong Wang <[email-masked]>
Date: Wed, 12 Jun 2019 16:25:48 +0100

Jiong Wang writes:

> Alexei Starovoitov writes:
>
>> On Wed, Jun 12, 2019 at 4:32 AM Naveen N. Rao
>> <[email-masked]> wrote:
>>>
>>> Currently, for constant blinding, we re-allocate the bpf program to
>>> account for its new size and adjust all branches to accommodate the
>>> same, for each BPF instruction that needs constant blinding. This is
>>> inefficient and can lead to soft lockup with sufficiently large
>>> programs, such as the new verifier scalability test (ld_dw: xor
>>> semi-random 64 bit imms, test 5 -- with net.core.bpf_jit_harden=2)
>>
>> Slowdown you see is due to patch_insn right?
>> In such case I prefer to fix the scaling issue of patch_insn instead.
>> This specific fix for blinding only is not addressing the core of the problem.
>> Jiong,
>> how is the progress on fixing patch_insn?

And what I have done is I have digested your conversion with Edward, and is
slightly incline to the BB based approach as it also exposes the inserted
insn to later pass in a natural way, then was trying to find a way that
could create BB info in little extra code based on current verifier code,
for example as a side effect of check_subprogs which is doing two insn
traversal already. (I had some such code before in the historical
wip/bpf-loop-detection branch, but feel it might be still too heavy for
just improving insn patching)

>
> I actually was about to reply this email as we have discussed exactly the
> same issue on jit blinding here:
>
>   https://www.spinics.net/lists/bpf/msg01836.html
>
> And sorry for the slow progress on fixing patch_insn, please give me one
> more week, I will try to send out a RFC for it.
>
> Regards,
> Jiong