Re: [PATCH bpf-next v2 1/2] bpf: Track aligned st store as imprecise spilled registers

[Yonghong Song <yonghong.song@linux.dev>]

On 1/8/24 11:06 AM, Eduard Zingerman wrote:
> On Mon, 2024-01-08 at 10:59 -0800, Yonghong Song wrote:
> [...]
>
>> I guess one way could be doing backtracking with "... = arr[i]"
>> is to have four ranges, [-32, -24), [-24, -16), [-16, -8), [-8, 0).
>> Later, when we see arr[i] = r0 and i has range [-32, 0). Since it covers [-32, -24), etc.,
>> precision marking can proceed with 'r0'. But I guess this can potentially
>> increase verifier backtracking states a lot and is not scalable. Conservatively
>> doing precision marking with 'r0' (in arr[i] = r0) is a better idea.
> In theory it should be possible to collapse this range to min/max pair.
> But it is a complication, and I'd say it shouldn't be implemented
> unless we have evidence that it significantly improves verification
> performance.

Ack. We do not need to introduce this yet as the variable index range
should be much much less common.

>
>> Andrii has similar comments in
>>     https://lore.kernel.org/bpf/CAEf4Bzb0LdSPnFZ-kPRftofA6LsaOkxXLN4_fr9BLR3iG-te-g@mail.gmail.com/
>>