Re: Kernel oops caused by signed divide

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

On 9/9/24 10:29 AM, Alexei Starovoitov wrote:
> On Mon, Sep 9, 2024 at 10:21 AM Zac Ecob <zacecob@protonmail.com> wrote:
>> Hello,
>>
>> I recently received a kernel 'oops' about a divide error.
>> After some research, it seems that the 'div64_s64' function used for the 'MOD'/'REM' instructions boils down to an 'idiv'.
>>
>> The 'dividend' is set to INT64_MIN, and the 'divisor' to -1, then because of two's complement, there is no corresponding positive value, causing the error (at least to my understanding).
>>
>>
>> Apologies if this is already known / not a relevant concern.
> Thanks for the report. This is a new issue.
>
> Yonghong,
>
> it's related to the new signed div insn.
> It sounds like we need to update chk_and_div[] part of
> the verifier to account for signed div differently.

In verifier, we have
   /* [R,W]x div 0 -> 0 */
   /* [R,W]x mod 0 -> [R,W]x */

What the value for
   Rx_a sdiv Rx_b -> ?
where Rx_a = INT64_MIN and Rx_b = -1?

Should we just do
   INT64_MIN sdiv -1 -> -1
or some other values?