Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

Yu, Yu-cheng
2020-09-01T18:24:25Z

On 9/1/2020 11:17 AM, Florian Weimer wrote:
> * Yu-cheng Yu:
>
>> On 9/1/2020 10:50 AM, Florian Weimer wrote:
>>> * Yu-cheng Yu:
>>>
>>>> Like other arch_prctl()'s, this parameter was 'unsigned long'
>>>> earlier. The idea was, since this arch_prctl is only implemented for
>>>> the 64-bit kernel, we wanted it to look as 64-bit only. I will change
>>>> it back to 'unsigned long'.
>>> What about x32? In general, long is rather problematic for x32.
>>
>> The problem is the size of 'long', right?
>> Because this parameter is passed in a register, and only the lower
>> bits are used, x32 works as well.
>
> The userspace calling convention leaves the upper 32-bit undefined.
> Therefore, this only works by accident if the kernel does not check that
> the upper 32-bit are zero, which is probably a kernel bug.
>
> It's unclear to me what you are trying to accomplish. Why do you want
> to use unsigned long here? The correct type appears to be unsigned int.
> This correctly expresses that the upper 32 bits of the register do not
> matter.

Yes, you are right. I will make it 'unsigned int'.

Thanks,
Yu-cheng