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

Yu, Yu-cheng
2020-09-01T17:49:11Z

On 8/27/2020 7:08 AM, H.J. Lu wrote:
> On Thu, Aug 27, 2020 at 7:07 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>>
>> On Thu, Aug 27, 2020 at 6:36 AM Florian Weimer <fweimer@redhat.com> wrote:
>>>
>>> * H. J. Lu:
>>>
>>>> On Thu, Aug 27, 2020 at 6:19 AM Florian Weimer <fweimer@redhat.com> wrote:
>>>>>
>>>>> * Dave Martin:
>>>>>
>>>>>> You're right that this has implications: for i386, libc probably pulls
>>>>>> more arguments off the stack than are really there in some situations.
>>>>>> This isn't a new problem though. There are already generic prctls with
>>>>>> fewer than 4 args that are used on x86.
>>>>>
>>>>> As originally posted, glibc prctl would have to know that it has to pull
>>>>> an u64 argument off the argument list for ARCH_X86_CET_DISABLE. But
>>>>> then the u64 argument is a problem for arch_prctl as well.
>>>>>
>>>>
>>>> Argument of ARCH_X86_CET_DISABLE is int and passed in register.
>>>
>>> The commit message and the C source say otherwise, I think (not sure
>>> about the C source, not a kernel hacker).
>>
>> It should read:
>>
>> arch_prctl(ARCH_X86_CET_DISABLE, unsigned long features)
>>
>
> Or
>
> arch_prctl(ARCH_X86_CET_DISABLE, unsigned int features)
>

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'.

Yu-cheng