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

H.J. Lu
2020-09-01T18:19:55Z

On Tue, Sep 1, 2020 at 11:17 AM Florian Weimer <fweimer@redhat.com> 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.
>

unsigned int is the correct type since only the lower 32 bits are used.

--
H.J.