[PATCH v3] x32: Implement prctl in assembly

[H.J. Lu]

On Mon, Dec 8, 2025 at 10:25 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Adhemerval Zanella Netto:
>
> > On 08/12/25 06:09, Florian Weimer wrote:
> >> * H. J. Lu:
> >>
> >>> On Mon, Dec 8, 2025 at 4:11 PM Florian Weimer <fweimer@redhat.com> wrote:
> >>>>
> >>>> * H. J. Lu:
> >>>>> Here is the v2 patch to implement prctl in assembly for x32.
> >>>>>
> >>>>> Since the variadic prctl function takes at most 5 integer arguments which
> >>>>> are passed in the same integer registers on x32 as the function with 5
> >>>>> integer arguments, we can use assembly for prctl.  Since upper 32-bits in
> >>>>> the last 4 arguments of prctl must be cleared to match the x32 prctl
> >>>>> syscall interface where the last 4 arguments are unsigned 64 bit longs,
> >>>>> implement prctl in assembly to clear upper 32-bits in the last 4 arguments
> >>>>> and add a test to verify it.
> >>>>
> >>>> What's the advantage of the assembler implementation over the C
> >>>> implementation?  I'm missing the context for this change.
> >>>>
> >>>
> >>> It is inspired by
> >>>
> >>> commit 6a04404521ac4119ae36827eeb288ea84eee7cf6
> >>> Author: Florian Weimer <fweimer@redhat.com>
> >>> Date:   Sat Feb 17 09:17:04 2024 +0100
> >>>
> >>>     Linux: Switch back to assembly syscall wrapper for prctl (bug 29770)
> >>
> >> The justification for that does not apply to x32, though, because prctl
> >> doesn't take floating point arguments.  I don't have a strong opinion,
> >> the C and assembler versions are of similar complexity.
> >
> > The main justification is UB to va_args *all* the arguments without taking
> > in the consideration which option is passed.  If x32 requires additional
> > argument handling to clear the upper 32-bits, there is no advantage of
> > using the assembly wrapper.
>
> I'm okay with making this change to avoid UB.
>
> Patch looks okay to me.
>
> Reviewed-by: Florian Weimer <fweimer@redhat.com>
>
> Minor nit:
>
> +weak_alias (__prctl, __prctl_time64)
> +hidden_weak (__prctl_time64)
>
> This isn't necessary because there is no __prctl_time64 on x32.

Fixed in the v3 patch I am checking in.

Thanks.

-- 
H.J.
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