Re: [PATCH v2] x32: Implement prctl in assembly

On 08/12/25 06:09, Florian Weimer wrote:
> * H. J. Lu:
> 
>> On Mon, Dec 8, 2025 at 4:11 PM Florian Weimer <[email protected]> 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 <[email protected]>
>> 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.