Re: Preliminary results - was: Re: Question on BIGGEST_ALIGNMENT in GCC on NetBSD/m68k

Hi Arnd,

On Tue, 2026-01-06 at 16:37 +0100, Arnd Bergmann wrote:
> On Tue, Jan 6, 2026, at 14:40, John Paul Adrian Glaubitz wrote:
> > On Tue, 2026-01-06 at 14:34 +0100, Kolbj=C3=B8rn Barmen wrote:
> > > On Mon, 16 Jun 2025, John Paul Adrian Glaubitz wrote:
> > >=20
> > > Does this patch exist somewhere? I ask because I am migrating to
> > > 32bit-aligned userland (https://www.gentoo.org/downloads/#m68k) and t=
he
> > > gcc provided fails building kernel, stopping at arch/m68k/kernel/sign=
al.c
> >=20
> > I haven't created that patch yet as my work on this effort is currently=
=20
> > paused. However, it should
> > be straight-forward to update the asserts in the kernel code.
> >=20
> > According to Geert, Arnd Bergmann made some changes to the kernel uapi=
=20
> > that would allow the kernel
> > to be built with either alignment. Not sure what the current state of=
=20
> > things is.
> >=20
> > Let's CC Arnd to find out.
>=20
> I have these patches in my work-in-progress tree, and I still
> plan to submit them upstream, but haven't made any progress on
> it since I talked to Geert about it last month.
>=20
> See below for my current working version against roughly v6.18,
> which replaces all the implicit padding in uapi header files
> with explicit padding.
>=20
> What I do here is a combination of changes:
>=20
>  - add -Werror=3Dpadded to the header checks to catch any
>    implicit padding on any architecture
>  - add __uapi_arch_pad8 for any 8-bit pad that is the
>    same on any architecture
>  - add __uapi_arch_pad16 for any 16-bit pad that is the
>    same on any architecture other than m68k
>  - add __uapi_arch_pad32 for any 32-bit pad for
>    padding between __u32 and __u64 on all architectures
>    that naturally align __u64
>  - add __uapi_arch_pad_long between __u32 and long
>    (or pointer) on 64-bit architectures
>  - a few more specialized macros for less common corner
>    cases
>  - annotate structures as __uapi_arch_align to force 16-bit
>    alignment on m68k and larger alignment elsewhere,
>    regardless of -malign-int
>=20
> I've tested this by running the header check on all
> architectures, and running the abigail tests to ensure
> the ABI is unmodified (abigail is currently broken on a
> few architectures).
>=20
> With all the patches applied, we get two benefits:
>  - m68k kernels have a consistent ABI with and without
>    -malign-int, so kernel and userspace can use
>    different setting here, and we can eventually change
>    the kernel to always use -malign-int
>  - Any new patches that introduce implicit padding in the
>    uapi headers break the build and get fixed before they
>    cause problems. Almost all the instances I've seen
>    are unintentional.
>=20
> I have started splitting up the change into individual
> commits that explain the padding, but it will take more
> work to get through the entire patch, plus any additional
> changes for review comments.
>=20
>=20
> The diffstat is
>=20
>  407 files changed, 2433 insertions(+), 754 deletions(-)
>=20
> and I think this touches around 1500 structures, though
> most files only have a single one.

Wow, that's really impressive! Thanks a lot for working on this!

I will definitely give it a try.

Adrian

--=20
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer
`. `'   Physicist
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