Re: [PATCH 08/41] arm64: Replace __ASSEMBLY__ with __ASSEMBLER__ in uapi headers

[Will Deacon]

On Fri, Mar 14, 2025 at 01:05:15PM +0100, Arnd Bergmann wrote:
> On Fri, Mar 14, 2025, at 12:55, Will Deacon wrote:
> > On Fri, Mar 14, 2025 at 08:09:39AM +0100, Thomas Huth wrote:
> >> __ASSEMBLY__ is only defined by the Makefile of the kernel, so
> >> this is not really useful for uapi headers (unless the userspace
> >> Makefile defines it, too). Let's switch to __ASSEMBLER__ which
> >> gets set automatically by the compiler when compiling assembly
> >> code.
> >> 
> >> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> >> Cc: Will Deacon <will@xxxxxxxxxx>
> >> Signed-off-by: Thomas Huth <thuth@xxxxxxxxxx>
> >> ---
> >>  arch/arm64/include/uapi/asm/kvm.h        | 2 +-
> >>  arch/arm64/include/uapi/asm/ptrace.h     | 4 ++--
> >>  arch/arm64/include/uapi/asm/sigcontext.h | 4 ++--
> >>  3 files changed, 5 insertions(+), 5 deletions(-)
> >
> > Is there a risk of breaking userspace with this? I wonder if it would
> > be more conservative to do something like:
> >
> > #if !defined(__ASSEMBLY__) && !defined(__ASSEMBLER__)
> >
> > so that if somebody is doing '#define __ASSEMBLY__' then they get the
> > same behaviour as today.
> >
> > Or maybe we don't care?
> 
> I think the main risk we would have is user applications relying
> on the __ASSEMBLER__ checks in new kernel headers and not defining
> __ASSEMBLY__. This would result in the application not building
> against old kernel headers that only check against __ASSEMBLY__.

Hmm. I hadn't thought about the case of old headers :/

A quick Debian codesearch shows that glibc might #define __ASSEMBLY__
for some arch-specific headers:

https://codesearch.debian.net/search?q=%23define+__ASSEMBLY__&literal=1

which is what I was more worried about.

> Checking for both in the kernel headers does not solve this
> problem, and I think we can still decide that we don't care:
> in the worst case, an application using the headers from assembly
> will have to get fixed later when it needs to be built against
> old headers.

Old headers might also just be missing definitions that the application
wants, so I suppose there's always the potential for some manual effort
in that case.

Will