linux-kernel - Re: [PATCH 08/41] arm64: Replace __ASSEMBLY__ with __ASSEMBLER_

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Message-Id: <df30d093-c173-495a-8ed9-874857df7dee@app.fastmail.com>
Date: Fri, 14 Mar 2025 13:05:15 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Will Deacon" <will@...nel.org>, "Thomas Huth" <thuth@...hat.com>
Cc: linux-kernel@...r.kernel.org, Linux-Arch <linux-arch@...r.kernel.org>,
 "Catalin Marinas" <catalin.marinas@....com>
Subject: Re: [PATCH 08/41] arm64: Replace __ASSEMBLY__ with __ASSEMBLER__ in uapi
 headers

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@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Signed-off-by: Thomas Huth <thuth@...hat.com>
>> ---
>>  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__.

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.

I checked that old gcc versions pass __ASSEMBLY__ at least as
far back as gcc-2.95, and it should be completely safe to
assume that no older gcc versions would be used on kernel
headers, and they probably would choke on c99 features like
'long long'. I would also assume that any other compiler that
may be used to include kernel headers has to have enough
gcc compatibility to define all the common macros.

      Arnd