linux-kernel - Re: [patch V2 1/6] ARM: uaccess: Implement missing __get_user_asm

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Message-ID: <87ms6s990q.ffs@tglx>
Date: Wed, 17 Sep 2025 21:25:09 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: "Russell King (Oracle)" <linux@...linux.org.uk>, Nathan Chancellor
 <nathan@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linus Torvalds
 <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>,
 kernel test robot <lkp@...el.com>, linux-arm-kernel@...ts.infradead.org,
 Christophe Leroy <christophe.leroy@...roup.eu>, Darren Hart
 <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>, André Almeida
 <andrealmeid@...lia.com>, x86@...nel.org, Alexander Viro
 <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan
 Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org
Subject: Re: [patch V2 1/6] ARM: uaccess: Implement missing
 __get_user_asm_dword()

On Wed, Sep 17 2025 at 18:34, Russell King wrote:

> On Wed, Sep 17, 2025 at 10:14:24AM -0700, Nathan Chancellor wrote:
>> On Wed, Sep 17, 2025 at 04:17:38PM +0100, Russell King (Oracle) wrote:
>> > For me, this produces:
>> > 
>> > get-user-test.c:41:16: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
>> >    41 |         (x) = *(__force __typeof__(*(ptr)) *) &__gu_val;                \
>> >       |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> > 
>> > with arm-linux-gnueabihf-gcc (Debian 14.2.0-19) 14.2.0
>> > 
>> > Maybe you're using a different compiler that doesn't issue that warning?
>> 
>> Maybe because the kernel uses -fno-strict-aliasing, which presumably
>> turns off -Wstrict-aliasing?
>
> Thanks, I'd forgotten to pick the -f flags for building the out of tree
> test. Yes, that does work, but I wonder whether the powerpc 32-bit
> approach with __long_type() that Christophe mentioned would be better.
> That also appears to avoid all issues, and doesn't need the use of
> the nasty __force, address-of and deref trick.

Hmm. I just noticed that include/asm-generic/uaccess.h does exactly the
same what I did with the per size case variables and that seems to be
not subject to endless bot complaints either.

But sure, the PPC trick is neat too. No strong preference, just that I'm
leaning towards the per case split up as it's less obfuscated IMO.

Thanks,

        tglx