linux-kernel - Re: [PATCH v4 07/14] arm64: Add support for suppressing warning backtraces

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Message-ID: <a64bf821-ea90-4fd9-92ec-13bf7b7a3067@csgroup.eu>
Date: Wed, 19 Mar 2025 09:05:27 +0100
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Will Deacon <will@...nel.org>, Alessandro Carminati <acarmina@...hat.com>
Cc: linux-kselftest@...r.kernel.org, David Airlie <airlied@...il.com>,
 Arnd Bergmann <arnd@...db.de>, Maíra Canal
 <mcanal@...lia.com>, Dan Carpenter <dan.carpenter@...aro.org>,
 Kees Cook <keescook@...omium.org>, Daniel Diaz <daniel.diaz@...aro.org>,
 David Gow <davidgow@...gle.com>, Arthur Grillo <arthurgrillo@...eup.net>,
 Brendan Higgins <brendan.higgins@...ux.dev>,
 Naresh Kamboju <naresh.kamboju@...aro.org>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Maxime Ripard
 <mripard@...nel.org>, Ville Syrjälä
 <ville.syrjala@...ux.intel.com>, Daniel Vetter <daniel@...ll.ch>,
 Thomas Zimmermann <tzimmermann@...e.de>, Guenter Roeck <linux@...ck-us.net>,
 Alessandro Carminati <alessandro.carminati@...il.com>,
 Jani Nikula <jani.nikula@...el.com>, dri-devel@...ts.freedesktop.org,
 kunit-dev@...glegroups.com, linux-arch@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
 linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
 loongarch@...ts.linux.dev, x86@...nel.org,
 Linux Kernel Functional Testing <lkft@...aro.org>,
 Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v4 07/14] arm64: Add support for suppressing warning
 backtraces



Le 18/03/2025 à 16:59, Will Deacon a écrit :
> On Thu, Mar 13, 2025 at 05:40:59PM +0100, Alessandro Carminati wrote:
>> On Thu, Mar 13, 2025 at 1:25 PM Will Deacon <will@...nel.org> wrote:
>>>
>>> On Thu, Mar 13, 2025 at 11:43:22AM +0000, Alessandro Carminati wrote:
>>>> diff --git a/arch/arm64/include/asm/bug.h b/arch/arm64/include/asm/bug.h
>>>> index 28be048db3f6..044c5e24a17d 100644
>>>> --- a/arch/arm64/include/asm/bug.h
>>>> +++ b/arch/arm64/include/asm/bug.h
>>>> @@ -11,8 +11,14 @@
>>>>
>>>>   #include <asm/asm-bug.h>
>>>>
>>>> +#ifdef HAVE_BUG_FUNCTION
>>>> +# define __BUG_FUNC  __func__
>>>> +#else
>>>> +# define __BUG_FUNC  NULL
>>>> +#endif
>>>> +
>>>>   #define __BUG_FLAGS(flags)                           \
>>>> -     asm volatile (__stringify(ASM_BUG_FLAGS(flags)));
>>>> +     asm volatile (__stringify(ASM_BUG_FLAGS(flags, %c0)) : : "i" (__BUG_FUNC));
>>>
>>> Why is 'i' the right asm constraint to use here? It seems a bit odd to
>>> use that for a pointer.
>>
>> I received this code as legacy from a previous version.
>> In my review, I considered the case when HAVE_BUG_FUNCTION is defined:
>> Here, __BUG_FUNC is defined as __func__, which is the name of the
>> current function as a string literal.
>> Using the constraint "i" seems appropriate to me in this case.
>>
>> However, when HAVE_BUG_FUNCTION is not defined:
>> __BUG_FUNC is defined as NULL. Initially, I considered it literal 0,
>> but after investigating your concern, I found:
>>
>> ```
>> $ echo -E "#include <stdio.h>\n#include <stddef.h>\nint main()
>> {\nreturn 0;\n}" | aarch64-linux-gnu-gcc -E -dM - | grep NULL
>> #define NULL ((void *)0)
>> ```
>>
>> I realized that NULL is actually a pointer that is not a link time
>> symbol, and using the "i" constraint with NULL may result in undefined
>> behavior.
>>
>> Would the following alternative definition for __BUG_FUNC be more convincing?
>>
>> ```
>> #ifdef HAVE_BUG_FUNCTION
>>      #define __BUG_FUNC __func__
>> #else
>>      #define __BUG_FUNC (uintptr_t)0
>> #endif
>> ```
>> Let me know your thoughts.
> 
> Thanks for the analysis; I hadn't noticed this specific issue, it just
> smelled a bit fishy. Anyway, the diff above looks better, thanks.

That propably deserves a comment.

Doesn't sparse and/or checkpatch complain about 0 being used in lieu of 
NULL ?

By the way I had similar problem in the past with GCC not seeing NULL as 
a __builtin_constant_p(), refer commit 1d8f739b07bd ("powerpc/kuap: Fix 
set direction in allow/prevent_user_access()")

Christophe