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

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Message-ID: <9030a298-fcb6-48ea-a941-ad6916bf9fa7@roeck-us.net>
Date: Wed, 19 Mar 2025 06:11:28 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Christophe Leroy <christophe.leroy@...roup.eu>,
 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>,
 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

On 3/19/25 01:05, Christophe Leroy wrote:
> 
> 
> 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 ?
> 

__BUG_FUNC is only used as parameter to asm code, not as pointer.

 From the diff:

-                    : : "i" (__FILE__), "i" (__LINE__),                \
+                    : : "i" (__FILE__), "i" (__BUG_FUNC), "i" (__LINE__),\

The use is quite similar to __FILE__ and __LINE__. It might even be possible
and appropriate to just define __BUG_FUNC as 0 if HAVE_BUG_FUNCTION is not defined.

Guenter