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([2600:1700:e321:62f0:da43:aeff:fecc:bfd5]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-225c68aa906sm97145285ad.88.2025.03.18.09.39.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 18 Mar 2025 09:39:38 -0700 (PDT) Sender: Guenter Roeck Message-ID: <17a383b4-2add-4e74-b7ca-d7ef2baac4f9@roeck-us.net> Date: Tue, 18 Mar 2025 09:39:35 -0700 Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 07/14] arm64: Add support for suppressing warning backtraces To: Will Deacon , Alessandro Carminati Cc: linux-kselftest@vger.kernel.org, David Airlie , Arnd Bergmann , =?UTF-8?Q?Ma=C3=ADra_Canal?= , Dan Carpenter , Kees Cook , Daniel Diaz , David Gow , Arthur Grillo , Brendan Higgins , Naresh Kamboju , Maarten Lankhorst , Andrew Morton , Maxime Ripard , =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= , Daniel Vetter , Thomas Zimmermann , Alessandro Carminati , Jani Nikula , dri-devel@lists.freedesktop.org, kunit-dev@googlegroups.com, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, loongarch@lists.linux.dev, x86@kernel.org, Linux Kernel Functional Testing , Catalin Marinas References: <20250313114329.284104-1-acarmina@redhat.com> <20250313114329.284104-8-acarmina@redhat.com> <20250313122503.GA7438@willie-the-truck> <20250318155946.GC13829@willie-the-truck> Content-Language: en-US From: Guenter Roeck Autocrypt: addr=linux@roeck-us.net; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 3/18/25 08:59, Will Deacon wrote: > On Thu, Mar 13, 2025 at 05:40:59PM +0100, Alessandro Carminati wrote: >> On Thu, Mar 13, 2025 at 1:25 PM Will Deacon 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 >>>> >>>> +#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 \n#include \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. > It has been a long time, but I seem to recall that I ran into trouble when trying to use a different constraint. Guenter