linux-kernel - Re: [PATCH] bug: Fix no-return-statement warning with !CONFIG

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Date: Thu, 11 Apr 2024 08:13:00 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Adrian Hunter <adrian.hunter@...el.com>, Arnd Bergmann <arnd@...db.de>
CC: Naresh Kamboju <naresh.kamboju@...aro.org>, Michael Ellerman
	<mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>, Aneesh Kumar K.V
	<aneesh.kumar@...nel.org>, "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
	Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>, Christian Borntraeger
	<borntraeger@...ux.ibm.com>, Sven Schnelle <svens@...ux.ibm.com>, Ingo Molnar
	<mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
	<dave.hansen@...ux.intel.com>, "x86@...nel.org" <x86@...nel.org>, "H. Peter
 Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>, Vincenzo Frascino
	<vincenzo.frascino@....com>, John Stultz <jstultz@...gle.com>, Stephen Boyd
	<sboyd@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Randy Dunlap
	<rdunlap@...radead.org>, Bjorn Helgaas <bhelgaas@...gle.com>, Anna-Maria
 Gleixner <anna-maria@...utronix.de>, "linuxppc-dev@...ts.ozlabs.org"
	<linuxppc-dev@...ts.ozlabs.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-s390@...r.kernel.org"
	<linux-s390@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] bug: Fix no-return-statement warning with !CONFIG_BUG



Le 11/04/2024 à 09:16, Adrian Hunter a écrit :
> On 11/04/24 10:04, Arnd Bergmann wrote:
>> On Wed, Apr 10, 2024, at 17:32, Adrian Hunter wrote:
>>> BUG() does not return, and arch implementations of BUG() use unreachable()
>>> or other non-returning code. However with !CONFIG_BUG, the default
>>> implementation is often used instead, and that does not do that. x86 always
>>> uses its own implementation, but powerpc with !CONFIG_BUG gives a build
>>> error:
>>>
>>>    kernel/time/timekeeping.c: In function ‘timekeeping_debug_get_ns’:
>>>    kernel/time/timekeeping.c:286:1: error: no return statement in function
>>>    returning non-void [-Werror=return-type]
>>>
>>> Add unreachable() to default !CONFIG_BUG BUG() implementation.
>>
>> I'm a bit worried about this patch, since we have had problems
>> with unreachable() inside of BUG() in the past, and as far as I
>> can remember, the current version was the only one that
>> actually did the right thing on all compilers.
>>
>> One problem with an unreachable() annotation here is that if
>> a compiler misanalyses the endless loop, it can decide to
>> throw out the entire code path leading up to it and just
>> run into undefined behavior instead of printing a BUG()
>> message.
>>
>> Do you know which compiler version show the warning above?
> 
> Original report has a list
> 
> 	https://lore.kernel.org/all/CA+G9fYvjdZCW=7ZGxS6A_3bysjQ56YF7S-+PNLQ_8a4DKh1Bhg@mail.gmail.com/
> 

Looking at the report, I think the correct fix should be to use 
BUILD_BUG() instead of BUG()