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

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Message-Id: <cac3f357-0dc2-46ba-9ea0-7b1f4278e8ff@app.fastmail.com>
Date: Thu, 11 Apr 2024 09:56:02 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Adrian Hunter" <adrian.hunter@...el.com>
Cc: "Peter Zijlstra" <peterz@...radead.org>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>,
 "John Stultz" <jstultz@...gle.com>, "H. Peter Anvin" <hpa@...or.com>,
 "Alexander Gordeev" <agordeev@...ux.ibm.com>,
 "Vincenzo Frascino" <vincenzo.frascino@....com>,
 linux-s390@...r.kernel.org, "Naresh Kamboju" <naresh.kamboju@...aro.org>,
 x86@...nel.org, "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 "Ingo Molnar" <mingo@...hat.com>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 "Christian Borntraeger" <borntraeger@...ux.ibm.com>,
 "Vasily Gorbik" <gor@...ux.ibm.com>,
 "Heiko Carstens" <hca@...ux.ibm.com>,
 "Nicholas Piggin" <npiggin@...il.com>, "Borislav Petkov" <bp@...en8.de>,
 "Andy Lutomirski" <luto@...nel.org>,
 "Bjorn Helgaas" <bhelgaas@...gle.com>,
 "Thomas Gleixner" <tglx@...utronix.de>,
 "Anna-Maria Gleixner" <anna-maria@...utronix.de>,
 "Stephen Boyd" <sboyd@...nel.org>,
 "Randy Dunlap" <rdunlap@...radead.org>, linux-kernel@...r.kernel.org,
 "Sven Schnelle" <svens@...ux.ibm.com>, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] bug: Fix no-return-statement warning with !CONFIG_BUG

On Thu, Apr 11, 2024, at 09:16, Adrian Hunter wrote:
> 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
>

It looks like it's all versions of gcc, though no versions
of clang show the warnings. I did a few more tests and could
not find any differences on actual code generation, but
I'd still feel more comfortable changing the caller than
the BUG() macro. It's trivial to add a 'return 0' there.

Another interesting observation is that clang-11 and earlier
versions end up skipping the endless loop, both with and
without the __builtin_unreachable, see
https://godbolt.org/z/aqa9zqz8x

clang-12 and above do work like gcc, so I guess that is good.

     Arnd