Re: [PATCH] bug: Fix no-return-statement warning with !CONFIG_BUG

[Christophe Leroy]

Le 11/04/2024 à 10:12, Christophe Leroy a écrit :
> 
> 
> 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@xxxxxxxxxxxxxx/
>>
> 
> Looking at the report, I think the correct fix should be to use 
> BUILD_BUG() instead of BUG()

I confirm the error goes away with the following change to next-20240411 
on powerpc tinyconfig with gcc 13.2

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 4e18db1819f8..3d5ac0cdd721 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -282,7 +282,7 @@ static inline void timekeeping_check_update(struct 
timekeeper *tk, u64 offset)
  }
  static inline u64 timekeeping_debug_get_ns(const struct tk_read_base *tkr)
  {
-	BUG();
+	BUILD_BUG();
  }
  #endif