Re: [PATCH 11/11] Use unreachable() in asm-generic/bug.h for !CONFIG_BUG case.

[David Daney <ddaney@caviumnetworks.com>]

Brian Gerst wrote:
> On Mon, Sep 14, 2009 at 5:55 PM, David Daney <ddaney@caviumnetworks.com> wrote:
>> The subject says it all (most).  The only drawback here is that for a
>> pre-GCC-5.4 compiler, instead of expanding to nothing we now expand
>> BUG() to an endless loop.  Before the patch when configured with
>> !CONFIG_BUG() you might get some warnings, but the code would be
>> small.  After the patch there are no warnings, but there is an endless
>> loop at each BUG() site.
>>
>> Of course for the GCC-4.5 case we get the best of both worlds.
>>
>> Signed-off-by: David Daney <ddaney@caviumnetworks.com>
>> Suggested-by: Ingo Molnar <mingo@elte.hu>
>> CC: Ingo Molnar <mingo@elte.hu>
>> ---
>>  include/asm-generic/bug.h |    4 ++--
>>  1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
>> index 4b67559..e952242 100644
>> --- a/include/asm-generic/bug.h
>> +++ b/include/asm-generic/bug.h
>> @@ -89,11 +89,11 @@ extern void warn_slowpath_null(const char *file, const int line);
>>
>>  #else /* !CONFIG_BUG */
>>  #ifndef HAVE_ARCH_BUG
>> -#define BUG() do {} while(0)
>> +#define BUG() unreachable()
>>  #endif
>>
>>  #ifndef HAVE_ARCH_BUG_ON
>> -#define BUG_ON(condition) do { if (condition) ; } while(0)
>> +#define BUG_ON(condition) do { if (condition) unreachable(); } while (0)
>>  #endif
>>
>>  #ifndef HAVE_ARCH_WARN_ON
>> --
> 
> This seems wrong to me.  Wouldn't you always want to do the endless
> loop?  In the absence of an arch-specific method to jump to an
> exception handler, it isn't really unreachable.  On gcc 4.5 this would
> essentially become a no-op.
> 

Several points:

* When you hit a BUG() you are screwed.

* When you configure with !CONFIG_BUG you are asserting that you don't 
want to try to trap on BUG();.

The existing code just falls through to whatever happens to follow the 
BUG().  This is not what the programmer intended, but the person that 
chose !CONFIG_BUG decided that they would like undefined behavior in 
order to save a few bytes of code.

With the patch one of two things will happen:

pre-GCC-4.5) We will now enter an endless loop and not fall through. 
This makes the code slightly larger than pre patch.

post-GCC-4.5) We do something totally undefined.  It will not 
necessarily fall through to the code after the BUG()  It could really 
end up doing almost anything.  On the plus side, we save a couple of 
bytes of code and eliminate some compiler warnings.

If you don't like it, don't configure with !CONFIG_BUG.  But the patch 
doesn't really change the fact that hitting a BUG() with !CONFIG_BUG 
leads to undefined behavior.  It only makes the case where you don't hit 
BUG() nicer.

David Daney