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

[David Daney]

Brian Gerst wrote:
> On Mon, Sep 14, 2009 at 5:55 PM, David Daney <ddaney@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> > Suggested-by: Ingo Molnar <mingo@xxxxxxx>
> > CC: Ingo Molnar <mingo@xxxxxxx>
> > ---
> >  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
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