On Thu, Mar 30, 2017 at 09:17:07AM +0200, Ingo Molnar wrote:
>
> * Peter Zijlstra <pet...@infradead.org> wrote:
>
> > On Wed, Mar 29, 2017 at 11:16:31PM +0200, Arnd Bergmann wrote:
> > > The latest change to the BUG() macro inadvertently reverted the earlier
> > > commit b06dd879f5db ("x86: always define BUG() and HAVE_ARCH_BUG, even
> > > with !CONFIG_BUG") that sanitized the behavior with CONFIG_BUG=n.
> > >
> > > I noticed this as some warnings have appeared again that were previously
> > > fixed as a side effect of that patch:
> > >
> > > kernel/seccomp.c: In function '__seccomp_filter':
> > > kernel/seccomp.c:670:1: error: no return statement in function returning
> > > non-void [-Werror=return-type]
> > >
> > > drivers/gpu/drm/i915/intel_sprite.c: In function
> > > 'intel_check_sprite_plane':
> > > drivers/gpu/drm/i915/intel_sprite.c:936:20: error: 'src_h' may be used
> > > uninitialized in this function [-Werror=maybe-uninitialized]
> > > src->y2 = (src_y + src_h) << 16;
> > > ~~~~~~~^~~~~~~~
> > > drivers/gpu/drm/i915/intel_sprite.c:934:20: error: 'src_w' may be used
> > > uninitialized in this function [-Werror=maybe-uninitialized]
> > > src->x2 = (src_x + src_w) << 16;
> > > ~~~~~~~^~~~~~~~
> > > drivers/gpu/drm/i915/intel_sprite.c:936:20: error: 'src_y' may be used
> > > uninitialized in this function [-Werror=maybe-uninitialized]
> > > src->y2 = (src_y + src_h) << 16;
> > > ~~~~~~~^~~~~~~~
> > > drivers/gpu/drm/i915/intel_sprite.c:934:20: error: 'src_x' may be used
> > > uninitialized in this function [-Werror=maybe-uninitialized]
> > > src->x2 = (src_x + src_w) << 16;
> > > ~~~~~~~^~~~~~~~
> > >
> > > This combines the two patches and uses the ud2 macro to define BUG()
> > > in case of CONFIG_BUG=n.
> >
> > OK, fair enough I suppose. However, I cribbed this from arm64. What does
> > that do for BUG=n ?
>
> I think we'll get a U2D crash in this case, without any bug information.
>
> I.e. only marginally debuggable, but it's a deterministic outcome - instead
> of the
> crazy GCC code generation variant of the day when the warning triggers, or
> the
> similarly crazy infinite loop hang.
>
> I'm not entirely sure though, I don't think many people actually _use_
> CONFIG_BUG=n, it's essentially a crazy thing to do even on constrainted
> hardware.
> Debugging and maintenance costs almost always trump marginal hardware costs
> of a
> bit more debugging code.
So should we then, for x86, disable BUG=n instead?
