linux-kernel - Re: [PATCH] x86/debug: define BUG() againfor !CONFIG

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Date:   Thu, 30 Mar 2017 09:17:07 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/debug: define BUG() againfor !CONFIG_BUG


* Peter Zijlstra <peterz@...radead.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.

Thanks,

	Ingo