linux-kernel - Re: [PATCH] ARM: Convert BUG() to use unreachable()

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Date:	Thu, 17 Dec 2009 18:17:18 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Richard Guenther <richard.guenther@...il.com>
Cc:	David Daney <ddaney@...iumnetworks.com>,
	"gcc@....gnu.org" <gcc@....gnu.org>,
	Jamie Lokier <jamie@...reable.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH] ARM: Convert BUG() to use unreachable()

On Thu, Dec 17, 2009 at 06:17:11PM +0100, Richard Guenther wrote:
> On Thu, Dec 17, 2009 at 6:09 PM, David Daney <ddaney@...iumnetworks.com> wrote:
> > Jamie Lokier wrote:
> >>
> >> Uwe Kleine-König wrote:
> >>>
> >>> Use the new unreachable() macro instead of for(;;);
> >>>        *(int *)0 = 0;
> >>>          /* Avoid "noreturn function does return" */
> >>> -       for (;;);
> >>> +       unreachable();
> >>
> >> Will GCC-4.5 remove ("optimise away") the *(int *)0 = 0 because it
> >> knows the branch of the code leading to unreachable can never be reached?
> >>
> >
> > I don't know the definitive answer, so I am sending to gcc@...
> >
> > FYI: #define unreachable() __builtin_unreachable()
> 
> It shouldn't as *(int *)0 = 0; might trap.  But if you want to be sure
> use
>    __builtin_trap ();
> instead for the whole sequence (the unreachable is implied then).
> GCC choses a size-optimal trap representation for your target then.

How is "size-optimal trap" defined?  The point of "*(int *)0 = 0;" is
to cause a NULL pointer dereference which is trapped by the kernel to
produce a full post mortem and backtrace which is easily recognised
as a result of this code.

Having gcc decide on, maybe, an undefined instruction instead would be
confusing.

Let me put it another way: I want this function to terminate with an
explicit NULL pointer dereference in every case.
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