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

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Message-ID: <4B2A877C.3000504@caviumnetworks.com>
Date:	Thu, 17 Dec 2009 11:33:16 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Joe Buck <Joe.Buck@...opsys.COM>
CC:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Richard Guenther <richard.guenther@...il.com>,
	"gcc@....gnu.org" <gcc@....gnu.org>,
	Jamie Lokier <jamie@...reable.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH] ARM: Convert BUG() to use unreachable()

Joe Buck wrote:
> On Thu, Dec 17, 2009 at 11:06:13AM -0800, Russell King - ARM Linux wrote:
>> On Thu, Dec 17, 2009 at 10:35:17AM -0800, Joe Buck wrote:
>>> Besides, didn't I see a whole bunch of kernel security patches related
>>> to null pointer dereferences lately?  If page 0 can be mapped, you
>>> suddenly won't get your trap.
>> Page 0 can not be mapped on ARM kernels since the late 1990s, and this
>> protection is independent of the generic kernel.
>>
>> Milage may vary on other architectures, but that's not a concern here.
> 
> I don't understand, though, why you would want to implement a generally
> useful facility (make the kernel trap so you can do a post-mortem
> analysis) in a way that's only safe for the ARM port.
> 

Each Linux kernel architecture has in its architecture specific bug.h an 
implementation that is deemed by the architecture maintainers to work. 
As far as I know, few if any of these use __builtin_trap().

Some could be converted to __builtin_trap(), others cannot (x86 for 
example).  If we enhanced __builtin_trap() to take an argument for the 
trap code, MIPS could be converted.  But as it stands now 
__builtin_trap() is not very useful.

As more architectures start adding funky tables that get generated by 
the inline asm (as in x86), __builtin_trap() becomes less useful.

David Daney
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