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

From: David Daney <ddaney at caviumnetworks dot com>
To: Joe Buck <Joe dot Buck at synopsys dot COM>
Cc: Russell King - ARM Linux <linux at arm dot linux dot org dot uk>, Richard Guenther <richard dot guenther at gmail dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, Jamie Lokier <jamie at shareable dot org>, "linux-kernel at vger dot kernel dot org" <linux-kernel at vger dot kernel dot org>, "linux-arm-kernel at lists dot infradead dot org" <linux-arm-kernel at lists dot infradead dot org>, Uwe Kleine-König <u dot kleine-koenig at pengutronix dot de>
Date: Thu, 17 Dec 2009 11:33:16 -0800
Subject: Re: [PATCH] ARM: Convert BUG() to use unreachable()
References: <1260266138-17684-1-git-send-email-u.kleine-koenig@pengutronix.de> <20091217150120.GD24967@shareable.org> <4B2A65C6.7080009@caviumnetworks.com> <84fc9c000912170917y4b2772d0n47bb4dabc74b91ef@mail.gmail.com> <20091217181718.GA4440@n2100.arm.linux.org.uk> <20091217183517.GC10707@synopsys.com> <20091217190613.GB4440@n2100.arm.linux.org.uk> <20091217191401.GD10707@synopsys.com>

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

References:
- Re: [PATCH] ARM: Convert BUG() to use unreachable()
  - From: David Daney
- Re: [PATCH] ARM: Convert BUG() to use unreachable()
  - From: Richard Guenther
- Re: [PATCH] ARM: Convert BUG() to use unreachable()
  - From: Russell King - ARM Linux
- Re: [PATCH] ARM: Convert BUG() to use unreachable()
  - From: Joe Buck
- Re: [PATCH] ARM: Convert BUG() to use unreachable()
  - From: Russell King - ARM Linux
- Re: [PATCH] ARM: Convert BUG() to use unreachable()
  - From: Joe Buck

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