linux-kernel - Re: [PATCH] allow placing exception table in .rodata (and do so on x86)

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Date:	Thu, 28 Apr 2011 15:44:13 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>, mingo@...e.hu,
	tglx@...utronix.de, linux-arch@...r.kernel.org,
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
	hpa@...or.com
Subject: Re: [PATCH] allow placing exception table in .rodata (and do so on x86)

On Thu, Apr 28, 2011 at 15:31, Jan Beulich <JBeulich@...ell.com> wrote:
>>>> On 28.04.11 at 14:53, Heiko Carstens <heiko.carstens@...ibm.com> wrote:
>> On Thu, Apr 28, 2011 at 01:07:07PM +0100, Jan Beulich wrote:
>>> >>> On 28.04.11 at 13:47, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>>> > On Thu, Apr 28, 2011 at 13:40, Jan Beulich <JBeulich@...ell.com> wrote:
>>> >>>>> On 28.04.11 at 12:43, Heiko Carstens <heiko.carstens@...ibm.com> wrote:
>>> >>> On Wed, Apr 27, 2011 at 04:36:04PM +0100, Jan Beulich wrote:
>>> >>> That's odd. The kernel actually writes to it (sort_main_extable()), so
>>> >>> it shouldn't be in the ro data section, but the data section.
>>> >>
>>> >> This area does get written, but only at boot time, before read-only
>>> >> data gets set to r/o (on x86 at least). With this in mind, it's better
>>> >> to place it in .rodata, as that way run-time protection will be in place
>>> >> (and I think you agree that it was misplaced in .text in any case).
>>> >
>>> > Which means it may be in ROM (which is really read-only) on some embedded
>>> > devices, so it cannot be sorted?
>>>
>>> Perhaps - but since sorting is a requirement, people building such
>>> systems must have found a way... Anyway, I don't see where both
>>
>> Yes, we found a way on s390: we put the exception table in the data section.
>>
>>> your and Heiko's comment are heading, since the situation is even
>>> worse without the patch afaics (since .text gets marked read-only
>>> as much as .rodata does, and could equally be placed in ROM).
>>
>> My point is that your default is wrong. If it makes sense to put the extable
>> into the rodata section then an architecture could do so. However making the
>> default to put data into the rodata section that is actually written to is
>> the wrong approach.
>> It just asks for breakage.
>
> The patch doesn't make this the default - it just makes it possible
> for an architecture to do so.

"asm-generic" is the default for new architectures.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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