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

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Message-Id: <4DB98854020000780003EA4D@vpn.id2.novell.com>
Date:	Thu, 28 Apr 2011 14:31:32 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Heiko Carstens" <heiko.carstens@...ibm.com>
Cc:	<mingo@...e.hu>, <tglx@...utronix.de>,
	"Geert Uytterhoeven" <geert@...ux-m68k.org>,
	<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 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.

Jan

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