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

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Date:	Thu, 28 Apr 2011 12:40:58 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Heiko Carstens" <heiko.carstens@...ibm.com>
Cc:	<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 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:
>> This is since the table is really a set of pointers, i.e. misplaced in
>> .text.
>> 
>> Quite likely other architectures would want to follow.
>> 
>> Signed-off-by: Jan Beulich <jbeulich@...ell.com>
>> 
> 
> [...]
> 
>> --- 2.6.39-rc5/include/asm-generic/vmlinux.lds.h
>> +++ 2.6.39-rc5-extable-in-rodata/include/asm-generic/vmlinux.lds.h
>> @@ -226,6 +226,7 @@
>>  		*(.rodata1)						\
>>  	}								\
>>  									\
>> +	EXCEPTION_TABLE_RO						\
> 
> 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).

Jan

>>  	JUMP_TABLE							\
> 
> same here.



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