linux-kernel - Re: [PATCH v2] ARM: mm: mark section-aligned portion of rodata NX

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Date:	Fri, 12 Feb 2016 10:45:55 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Kees Cook <keescook@...omium.org>,
	Vladimir Murzin <vladimir.murzin@....com>,
	Victor Kamensky <victor.kamensky@...aro.org>,
	Nicolas Pitre <nico@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Laura Abbott <labbott@...oraproject.org>
Subject: Re: [PATCH v2] ARM: mm: mark section-aligned portion of rodata NX

On Thursday 11 February 2016 18:00:32 Russell King - ARM Linux wrote:
> On Fri, Jan 22, 2016 at 01:19:03PM -0800, Kees Cook wrote:
> > On Tue, Dec 8, 2015 at 10:38 AM, Kees Cook <keescook@...omium.org> wrote:
> > > On Mon, Dec 7, 2015 at 11:47 PM, Ard Biesheuvel
> > > <ard.biesheuvel@...aro.org> wrote:
> > >> On 7 December 2015 at 23:35, Kees Cook <keescook@...omium.org> wrote:
> > >>>  /*
> > >>> + * Without CONFIG_DEBUG_ALIGN_RODATA, __start_rodata_section_aligned will
> > >>> + * be the first section-aligned location after __start_rodata. Otherwise,
> > >>> + * it will be equal to __start_rodata.
> > >>> + */
> > >>> +__start_rodata_section_aligned = ALIGN(__start_rodata, 1 << SECTION_SHIFT);
> ...
> 
> I'm afraid this causes build errors on two configurations:
> 
> ./arch/arm/kernel/vmlinux.lds:701: undefined symbol `SECTION_SHIFT' referenced in expression
> 
> which I think is down to the new __start_rodata_section_aligned line.
> 
> Olof's builder errored out with this for both allnoconfig and
> lpc18xx_defconfig builds.  Both probably due to CONFIG_MMU being
> unset, and hence no definition for SECTION_SHIFT.
> 
> I think you need to make this conditional on CONFIG_MMU.

I've hit another problem with this patch, when using XIP_KERNEL, I now see

arch/arm/mm/built-in.o:(.data+0x4bc): undefined reference to `__start_rodata_section_aligned'

in linux-next, apparently because of the combination with "ARM: 8513/1: xip: Move
XIP linking to a separate file".

	Arnd