linux-kernel - Re: TASK

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Date:	Tue, 3 Jun 2014 16:11:38 +0200
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Greg Ungerer <gerg@...inux.org>
Cc:	Rabin Vincent <rabin@....in>, Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-xtensa@...ux-xtensa.org, linux-m32r@...linux-m32r.org,
	linux-c6x-dev@...ux-c6x.org, microblaze-uclinux@...e.uq.edu.au,
	linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
	linux-mm@...ck.org, linux-m68k@...ts.linux-m68k.org,
	kernel@...gutronix.de, uclinux-dist-devel@...ckfin.uclinux.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	panchaxari <panchaxari.prasannamurthy@...aro.org>,
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: TASK_SIZE for !MMU

Hello Greg,

thanks for your reply.

On Tue, Jun 03, 2014 at 10:14:55PM +1000, Greg Ungerer wrote:
> >>I think it would be OK to define TASK_SIZE to 0xffffffff for !MMU.
> >>blackfin, frv and m68k also do this. c6x does define it to 0xFFFFF000 to
> >>leave space for error codes.
> 
> I did that same change for m68k in commit cc24c40 ("m68knommu: remove
> size limit on non-MMU TASK_SIZE"). For similar reasons as you need to
> now.
ok.
 
> >>Thoughts?
> >The problem is that current linus/master (and also next) doesn't boot on
> >my ARM-nommu machine because the user string functions (strnlen_user,
> >strncpy_from_user et al.) refuse to work on strings above TASK_SIZE
> >which in my case also includes the XIP kernel image.
> 
> I seem to recall that we were not considering flash or anything else
> other than RAM when defining that original TASK_SIZE (back many, many
> years ago). Some of the address checks you list above made some sense
> if you had everything in RAM (though only upper bounds are checked).
> The thinking was some checking is better than none I suppose.
What is the actual meaning of TASK_SIZE? The maximal value of a valid
userspace address?

> Setting a hard coded memory size in CONFIG_DRAM_SIZE is not all that
> fantastic either...
Not sure what you mean? Having CONFIG_DRAM_SIZE at all or use it for
boundary checking?

CONFIG_DRAM_SIZE is hardly used apart from defining TASK_SIZE:

 - #define END_MEM (UL(CONFIG_DRAM_BASE) + CONFIG_DRAM_SIZE)
   which is only used to define MODULES_END. Ap
 - Some memory configuration using cp15 registers in
   arch/arm/mm/proc-arm{740,940,946}.S

For the former I'd say better use 0xffffffff, too. For the latter I
wonder if we should just drop CPU_ARM740T, CPU_ARM940T and CPU_ARM946E.
These are only selectable if ARCH_INTEGRATOR and are not selected by
other symbols. As ARCH_INTEGRATOR selects ARM_PATCH_PHYS_VIRT since
commit fe9891454473 (ARM: integrator: Default enable
ARM_PATCH_PHYS_VIRT, AUTO_ZRELADDR) for Linux 3.13 and
ARM_PATCH_PHYS_VIRT depends on MMU the Integrator-noMMU targets are
broken anyhow.

I will prepare a patch series with some cleanups.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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