Re: TASK_SIZE for !MMU

[Greg Ungerer]

Hi Uwe,

On 04/06/14 00:11, Uwe Kleine-König wrote:
> 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?

Yes (as Geert pointed out :-)
The limit of virtual userspace addresses.


> > 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?

Having the DRAM size be a configure time constant. And as you have
found RAM isn't the only place in the physical address space that
code will necessarily access.


> 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.

I have no idea how many people would be using those older ARM CPU types.
It was hard to get much interest for them in mainline even years ago.

Regards
Greg



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