[PATCH] ARM: fix string functions on !MMU

[u.kleine-koenig@pengutronix.de (Uwe Kleine-König)]

Hello Rabin,

On Mon, Jun 02, 2014 at 06:53:43PM +0200, Rabin Vincent wrote:
> On Mon, Apr 28, 2014 at 09:51:49AM +0200, Uwe Kleine-K?nig wrote:
> > On Mon, Apr 21, 2014 at 08:10:08PM +0200, Rabin Vincent wrote:
> > > 8c56cc8be5b38e ("ARM: 7449/1: use generic strnlen_user and
> > > strncpy_from_user functions") apparently broken those string operations
> > > for !MMU.  USER_DS == KERNEL_DS on !MMU, so user_addr_max() always
> > > restricts the addresses to TASK_SIZE.
> > > 
> > > TASK_SIZE has anyway no meaning on !MMU, so make user_addr_max() not
> > > restrict anything.
> > > 
> > > Signed-off-by: Rabin Vincent <rabin@rab.in>
> > I tested this on my efm32 machine and it booted just fine. Before I used
> > a patch that did:
> > 
> > diff --git a/arch/arm/include/asm/memory.h b/arch/arm/include/asm/memory.h
> > index 02fa2558f662..f25c7f4c5a44 100644
> > --- a/arch/arm/include/asm/memory.h
> > +++ b/arch/arm/include/asm/memory.h
> > @@ -92,9 +92,12 @@
> >   * It is difficult to define and perhaps will never meet the original meaning
> >   * of this define that was meant to.
> >   * Fortunately, there is no reference for this in noMMU mode, for now.
> > + *
> > + * HACK: copy_from_user must even handle copying from flash. So don't impose a
> > + * limit at all. Not sure this is correct ...
> >   */
> >  #ifndef TASK_SIZE
> > -#define TASK_SIZE              (CONFIG_DRAM_SIZE)
> > +#define TASK_SIZE              (~0UL)
> >  #endif
> 
> The current code for user_addr_max() for !MMU is essentialy:
> 
> 	#define user_addr_max() TASK_SIZE
> 
> which is obviously wrong for the KERNEL_DS case, since it should be
> ~0UL.  And user space can access all that the kernel does, so there
> should be no restriction for USER_DS either (which is anyway equivalent
> to KERNEL_DS).  Hence, I think my patch, which removes the usage of
> TASK_SIZE in user_addr_max() for !MMU, is correct regardless of what the
> correct definition or meaning of TASK_SIZE for !MMU is.
> 
> If you make TASK_SIZE to ~0UL (which is probably what it should be on
> !MMU), then the result is equivalent to my patch but it is not
> semantically correct since you are restricting user_addr_max() to
> TASK_SIZE even for the KERNEL_DS.
I'd prefer to share as much code as possible between MMU and !MMU, so my
preferred solution is:

	#ifndef CONFIG_MMU
	#define TASK_SIZE	~0UL	/* do we need parentesis? */
	#endif

	#define user_addr_max()	\
		(segment_eq(get_fs(), KERNEL_DS) ? ~0UL : TASK_SIZE)

which should be correct and address your concern.

Best regards
Uwe


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