[PATCH] ARM: fix string functions on !MMU

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

Hello,

On Tue, Jun 03, 2014 at 09:51:33AM +0200, Uwe Kleine-K?nig wrote:
> 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)

After looking into that a bit more I wonder if the correct version is
(maybe an equivalent to):

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

That is because in the MMU case get_fs() is #defined as:

	#define get_fs()        (current_thread_info()->addr_limit)

and .addr_limit is changeable via set_fs. This would also mean that the
current definition:

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

might return ~0UL even though there is a limit which just happens not to
be TASK_SIZE. (BTW, alpha, m68k, openrisc and sparc use the same
definition.)

Thoughts?

Best regards
Uwe

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