[PATCH v2 04/10] ARM: lib: add switch_stack function for safely changing stack

[dave.martin@linaro.org (Dave Martin)]

On Thu, Jun 09, 2011 at 05:11:13PM +0100, Dave Martin wrote:
> On Thu, Jun 09, 2011 at 04:58:56PM +0100, Will Deacon wrote:
> > When disabling the MMU, it is necessary to take out a 1:1 identity map
> > of the reset code so that it can safely be executed with and without
> > the MMU active. To avoid the situation where the physical address of the
> > reset code aliases with the virtual address of the active stack (which
> > cannot be included in the 1:1 mapping), it is desirable to change to a
> > new stack at a location which is less likely to alias.
> > 
> > This code adds a new lib function, switch_stack:
> > 
> > void switch_stack(void (*fn)(void *), void *arg, void *sp);
> > 
> > which changes the stack to point at the sp parameter, before invoking
> > fn(arg) with the new stack selected.
> > 
> > Signed-off-by: Dave Martin <dave.martin@linaro.org>
> > Signed-off-by: Will Deacon <will.deacon@arm.com>
> > ---
> >  arch/arm/lib/Makefile       |    3 +-
> >  arch/arm/lib/switch_stack.S |   44 +++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 46 insertions(+), 1 deletions(-)
> >  create mode 100644 arch/arm/lib/switch_stack.S
> > 
> > diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
> > index 59ff42d..5fa67de 100644
> > --- a/arch/arm/lib/Makefile
> > +++ b/arch/arm/lib/Makefile
> > @@ -13,7 +13,8 @@ lib-y		:= backtrace.o changebit.o csumipv6.o csumpartial.o   \
> >  		   testchangebit.o testclearbit.o testsetbit.o        \
> >  		   ashldi3.o ashrdi3.o lshrdi3.o muldi3.o             \
> >  		   ucmpdi2.o lib1funcs.o div64.o sha1.o               \
> > -		   io-readsb.o io-writesb.o io-readsl.o io-writesl.o
> > +		   io-readsb.o io-writesb.o io-readsl.o io-writesl.o  \
> > +		   switch_stack.o
> >  
> >  mmu-y	:= clear_user.o copy_page.o getuser.o putuser.o
> >  
> > diff --git a/arch/arm/lib/switch_stack.S b/arch/arm/lib/switch_stack.S
> > new file mode 100644
> > index 0000000..76e3844
> > --- /dev/null
> > +++ b/arch/arm/lib/switch_stack.S
> > @@ -0,0 +1,44 @@
> > +/*
> > + * arch/arm/lib/switch_stack.S
> > + *
> > + * Copyright (C) 2011 ARM Ltd.
> > + * Written by Will Deacon <will.deacon@arm.com>
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2 as
> > + * published by the Free Software Foundation.
> > + *
> > + * This program is distributed in the hope that it will be useful,
> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> > + * GNU General Public License for more details.
> > + *
> > + * You should have received a copy of the GNU General Public License
> > + * along with this program; if not, write to the Free Software
> > + * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
> > + */
> > +
> > +#include <linux/linkage.h>
> > +#include <asm/assembler.h>
> > +
> > +/*
> > + * void switch_stack(void (*fn)(void *), void *arg, void *sp)
> > + *
> > + * Change the stack to that pointed at by sp, then invoke fn(arg) with
> > + * the new stack.
> > + */
> > +ENTRY(switch_stack)
> > +	str	sp, [r2, #-4]!
> > +	str	lr, [r2, #-4]!
> > +
> > +	mov	sp, r2
> > +	mov	r2, r0
> > +	mov	r0, r1
> > +
> > +	adr	lr, BSYM(1f)
> > +	mov	pc, r2
> > +
> > +1:	ldr	lr, [sp, #4]
> 
> Looks like this should be
> 
> 	ldr	lr, [sp, #4]!
> 
> ...otherwise we'll also load the lr value into sp.
> Looks like that may have been my mistake.

Argh

I really meant:

	ldr	lr, [sp], #4

Apologies for the confusion!

Cheers
---Dave