Subject: [PATCH] [RFC] arm: fix memset-related crashes caused by recent GCC (4.7.2) optimizations

From: [email-masked] (Ivan Djelic)
Date: Mon, 11 Feb 2013 22:39:25 +0100

On Mon, Feb 11, 2013 at 06:17:32PM +0000, Ben Dooks wrote:
> On 02/02/13 08:33, Ivan Djelic wrote:
> > Recent GCC versions (e.g. GCC-4.7.2) perform optimizations based on
> > assumptions about the implementation of memset and similar functions.
> > The current ARM optimized memset code does not return the value of
> > its first argument, as is usually expected from standard implementations.
> >
> > For instance in the following function:
> >
> > void debug_mutex_lock_common(struct mutex *lock, struct mutex_waiter *waiter)
> > {
> > 	memset(waiter, MUTEX_DEBUG_INIT, sizeof(*waiter));
> > 	waiter->magic = waiter;
> > 	INIT_LIST_HEAD(&waiter->list);
> > }
> >
> > compiled as:
> >
> > 800554d0<debug_mutex_lock_common>:
> > 800554d0:       e92d4008        push    {r3, lr}
> > 800554d4:       e1a00001        mov     r0, r1
> > 800554d8:       e3a02010        mov     r2, #16 ; 0x10
> > 800554dc:       e3a01011        mov     r1, #17 ; 0x11
> > 800554e0:       eb04426e        bl      80165ea0<memset>
> > 800554e4:       e1a03000        mov     r3, r0
> > 800554e8:       e583000c        str     r0, [r3, #12]
> > 800554ec:       e5830000        str     r0, [r3]
> > 800554f0:       e5830004        str     r0, [r3, #4]
> > 800554f4:       e8bd8008        pop     {r3, pc}
> >
> > GCC assumes memset returns the value of pointer 'waiter' in register r0; causing
> > register/memory corruptions.
> 
> 
> > @@ -43,29 +47,28 @@ ENTRY(memset)
> >   #if ! CALGN(1)+0
> >
> >   /*
> > - * We need an extra register for this loop - save the return address and
> > - * use the LR
> > + * We need an 2 extra registers for this loop - use r8 and the LR
> >    */
> > -	str	lr, [sp, #-4]!
> > -	mov	ip, r1
> > +	stmfd	sp!, {r8, lr}
> > +	mov	r8, r1
> >   	mov	lr, r1
> 
> Out of interest, why not save {r0, lr} and avoid having to
> re-write the entirety of the inner loop?

Because at the inner loop entry, r0 no longer contains the current write pointer: ip is
the current pointer, possibly different from r0 because of the unaligned fixup code.
The idea was to avoid introducing an extra load+store on load/store-free paths; thereby
using a register other than r0 for the current pointer...

BR,
--
Ivan

> 
> >
> >   2:	subs	r2, r2, #64
> > -	stmgeia	r0!, {r1, r3, ip, lr}	@ 64 bytes at a time.
> > -	stmgeia	r0!, {r1, r3, ip, lr}
> > -	stmgeia	r0!, {r1, r3, ip, lr}
> > -	stmgeia	r0!, {r1, r3, ip, lr}
> > +	stmgeia	ip!, {r1, r3, r8, lr}	@ 64 bytes at a time.
> > +	stmgeia	ip!, {r1, r3, r8, lr}
> > +	stmgeia	ip!, {r1, r3, r8, lr}
> > +	stmgeia	ip!, {r1, r3, r8, lr}
> >   	bgt	2b
> > -	ldmeqfd	sp!, {pc}		@ Now<64 bytes to go.
> > +	ldmeqfd	sp!, {r8, pc}		@ Now<64 bytes to go.
> 
> 
> -- 
> Ben Dooks				http://www.codethink.co.uk/
> Senior Engineer				Codethink - Providing Genius