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

[ivan.djelic@parrot.com (Ivan Djelic)]

On Sat, Feb 09, 2013 at 02:48:31PM +0000, Nicolas Pitre wrote:
> On Sat, 2 Feb 2013, 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.
> > 
> > This patch fixes the return value of the assembly version of memset.
> > Could you please review, or suggest better alternatives ?
> > 
> > Thanks,
> > 
> > --
> > Ivan
> > 
> > (this is a shorter and (hopefully) clearer repost of
> > http://lists.infradead.org/pipermail/linux-arm-kernel/2013-January/144916.html)
> > 
> > The patch adds a 'mov' instruction and merges an additional load+store into
> > existing load/store instructions.
> > For ease of review, here is a breakdown of the patch into 4 simple steps:
> > 
> > Step 1
> > ======
> > Perform the following substitutions:
> > ip -> r8, then
> > r0 -> ip,
> > and insert 'mov ip, r0' as the first statement of the function.
> > At this point, we have a memset() implementation returning the proper result,
> > but corrupting r8 on some paths (the ones that were using ip).
> > 
> > Step 2
> > ======
> > Make sure r8 is saved and restored when (! CALGN(1)+0) == 1:
> > 
> > save r8:
> > -       str     lr, [sp, #-4]!
> > +       stmfd   sp!, {r8, lr}
> > 
> > and restore r8 on both exit paths:
> > -       ldmeqfd sp!, {pc}               @ Now <64 bytes to go.
> > +       ldmeqfd sp!, {r8, pc}           @ Now <64 bytes to go.
> > (...)
> >         tst     r2, #16
> >         stmneia ip!, {r1, r3, r8, lr}
> > -       ldr     lr, [sp], #4
> > +       ldmfd   sp!, {r8, lr}
> > 
> > Step 3
> > ======
> > Make sure r8 is saved and restored when (! CALGN(1)+0) == 0:
> > 
> > save r8:
> > -       stmfd   sp!, {r4-r7, lr}
> > +       stmfd   sp!, {r4-r8, lr}
> > 
> > and restore r8 on both exit paths:
> >         bgt     3b
> > -       ldmeqfd sp!, {r4-r7, pc}
> > +       ldmeqfd sp!, {r4-r8, pc}
> > (...)
> >         tst     r2, #16
> >         stmneia ip!, {r4-r7}
> > -       ldmfd   sp!, {r4-r7, lr}
> > +       ldmfd   sp!, {r4-r8, lr}
> > 
> > Step 4
> > ======
> > Rewrite register list "r4-r7, r8" as "r4-r8".
> > 
> > Signed-off-by: Ivan Djelic <ivan.djelic@parrot.com>
> 
> Reviewed-by: Nicolas Pitre <nico@linaro.org>
> 
> This is good for the stable tree as well.
> 
> Please include in your official commit text the above breakdown 
> explanation as well.

OK, will do.
Thanks a lot for reviewing,
--
Ivan