[PATCH] ARM: fix v7-M signal return

[Uwe Kleine-König]

Hello Rabin,

On Sat, May 03, 2014 at 07:39:03PM +0200, Rabin Vincent wrote:
> On Mon, Apr 28, 2014 at 10:27:57AM +0200, Uwe Kleine-König wrote:
> > This is a valid fix, but it seems on my efm32 the unpredictable
> > behaviour is to just discard the LSB. How did you find that? Is it an
> > issue on your machine? Which cpu are you using?
> 
> I'm running this on QEMU.  Here is an old qemu-devel thread on this
> topic if you are interested:
> http://lists.gnu.org/archive/html/qemu-devel/2012-03/msg00158.html
I'm interested in your setup and (if applicable) additional kernel
patches.

> > I'd like to have the instruction clearing the thumb bit above the
> > comment about the basic exception frame and please add a comment for
> > your instruction, too.
> 
> OK, here is a v2 with those changes:
> 
> 8<------------------
> From 4aa76f95a6ecf781eec89dba8a3884e5e4339182 Mon Sep 17 00:00:00 2001
> From: Rabin Vincent <rabin at rab.in>
> Date: Sat, 3 May 2014 19:27:09 +0200
> Subject: [PATCHv2] ARM: fix v7-M signal return
> 
> According to the ARM ARM, the behaviour is UNDPREDICTABLE if the PC read
> from the exception return stack is not half word aligned.  See the
> pseudo code for ExceptionReturn() and PopStack().
> 
> The signal handler's address has the bit 0 set, and setup_return()
> directly writes this to regs->ARM_pc.  Mask out bit 0 before the
> exception return to get predictable behaviour.
> 
> Signed-off-by: Rabin Vincent <rabin at rab.in>
> ---
>  arch/arm/kernel/entry-header.S | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S
> index 1420725..743dff6 100644
> --- a/arch/arm/kernel/entry-header.S
> +++ b/arch/arm/kernel/entry-header.S
> @@ -132,6 +132,9 @@
>  	orrne	r5, V7M_xPSR_FRAMEPTRALIGN
>  	biceq	r5, V7M_xPSR_FRAMEPTRALIGN
>  
> +	@ ensure bit 0 is cleared in the PC
Maybe add: , otherwise behaviour is unpredictable

> +	bic	r4, r4, #1
I just notice that the coding style is inconsitent here, the
instructions above don't repeat the dest register. Can you please make
this "bic r4, #1", too?

With these two changes you can have my ack.

What happens on qemu without this fix? If it crashes I'd suggest to get
this patch into 3.15 with a stable annotation. If not I think 3.16-rc1
is enough.

Best regards
Uwe

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