[3.13.y.z extended stable] Patch "ARM: 8064/1: fix v7-M signal return" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    ARM: 8064/1: fix v7-M signal return

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.4.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From d7e2eb195e1d4712a891c00008c06fd64c3b8524 Mon Sep 17 00:00:00 2001
From: Rabin Vincent <rabin at rab.in>
Date: Sat, 24 May 2014 17:38:01 +0100
Subject: ARM: 8064/1: fix v7-M signal return
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

commit 483a6c9d447f625b991fa04a1530493d893984db upstream.

According to the ARM ARM, the behaviour is UNPREDICTABLE 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.  Current hardware happens to
discard this bit, but QEMU's emulation doesn't and this makes processes
crash.  Mask out bit 0 before the exception return in order to get
predictable behaviour.

Fixes: 19c4d593f0b4 ("ARM: ARMv7-M: Add support for exception handling")

Acked-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>
Signed-off-by: Rabin Vincent <rabin at rab.in>
Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/arm/kernel/entry-header.S | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S
index 39f89fb..88c6bab 100644
--- a/arch/arm/kernel/entry-header.S
+++ b/arch/arm/kernel/entry-header.S
@@ -132,6 +132,10 @@
 	orrne	r5, V7M_xPSR_FRAMEPTRALIGN
 	biceq	r5, V7M_xPSR_FRAMEPTRALIGN

+	@ ensure bit 0 is cleared in the PC, otherwise behaviour is
+	@ unpredictable
+	bic	r4, #1
+
 	@ write basic exception frame
 	stmdb	r2!, {r1, r3-r5}
 	ldmia	sp, {r1, r3-r5}
--
1.9.1