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

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")

Cc: stable@kernel.org
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

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}