ARM: Thumb-2: Fix exception return sequence to restore stack correctly

The implementation of svc_exit didn't take into account any stack hole
created by svc_entry; as happens with the undef handler when kprobes are
configured. The fix is to read the saved value of SP rather than trying
to calculate it.
Signed-off-by: Jon Medhurst <tixy@yxit.co.uk>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>

arch/arm/kernel/entry-header.S

@@ -121,15 +121,13 @@
 	.endm
 #else	/* CONFIG_THUMB2_KERNEL */
 	.macro	svc_exit, rpsr
+	ldr	lr, [sp, #S_SP]			@ top of the stack
+	ldrd	r0, r1, [sp, #S_LR]		@ calling lr and pc
 	clrex					@ clear the exclusive monitor
-	ldr	r0, [sp, #S_SP]			@ top of the stack
-	ldr	r1, [sp, #S_PC]			@ return address
-	tst	r0, #4				@ orig stack 8-byte aligned?
-	stmdb	r0, {r1, \rpsr}			@ rfe context
+	stmdb	lr!, {r0, r1, \rpsr}		@ calling lr and rfe context
 	ldmia	sp, {r0 - r12}
-	ldr	lr, [sp, #S_LR]
-	addeq	sp, sp, #S_FRAME_SIZE - 8	@ aligned
-	addne	sp, sp, #S_FRAME_SIZE - 4	@ not aligned
+	mov	sp, lr
+	ldr	lr, [sp], #4
 	rfeia	sp!
 	.endm