powerpc/kprobe: Complete kprobe and migrate exception frame

We can't emulate stwu since that may corrupt current exception stack.
So we will have to do real store operation in the exception return code.

Firstly we'll allocate a trampoline exception frame below the kprobed
function stack and copy the current exception frame to the trampoline.
Then we can do this real store operation to implement 'stwu', and reroute
the trampoline frame to r1 to complete this exception migration.
Signed-off-by: Tiejun Chen <tiejun.chen@windriver.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/entry_32.S

@@ -831,19 +831,56 @@ restore_user:
 	bnel-	load_dbcr0
 #endif
 
-#ifdef CONFIG_PREEMPT
 	b	restore
 
 /* N.B. the only way to get here is from the beq following ret_from_except. */
 resume_kernel:
-	/* check current_thread_info->preempt_count */
+	/* check current_thread_info, _TIF_EMULATE_STACK_STORE */
 	CURRENT_THREAD_INFO(r9, r1)
+	lwz	r8,TI_FLAGS(r9)
+	andis.	r8,r8,_TIF_EMULATE_STACK_STORE@h
+	beq+	1f
+
+	addi	r8,r1,INT_FRAME_SIZE	/* Get the kprobed function entry */
+
+	lwz	r3,GPR1(r1)
+	subi	r3,r3,INT_FRAME_SIZE	/* dst: Allocate a trampoline exception frame */
+	mr	r4,r1			/* src:  current exception frame */
+	mr	r1,r3			/* Reroute the trampoline frame to r1 */
+
+	/* Copy from the original to the trampoline. */
+	li	r5,INT_FRAME_SIZE/4	/* size: INT_FRAME_SIZE */
+	li	r6,0			/* start offset: 0 */
+	mtctr	r5
+2:	lwzx	r0,r6,r4
+	stwx	r0,r6,r3
+	addi	r6,r6,4
+	bdnz	2b
+
+	/* Do real store operation to complete stwu */
+	lwz	r5,GPR1(r1)
+	stw	r8,0(r5)
+
+	/* Clear _TIF_EMULATE_STACK_STORE flag */
+	lis	r11,_TIF_EMULATE_STACK_STORE@h
+	addi	r5,r9,TI_FLAGS
+0:	lwarx	r8,0,r5
+	andc	r8,r8,r11
+#ifdef CONFIG_IBM405_ERR77
+	dcbt	0,r5
+#endif
+	stwcx.	r8,0,r5
+	bne-	0b
+1:
+
+#ifdef CONFIG_PREEMPT
+	/* check current_thread_info->preempt_count */
 	lwz	r0,TI_PREEMPT(r9)
 	cmpwi	0,r0,0		/* if non-zero, just restore regs and return */
 	bne	restore
-	lwz	r0,TI_FLAGS(r9)
-	andi.	r0,r0,_TIF_NEED_RESCHED
+	andi.	r8,r8,_TIF_NEED_RESCHED
 	beq+	restore
+	lwz	r3,_MSR(r1)
 	andi.	r0,r3,MSR_EE	/* interrupts off? */
 	beq	restore		/* don't schedule if so */
 #ifdef CONFIG_TRACE_IRQFLAGS
@@ -864,8 +901,6 @@ resume_kernel:
 	 */
 	bl	trace_hardirqs_on
 #endif
-#else
-resume_kernel:
 #endif /* CONFIG_PREEMPT */
 
 	/* interrupts are hard-disabled at this point */

arch/powerpc/kernel/entry_64.S

@@ -593,6 +593,41 @@ _GLOBAL(ret_from_except_lite)
 	b	.ret_from_except
 
 resume_kernel:
+	/* check current_thread_info, _TIF_EMULATE_STACK_STORE */
+	CURRENT_THREAD_INFO(r9, r1)
+	ld	r8,TI_FLAGS(r9)
+	andis.	r8,r8,_TIF_EMULATE_STACK_STORE@h
+	beq+	1f
+
+	addi	r8,r1,INT_FRAME_SIZE	/* Get the kprobed function entry */
+
+	lwz	r3,GPR1(r1)
+	subi	r3,r3,INT_FRAME_SIZE	/* dst: Allocate a trampoline exception frame */
+	mr	r4,r1			/* src:  current exception frame */
+	mr	r1,r3			/* Reroute the trampoline frame to r1 */
+
+	/* Copy from the original to the trampoline. */
+	li	r5,INT_FRAME_SIZE/8	/* size: INT_FRAME_SIZE */
+	li	r6,0			/* start offset: 0 */
+	mtctr	r5
+2:	ldx	r0,r6,r4
+	stdx	r0,r6,r3
+	addi	r6,r6,8
+	bdnz	2b
+
+	/* Do real store operation to complete stwu */
+	lwz	r5,GPR1(r1)
+	std	r8,0(r5)
+
+	/* Clear _TIF_EMULATE_STACK_STORE flag */
+	lis	r11,_TIF_EMULATE_STACK_STORE@h
+	addi	r5,r9,TI_FLAGS
+	ldarx	r4,0,r5
+	andc	r4,r4,r11
+	stdcx.	r4,0,r5
+	bne-	0b
+1:
+
 #ifdef CONFIG_PREEMPT
 	/* Check if we need to preempt */
 	andi.	r0,r4,_TIF_NEED_RESCHED