[PATCH] ppc64: better stack traces

This improves the stack traces we get on PPC64 by putting a marker in
those stack frames that are created as a result of an interrupt or
exception.  The marker is "regshere" (0x7265677368657265).

With this, stack traces show where exceptions have occurred, which can
be very useful.  This also improves the accuracy of the trace because
the relevant return address can be in the link register at the time of
the exception rather than on the stack.  We now print the PC and
exception type for each exception frame, and then the link register if
appropriate as the next item in the trace.

arch/ppc64/kernel/head.S

@@ -1281,6 +1281,10 @@ _GLOBAL(save_remaining_regs)
 	SAVE_4GPRS(16, r1)
 	SAVE_8GPRS(24, r1)
 
+	/* Set the marker value "regshere" just before the reg values */
+	SET_REG_TO_CONST(r22, 0x7265677368657265)
+	std	r22,STACK_FRAME_OVERHEAD-16(r1)
+
 	/*
 	 * Clear the RESULT field
 	 */

arch/ppc64/kernel/process.c

@@ -457,16 +457,16 @@ int sys_execve(unsigned long a0, unsigned long a1, unsigned long a2,
 
 static int kstack_depth_to_print = 64;
 
-static inline int validate_sp(unsigned long sp, struct task_struct *p)
+static int validate_sp(unsigned long sp, struct task_struct *p,
+		       unsigned long nbytes)
 {
 	unsigned long stack_page = (unsigned long)p->thread_info;
 
-	if (sp < stack_page + sizeof(struct thread_struct))
-		return 0;
-	if (sp >= stack_page + THREAD_SIZE)
-		return 0;
-
+	if (sp >= stack_page + sizeof(struct thread_struct)
+	    && sp <= stack_page + THREAD_SIZE - nbytes)
 		return 1;
+
+	return 0;
 }
 
 unsigned long get_wchan(struct task_struct *p)
@@ -478,12 +478,12 @@ unsigned long get_wchan(struct task_struct *p)
 		return 0;
 
 	sp = p->thread.ksp;
-	if (!validate_sp(sp, p))
+	if (!validate_sp(sp, p, 112))
 		return 0;
 
 	do {
 		sp = *(unsigned long *)sp;
-		if (!validate_sp(sp, p))
+		if (!validate_sp(sp, p, 112))
 			return 0;
 		if (count > 0) {
 			ip = *(unsigned long *)(sp + 16);
@@ -496,9 +496,10 @@ unsigned long get_wchan(struct task_struct *p)
 
 void show_stack(struct task_struct *p, unsigned long *_sp)
 {
-	unsigned long ip;
+	unsigned long ip, newsp, lr;
 	int count = 0;
 	unsigned long sp = (unsigned long)_sp;
+	int firstframe = 1;
 
 	if (sp == 0) {
 		if (p) {
@@ -509,17 +510,40 @@ void show_stack(struct task_struct *p, unsigned long *_sp)
 		}
 	}
 
-	if (!validate_sp(sp, p))
-		return;
-
+	lr = 0;
 	printk("Call Trace:\n");
 	do {
-		sp = *(unsigned long *)sp;
-		if (!validate_sp(sp, p))
+		if (!validate_sp(sp, p, 112))
 			return;
-		ip = *(unsigned long *)(sp + 16);
-		printk("[%016lx] ", ip);
-		print_symbol("%s\n", ip);
+
+		_sp = (unsigned long *) sp;
+		newsp = _sp[0];
+		ip = _sp[2];
+		if (!firstframe || ip != lr) {
+			printk("[%016lx] [%016lx] ", sp, ip);
+			print_symbol("%s", ip);
+			if (firstframe)
+				printk(" (unreliable)");
+			printk("\n");
+		}
+		firstframe = 0;
+
+		/*
+		 * See if this is an exception frame.
+		 * We look for the "regshere" marker in the current frame.
+		 */
+		if (validate_sp(sp, p, sizeof(struct pt_regs) + 400)
+		    && _sp[12] == 0x7265677368657265) {
+			struct pt_regs *regs = (struct pt_regs *)
+				(sp + STACK_FRAME_OVERHEAD);
+			printk("--- Exception: %lx", regs->trap);
+			print_symbol(" at %s\n", regs->nip);
+			lr = regs->link;
+			print_symbol("    LR = %s\n", lr);
+			firstframe = 1;
+		}
+
+		sp = newsp;
 	} while (count++ < kstack_depth_to_print);
 }
 

arch/ppc64/xmon/xmon.c

@@ -1399,8 +1399,7 @@ static void xmon_show_stack(unsigned long sp, unsigned long lr,
 
 		/* Look for "regshere" marker to see if this is
 		   an exception frame. */
-		if (newsp - sp == sizeof(struct pt_regs) + 400
-		    && mread(sp + 0x60, &marker, sizeof(unsigned long))
+		if (mread(sp + 0x60, &marker, sizeof(unsigned long))
 		    && marker == 0x7265677368657265) {
 			if (mread(sp + 0x70, &regs, sizeof(regs))
 			    != sizeof(regs)) {
@@ -1417,12 +1416,6 @@ static void xmon_show_stack(unsigned long sp, unsigned long lr,
 
 		if (newsp == 0)
 			break;
-		if (newsp < sp) {
-			printf("Stack chain goes %s: %.16lx\n",
-			       (newsp < KERNELBASE? "into userspace":
-				"backwards"), newsp);
-			break;
-		}
 
 		sp = newsp;
 	} while (count++ < xmon_depth_to_print);