[PATCH] Fix show_trace() in irq context with CONFIG_4KSTACKS

- valid_stack_ptr() erroneously assumes that stack always lives in
  task_struct->thread_info.

- the main loop in show_trace() does not recalc ebp after stack
  switching.  With CONFIG_FRAME_POINTER every call to print_context_stack()
  will produce the same output.

With this patch, show_trace() does not use task argument in the main loop. 
Instead, it converts stack to thread_info* context, and passes it to
print_context_stack() and (implicitly) to valid_stack_ptr().

valid_stack_ptr() now does bounds checking against proper context.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/traps.c

@@ -105,36 +105,27 @@ int register_die_notifier(struct notifier_block *nb)
 	return err;
 }
 
-static int valid_stack_ptr(struct task_struct *task, void *p)
+static inline int valid_stack_ptr(struct thread_info *tinfo, void *p)
 {
-	if (p <= (void *)task->thread_info)
-		return 0;
-	if (kstack_end(p))
-		return 0;
-	return 1;
+	return	p > (void *)tinfo &&
+		p < (void *)tinfo + THREAD_SIZE - 3;
 }
 
-#ifdef CONFIG_FRAME_POINTER
-static void print_context_stack(struct task_struct *task, unsigned long *stack,
-			 unsigned long ebp)
+static inline unsigned long print_context_stack(struct thread_info *tinfo,
+				unsigned long *stack, unsigned long ebp)
 {
 	unsigned long addr;
 
-	while (valid_stack_ptr(task, (void *)ebp)) {
+#ifdef	CONFIG_FRAME_POINTER
+	while (valid_stack_ptr(tinfo, (void *)ebp)) {
 		addr = *(unsigned long *)(ebp + 4);
 		printk(" [<%08lx>] ", addr);
 		print_symbol("%s", addr);
 		printk("\n");
 		ebp = *(unsigned long *)ebp;
 	}
-}
 #else
-static void print_context_stack(struct task_struct *task, unsigned long *stack,
-			 unsigned long ebp)
-{
-	unsigned long addr;
-
-	while (!kstack_end(stack)) {
+	while (valid_stack_ptr(tinfo, stack)) {
 		addr = *stack++;
 		if (__kernel_text_address(addr)) {
 			printk(" [<%08lx>]", addr);
@@ -142,8 +133,9 @@ static void print_context_stack(struct task_struct *task, unsigned long *stack,
 			printk("\n");
 		}
 	}
-}
 #endif
+	return ebp;
+}
 
 void show_trace(struct task_struct *task, unsigned long * stack)
 {
@@ -152,11 +144,6 @@ void show_trace(struct task_struct *task, unsigned long * stack)
 	if (!task)
 		task = current;
 
-	if (!valid_stack_ptr(task, stack)) {
-		printk("Stack pointer is garbage, not printing trace\n");
-		return;
-	}
-
 	if (task == current) {
 		/* Grab ebp right from our regs */
 		asm ("movl %%ebp, %0" : "=r" (ebp) : );
@@ -169,7 +156,7 @@ void show_trace(struct task_struct *task, unsigned long * stack)
 		struct thread_info *context;
 		context = (struct thread_info *)
 			((unsigned long)stack & (~(THREAD_SIZE - 1)));
-		print_context_stack(task, stack, ebp);
+		ebp = print_context_stack(context, stack, ebp);
 		stack = (unsigned long*)context->previous_esp;
 		if (!stack)
 			break;