x86: Add stack top margin for stack overflow checking

It seems that a margin for stack overflow checking is added to
top of a kernel stack but is not added to IRQ and exception
stacks in stack_overflow_check(). Therefore, the overflows of
IRQ and exception stacks are always detected only after they
actually occurred and data corruption might occur due to them.

This patch adds the margin to top of IRQ and exception stacks
as well as a kernel stack to enhance reliability.

Signed-off-by: Mitsuo Hayasaka <mitsuo.hayasaka.hu@hitachi.com>
Cc: yrl.pp-manager.tt@hitachi.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20111207082910.9847.3359.stgit@ltc219.sdl.hitachi.co.jp
[ removed the #undef - we typically don't do that for uncommon names ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/irq_64.c

@@ -38,6 +38,7 @@ int sysctl_panic_on_stackoverflow;
 static inline void stack_overflow_check(struct pt_regs *regs)
 {
 #ifdef CONFIG_DEBUG_STACKOVERFLOW
+#define STACK_TOP_MARGIN	128
 	struct orig_ist *oist;
 	u64 irq_stack_top, irq_stack_bottom;
 	u64 estack_top, estack_bottom;
@@ -47,17 +48,18 @@ static inline void stack_overflow_check(struct pt_regs *regs)
 		return;
 
 	if (regs->sp >= curbase + sizeof(struct thread_info) +
-				  sizeof(struct pt_regs) + 128 &&
+				  sizeof(struct pt_regs) + STACK_TOP_MARGIN &&
 	    regs->sp <= curbase + THREAD_SIZE)
 		return;
 
-	irq_stack_top = (u64)__get_cpu_var(irq_stack_union.irq_stack);
+	irq_stack_top = (u64)__get_cpu_var(irq_stack_union.irq_stack) +
+			STACK_TOP_MARGIN;
 	irq_stack_bottom = (u64)__get_cpu_var(irq_stack_ptr);
 	if (regs->sp >= irq_stack_top && regs->sp <= irq_stack_bottom)
 		return;
 
 	oist = &__get_cpu_var(orig_ist);
-	estack_top = (u64)oist->ist[0] - EXCEPTION_STKSZ;
+	estack_top = (u64)oist->ist[0] - EXCEPTION_STKSZ + STACK_TOP_MARGIN;
 	estack_bottom = (u64)oist->ist[N_EXCEPTION_STACKS - 1];
 	if (regs->sp >= estack_top && regs->sp <= estack_bottom)
 		return;