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Subject: Re: [PATCH] Fix  max-stack calculators to skip canary
From: Joe Perches <joe@perches.com>
To: Eric Sandeen <sandeen@sandeen.net>
Cc: Ingo Molnar <mingo@elte.hu>,
       linux-kernel Mailing List <linux-kernel@vger.kernel.org>,
       Arjan van de Ven <arjan@infradead.org>,
       Andrew Morton <akpm@linux-foundation.org>
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	 <480E15DC.5040301@sandeen.net>  <480E1DED.9040104@sandeen.net>
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Date: Tue, 22 Apr 2008 10:33:14 -0700
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On Tue, 2008-04-22 at 12:18 -0500, Eric Sandeen wrote:
> Index: linux-2.6.25/kernel/exit.c
> ===================================================================
> --- linux-2.6.25.orig/kernel/exit.c	2008-04-20 22:34:16.000000000 -0500
> +++ linux-2.6.25/kernel/exit.c	2008-04-22 11:38:05.769412824 -0500
> @@ -826,6 +826,8 @@ static void check_stack_usage(void)
>  	unsigned long *n = end_of_stack(current);
>  	unsigned long free;
>  
> +	n++;	/* skip over canary at end */
> +
>  	while (*n == 0)
>  		n++;
>  	free = (unsigned long)n - (unsigned long)end_of_stack(current);
> Index: linux-2.6.25/kernel/sched.c
> ===================================================================
> --- linux-2.6.25.orig/kernel/sched.c	2008-04-20 22:34:19.000000000 -0500
> +++ linux-2.6.25/kernel/sched.c	2008-04-22 11:48:06.975407495 -0500
> @@ -5190,6 +5190,8 @@ void sched_show_task(struct task_struct 
>  #ifdef CONFIG_DEBUG_STACK_USAGE
>  	{
>  		unsigned long *n = end_of_stack(p);
> +
> +		n++;	/* skip over canary at end */
>  		while (!*n)
>  			n++;
>  		free = (unsigned long)n - (unsigned long)end_of_stack(p);
> 

Wouldn't it be better to have exactly the same code?
How about using a statement expression macro?

#define DEBUG_STACK_FREE(process) \
({ \
	unsigned long *n = end_of_stack(process); \
	do { 	/* Skip over canary */\
		n++; \
	} while (!*n); \
	(unsigned long)n - (unsigned long)end_of_stack(process); \
})