linux-kernel - Re: [PATCH] Fix max-stack calculators to skip canary

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Message-Id: <1208885594.8636.23.camel@localhost>
Date:	Tue, 22 Apr 2008 10:33:14 -0700
From:	Joe Perches <joe@...ches.com>
To:	Eric Sandeen <sandeen@...deen.net>
Cc:	Ingo Molnar <mingo@...e.hu>,
	linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Fix  max-stack calculators to skip canary

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); \
})

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