linux-kernel - Re: [PATCH] mm: __nr_to_section - make it safe against overflow

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Date:	Mon, 5 Jan 2009 09:10:57 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Cyrill Gorcunov <gorcunov@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jirislaby@...il.com>
Subject: Re: [PATCH] mm: __nr_to_section - make it safe against overflow

On Mon, 5 Jan 2009, Cyrill Gorcunov wrote:

>  	/*
>  	 * This is, logically, a pointer to an array of struct
> @@ -980,9 +986,12 @@ extern struct mem_section mem_section[NR
>
>  static inline struct mem_section *__nr_to_section(unsigned long nr)
>  {
> -	if (!mem_section[SECTION_NR_TO_ROOT(nr)])
> +	unsigned long idx = SECTION_NR_TO_ROOT(nr);
> +	WARN_ON_ONCE(idx >= NR_SECTION_ROOTS);
> +
> +	if (idx >=NR_SECTION_ROOTS || !mem_section[idx])
>  		return NULL;
> -	return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK];
> +	return &mem_section[idx][nr & SECTION_ROOT_MASK];
>  }
>  extern int __section_nr(struct mem_section* ms);
>  extern unsigned long usemap_size(void);

Not that you are adding code to numerous hot code path. Plus this is a
frequently used inline. Code size is going to increase if you do this.

I would think that the code does not have the tests because of performance
and code size concerns. Can we just say that a sane nr must be passed to
__nr_section?

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