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

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Date:	Mon, 5 Jan 2009 12:00:58 +0200
From:	"Pekka Enberg" <penberg@...helsinki.fi>
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

Hi Cyrill,

On Mon, Jan 5, 2009 at 11:40 AM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> @@ -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;

Looks good to me but I have minor nitpick. You might want to write the
above like this:

    if (WARN_ON_ONCE(idx >= NR_SECTION_ROOTS))
            return NULL;

to separate the error condition from the normal case where we don't
have a mem section.

> -       return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK];
> +       return &mem_section[idx][nr & SECTION_ROOT_MASK];
>  }
--
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