linux-kernel - Re: [PATCH] mm: __nr_to_section - make it safe against overflow v2

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Date:	Tue, 6 Jan 2009 14:57:41 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>, npiggin@...e.de,
	riel@...hat.com, penberg@...helsinki.fi,
	linux-kernel@...r.kernel.org, jirislaby@...il.com
Subject: Re: [PATCH] mm: __nr_to_section - make it safe against overflow v2

On Mon, 5 Jan 2009 16:37:42 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Mon, 5 Jan 2009 13:31:32 +0300
> Cyrill Gorcunov <gorcunov@...il.com> wrote:
> 
> > __nr_to_section should check for array bound overflow.
> > We should better get NULL dereference then silently
> > pass some memory snippet out of bounds to a caller.
> > 
> 
> Are there actually any known problems here?
> 

IIRC, I never saw any problem. (But I may see in memory-hotplug development.)
I don't like adding any check here.

This __nr_to_section[] is very internal function and will not be accessed directly
by codes other than VM(of sparsemem and memory hotplug).

For general users,  this function will be called via pfn_to_page(). So, this means
what this patch should really do is checking "pfn" argugments to pfn_to_page().
Of course, I don't want any check in pfn_to_page()'s "pfn" range.But, pfn_to_page()
is usually called with pfn_valid() if necessary. pfn_valid() and pfn_present(),
other callers of this, check this in explicit way.

Cyrill, if you really want to do check "idx", please add

==
__nr_to_section_direct(unsigned long nr) (or some better name)
{
	BUG_ON(nr >NR_SECTION_ROOTS);
	return __nr_to_section(nr);
}
==
to, mm/internal.h or somewhere. and use this in sparse.c and memhotplug.c

So, Nack from me for now.

Thanks,
-Kame

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