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

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Date:	Mon, 5 Jan 2009 18:28:48 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
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

[Christoph Lameter - Mon, Jan 05, 2009 at 09:10:57AM -0600]
| 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.

yes, I know, that is why I've changed WARN_ON_ONCE to plain WARN_ON.

| 
| 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?
| 

If you mean did I test this patch for speed regresson then to be fair --
no, I didn't. BUT we have a number of macros wich are self protective
like present_section which is used havily too. On the other hand --
bad argument passed to __nr_to_section will be (and it is now) really
harmfull -- since it would allow to reference a memory outside the
valid bounds. The second -- SECTION_ROOT_MASK wich is fragile, any
attempt to modify mem_section structure will silently lead to insane
referencing, that is why it deserve a comment on top of structure.

Don't know Christoph, if it really that important to not spend a few
cycles here in a sake of safety -- we could easily drop this patch.

		- Cyrill -
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