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

[Cyrill Gorcunov <gorcunov@gmail.com>]

[Nick Piggin - Mon, Jan 05, 2009 at 04:34:06PM +0100]
| On Mon, Jan 05, 2009 at 06:28:48PM +0300, Cyrill Gorcunov wrote:
| > [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.
| 
| Still costs. Putting it under a config option would be nice.
| 
| 
| > | 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.
| 
| The problem with testing every little slowdown for a speed regression
| is that they are just going to be in the noise. But we *know* it will
| go slower. The problem is that they add up. We just have to be sensible
| about it.
| 
| Has there ever been a problem here before? Has it been a problem during
| development? (in which case putting it in a .config option might make
| sense).
| 

After checking all 'for' and 'against' -- I think I just overzealous
about it. Please drop it. (the case I was concerning to actually protected
by present_section macro anyway). Sorry for noise.

		- Cyrill -