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SubjectRe: [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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