Messages in this thread | | | Date | Tue, 25 Aug 2009 11:25:17 +0200 | From | Ingo Molnar <> | Subject | Re: WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (f6f6e1a4), by kmemleak's scan_block() |
| |
* Pekka Enberg <penberg@cs.helsinki.fi> wrote:
> On Tue, 2009-08-25 at 11:03 +0200, Vegard Nossum wrote: > > I don't know so much about the kmemleak internals, but this I can say > > about the kmemcheck part: According to your definition, an object is > > initialized if all the bytes of an object are initialized. > > > > Is it possible that because of this, if we have a partially > > uninitialized object, kmemleak will not record the pointers found in > > that object? If so, it might skip valid pointers, and deem an object > > unreferenced. Which could make kmemleak give false-positives. > > > > I think it would be better to ask kmemcheck on a per-pointer basis > > (i.e. for each pointer-sized word in the object), whether it is > > initialized or not. > > Yeah, makes sense. > > Pekka > > diff --git a/arch/x86/mm/kmemcheck/kmemcheck.c b/arch/x86/mm/kmemcheck/kmemcheck.c > index 2c55ed0..528bf95 100644 > --- a/arch/x86/mm/kmemcheck/kmemcheck.c > +++ b/arch/x86/mm/kmemcheck/kmemcheck.c > @@ -331,6 +331,20 @@ static void kmemcheck_read_strict(struct pt_regs *regs, > kmemcheck_shadow_set(shadow, size); > } > > +bool kmemcheck_is_obj_initialized(unsigned long addr, size_t size) > +{ > + enum kmemcheck_shadow status; > + void *shadow; > + > + shadow = kmemcheck_shadow_lookup(addr); > + if (!shadow) > + return true; > + > + status = kmemcheck_shadow_test(shadow, size); > + > + return status == KMEMCHECK_SHADOW_INITIALIZED; > +} > + > /* Access may cross page boundary */ > static void kmemcheck_read(struct pt_regs *regs, > unsigned long addr, unsigned int size) > diff --git a/include/linux/kmemcheck.h b/include/linux/kmemcheck.h > index 47b39b7..dc2fd54 100644 > --- a/include/linux/kmemcheck.h > +++ b/include/linux/kmemcheck.h > @@ -34,6 +34,8 @@ void kmemcheck_mark_initialized_pages(struct page *p, unsigned int n); > int kmemcheck_show_addr(unsigned long address); > int kmemcheck_hide_addr(unsigned long address); > > +bool kmemcheck_is_obj_initialized(unsigned long addr, size_t size); > + > #else > #define kmemcheck_enabled 0 > > @@ -99,6 +101,11 @@ static inline void kmemcheck_mark_initialized_pages(struct page *p, > { > } > > +static inline bool kmemcheck_is_obj_initialized(unsigned long addr, size_t size) > +{ > + return true; > +} > + > #endif /* CONFIG_KMEMCHECK */ > > /* > diff --git a/mm/kmemleak.c b/mm/kmemleak.c > index 6debe0d..b075bf0 100644 > --- a/mm/kmemleak.c > +++ b/mm/kmemleak.c > @@ -97,6 +97,7 @@ > #include <asm/processor.h> > #include <asm/atomic.h> > > +#include <linux/kmemcheck.h> > #include <linux/kmemleak.h> > > /* > @@ -885,7 +886,8 @@ static void scan_block(void *_start, void *_end, > > for (ptr = start; ptr < end; ptr++) { > unsigned long flags; > - unsigned long pointer = *ptr; > + unsigned long pointer; > + > struct kmemleak_object *object; > > if (allow_resched) > @@ -893,6 +895,13 @@ static void scan_block(void *_start, void *_end, > if (scan_should_stop()) > break; > > + /* Don't scan uninitialized memory. */ > + if (!kmemcheck_is_obj_initialized((unsigned long) ptr, > + sizeof(unsigned long))) > + continue;
Nice. In fact this improves kmemleak efficiency as it reduces the amount of false negatives: we wont interpret a random old pointer in already-freed memory as a true 'reference'.
kmemcheck+kmemleak combo bootups might be Da Bomb of the future, in terms of testing ;-)
Ingo
| |