lkml.org 
[lkml]   [2009]   [Aug]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: 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


\
 
 \ /
  Last update: 2009-08-25 11:27    [W:0.339 / U:0.232 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site