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    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


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