linux-kernel - Re: WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (f6f6e1a4), by kmemleak's scan

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Date:	Tue, 25 Aug 2009 10:21:08 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: WARNING: kmemcheck: Caught 32-bit read from uninitialized
	memory  (f6f6e1a4), by kmemleak's scan_block()

On Tue, 2009-08-25 at 12:11 +0300, Pekka Enberg 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.

I think this patch should work. With a few minor (aesthetic) things
below and assuming that Ingo tests it (I don't have x86 hardware at hand
now):

Acked-by: Catalin Marinas <catalin.marinas@....com>

> @@ -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;

An empty line here added which splits the local variables block.

>  		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)))

There is a BYTES_PER_POINTER macro defined in the kmemleak.c file, you
could use that instead of sizeof(unsigned long).

Thanks.

-- 
Catalin

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