linux-kernel - Re: Q, slab, kmemleak

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Date:	Tue, 01 Dec 2009 13:49:55 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	hooanon05@...oo.co.jp
CC:	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: Q, slab, kmemleak_erase() and redzone?

hooanon05@...oo.co.jp kirjoitti:
> Pekka Enberg:
>> We are setting an element in the per CPU array to NULL so the the
>> kmemleak code in ____cache_alloc() is safe. Red-zoning is done at the
>> _object_ which is not touched by kmemleak. Looking at the oops, it
>> does seem likely that you have a bug in your module (or in some other
>> part of the kernel).
> 
> Thanks for reply.
> In ____cache_alloc(), the variable 'ac' is assigned before
> cache_alloc_refill() call, and it is used for the parameter of
> kmemleak_erase(). The value may be changed by cache_alloc_refill(),
> isn't it?

No. The pointer returned by cpu_cache_get() is not changed by 
cache_alloc_refill(). The contents of the array might change, yes. That 
said, we should check if objp is NULL before calling kmemleak_erase(). 
Catalin?

> In this case, kmemleak_erase() receives the incorrect pointer and sets
> NULL to somewhere else which may be redzone?
> 
> How about this fix?
> If cpu_cache_get() call is heavy and we cannot ignore it when KMEMLEAK
> is disabled, then a new wrapper may be necessary.
> 
> diff --git a/mm/slab.c b/mm/slab.c
> index 71e0a1f..3f3e018 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3104,6 +3104,7 @@ static inline void *____cache_alloc(struct kmem_cache *cachep, gfp_t flags)
>  	} else {
>  		STATS_INC_ALLOCMISS(cachep);
>  		objp = cache_alloc_refill(cachep, flags);
> +		ac = cpu_cache_get(cachep);
>  	}
>  	/*
>  	 * To avoid a false negative, if an object that is in one of the
> 
> 
> 
> J. R. Okajima

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