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Message-ID: <4B1502E3.4000304@cs.helsinki.fi>
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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