linux-kernel - Re: [linux-next-20130422] Bug in SLAB?

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Date:	Thu, 9 May 2013 21:25:22 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	cl@...ux.com, glommer@...allels.com
Cc:	penberg@...nel.org, linux-kernel@...r.kernel.org,
	penguin-kernel@...ove.SAKURA.ne.jp
Subject: Re: [linux-next-20130422] Bug in SLAB?

Tetsuo Handa wrote:
> > Christoph Lameter wrote:
> > > What is MAX_ORDER on the architecture?
> > 
> > In my environment (x86_32), the constants are
> > 
> >   MAX_ORDER=11 PAGE_SHIFT=12 KMALLOC_SHIFT_HIGH=22 KMALLOC_MAX_SIZE=4194304
> > 
> 
> I don't know if any, but on an architecture with PAGE_SHIFT + MAX_ORDER > 26,
> 
>   static void init_node_lock_keys(int q)
>   {
>           int i;
>   
>           if (slab_state < UP)
>                   return;
>   
>           for (i = 1; i < PAGE_SHIFT + MAX_ORDER; i++) {
>                   struct kmem_cache_node *n;
>                   struct kmem_cache *cache = kmalloc_caches[i];
> 
> looks like out of bounds access due to
> 
>   #define KMALLOC_SHIFT_HIGH      ((MAX_ORDER + PAGE_SHIFT - 1) <= 25 ? \
>                                   (MAX_ORDER + PAGE_SHIFT - 1) : 25)
> 
> and
> 
>   struct kmem_cache *kmalloc_caches[KMALLOC_SHIFT_HIGH + 1];
> 
> .
> 

As of commit e0fd9aff on linux.git#master,
CONFIG_PPC_256K_PAGES=y (which makes PAGE_SHIFT 18) &&
CONFIG_FORCE_MAX_ZONEORDER=11 (which makes MAX_ORDER 11) &&
CONFIG_PROVE_LOCKING=y with below assertion

----------
diff --git a/mm/slab.c b/mm/slab.c
index 8ccd296..0401982 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -565,6 +565,7 @@ static void init_node_lock_keys(int q)
        if (slab_state < UP)
                return;

+       BUILD_BUG_ON(PAGE_SHIFT + MAX_ORDER != KMALLOC_SHIFT_HIGH + 1);
        for (i = 1; i < PAGE_SHIFT + MAX_ORDER; i++) {
                struct kmem_cache_node *n;
                struct kmem_cache *cache = kmalloc_caches[i];
----------

on powerpc triggers below error.

  CC      mm/slab.o
mm/slab.c: In function 'init_node_lock_keys':
mm/slab.c:568:2: error: call to '__compiletime_assert_568' declared with attribute error: BUILD_BUG_ON failed: PAGE_SHIFT + MAX_ORDER != KMALLOC_SHIFT_HIGH + 1
make[1]: *** [mm/slab.o] Error 1
make: *** [mm/slab.o] Error 2

This is an example of overrun at kmalloc_caches[26...PAGE_SHIFT+MAX_ORDER-1]
range which the compiler may not be able to detect.
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