linux-kernel - Re: [PATCH -mm] slab: update_memcg_params: explicitly check that old array != NULL

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Date:	Mon, 26 Jan 2015 13:45:34 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Dan Carpenter <dan.carpenter@...cle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm] slab: update_memcg_params: explicitly check that old
 array != NULL

On Mon, Jan 26, 2015 at 01:23:05PM +0300, Dan Carpenter wrote:
> On Mon, Jan 26, 2015 at 01:01:19PM +0300, Vladimir Davydov wrote:
> > This warning is false-positive, because @old equals NULL iff
> > @memcg_nr_cache_ids equals 0.
> 
> I don't see how it could be a false positive.  The "old" pointer is
> dereferenced inside the call to memset() so unless memset is a macro the
> compiler isn't going to optimize the dereference away.

old->entries is not dereferenced: memcg_cache_array->entries is not a
pointer - it is embedded to the memcg_cache_array struct.

> 
> 
> //----- test code
> 
> void frob(void *p){}
> 
> struct foo {
> 	int *x, *y, *z;
> };
> 
> int main(void)
> {
> 	struct foo *x = NULL;
> 
> 	frob(x->y);
> 
> 	return 0;
> }
> 
> //---- end
> 
> 
> If we compile with gcc test.c then it segfaults.  With -02 the compiler
> is able to tell that frob() is an empty function and it doesn't
> segfault.  In the kernel code, there is no way for the compiler to
> optimize the memset() away so it will Oops.

Just change

- 	int *x, *y, *z;
+	int *x, *z;
+	int *y[0];

and it won't.

Thanks,
Vladimir
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