linux-kernel - Re: [RFC][PATCH 0/5] do not dereference NULL pools in pools' destroy() functions

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Date:	Wed, 10 Jun 2015 11:04:44 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Minchan Kim <minchan@...nel.org>,
	Pekka Enberg <penberg@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, sergey.senozhatsky.work@...il.com,
	Joe Perches <joe@...ches.com>
Subject: Re: [RFC][PATCH 0/5] do not dereference NULL pools in pools'
 destroy() functions

On (06/09/15 20:11), Christoph Lameter wrote:
> On Tue, 9 Jun 2015, Andrew Morton wrote:
> 
> > Well I like it, even though it's going to cause a zillion little cleanup
> > patches.
> >
> > checkpatch already has a "kfree(NULL) is safe and this check is
> > probably not required" test so I guess Joe will need to get busy ;)
> >
> > I'll park these patches until after 4.1 is released - it's getting to
> > that time...
> 
> Why do this at all?

this makes things less fragile.

> I understand that kfree/kmem_cache_free can take a
> null pointer but this is the destruction of a cache and it usually
> requires multiple actions to clean things up and these actions have to be
> properly sequenced. All other processors have to stop referencing this
> cache before it can be destroyed. 

>I think failing

well, it's not just `failing', it's a NULL pointer deref.

> if someone does something strange like doing cache destruction with a
> NULL pointer is valuable.
> 

a missing check is not `something strange'. it's just happening.

(a very quick google search)
http://help.lockergnome.com/linux/PATCH-dlm-NULL-dereference-failure-kmem_cache_create--ftopict555436.html
http://linux-kernel.2935.n7.nabble.com/PATCH-2-6-30-rc6-Remove-kmem-cache-destroy-in-s3c24xx-dma-init-td460417.html
etc.

	-ss
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