linux-kernel - Re: [PATCH] dmapool: remove redundant NULL check for dev in dma_pool

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Message-Id: <20140430141924.8d84f7fdcac3ac3996802aa9@linux-foundation.org>
Date:	Wed, 30 Apr 2014 14:19:24 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Daeseok Youn <daeseok.youn@...il.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dmapool: remove redundant NULL check for dev in
 dma_pool_create()

On Tue, 29 Apr 2014 11:53:10 +0900 Daeseok Youn <daeseok.youn@...il.com> wrote:

> "dev" cannot be NULL because it is already checked before
> calling dma_pool_create().
> 
> Signed-off-by: Daeseok Youn <daeseok.youn@...il.com>
> ---
> If dev can be NULL, it has NULL deferencing when kmalloc_node()
> is called after enabling CONFIG_NUMA.

hm, this is unclear.

The code which handles the dev==NULL case was obviously put there
deliberately, presumably with the intention of permitting drivers to
call dma_pool_create() without a device*.  This code is very old.

A lot of drivers call dma_pool_create() (I doubt if you audited all of
them!) and perhaps there are some which use this feature and have never
been run on NUMA hardware.

I think I'll apply the patch anyway because such drivers (if they
exist) probably need some attending to.

I rewrote the changelog thusly:

: "dev" cannot be NULL because it is already checked before calling
: dma_pool_create().
: 
: If dev ever was NULL, the code would oops in dev_to_node() after enabling
: CONFIG_NUMA.
: 
: It is possible that some driver is using dev==NULL and has never been run
: on a NUMA machine.  Such a driver is probably outdated, possibly buggy and
: will need some attention if it starts triggering NULL derefs.

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