linux-kernel - Re: [PATCH] nfs: Don't deref potentially NULL pointer returned by strsep

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Date:	Mon, 4 Jul 2011 10:34:48 -0400
From:	Chuck Lever <chuck.lever@...cle.com>
To:	Jesper Juhl <jj@...osbits.net>
Cc:	linux-nfs@...r.kernel.org,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	linux-kernel@...r.kernel.org, Gero Kuhlmann <gero@...inix.han.de>
Subject: Re: [PATCH] nfs: Don't deref potentially NULL pointer returned by strsep


On Jul 4, 2011, at 5:04 AM, Jesper Juhl wrote:

> On Sun, 3 Jul 2011, Chuck Lever wrote:
> 
>> 
>> On Jul 3, 2011, at 3:40 AM, Jesper Juhl wrote:
>> 
>>> In fs/nfs/nfsroot.c:root_nfs_parse_options() we call strsep(), which
>>> may return NULL, but we do not test the return value before
>>> dereferencing the pointer.
>>> 
>>> Signed-off-by: Jesper Juhl <jj@...osbits.net>
>>> ---
>>> fs/nfs/nfsroot.c |    2 ++
>>> 1 files changed, 2 insertions(+), 0 deletions(-)
>>> 
>>> Compile tested only.
>>> 
>>> diff --git a/fs/nfs/nfsroot.c b/fs/nfs/nfsroot.c
>>> index c4744e1..b6ac860 100644
>>> --- a/fs/nfs/nfsroot.c
>>> +++ b/fs/nfs/nfsroot.c
>>> @@ -191,6 +191,8 @@ static int __init root_nfs_parse_options(char *incoming, char *exppath,
>>> 	 * Set the NFS remote path
>>> 	 */
>>> 	p = strsep(&incoming, ",");
>>> +	if (!p)
>>> +		return -1;
>> 
>> strsep() may return NULL only if the value of "incoming" is NULL.  But 
>> callers ensure that "incoming" always contains the address of a fixed 
>> buffer.  Thus if strsep() returns NULL here there is some kind of 
>> programming error; it's not the result of invalid input.
>> 
>> Do you have a reproducible test case to make this fail?
>> 
> Nope. I simply spotted the unchecked strsep() call and thought that it 
> would be better to be defensive and check it in case callers change in the 
> future and a bug creeps in that causes a NULL incoming to be passed. 
> Perhaps a BUG_ON(!incoming) would be better - or perhaps just forget about 
> it.

While I personally prefer an explicit assertion in cases like this one, the community preference is to avoid cluttering the code with BUG_ONs, since the code would crash anyway in that case.

--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com



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