linux-kernel - Re: [PATCH] Ceph: Fix a use-after-free bug in ceph_destroy

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Message-ID: <20101225224658.GC19019@bicker>
Date:	Sun, 26 Dec 2010 01:46:58 +0300
From:	Dan Carpenter <error27@...il.com>
To:	Jesper Juhl <jj@...osbits.net>
Cc:	richard -rw- weinberger <richard.weinberger@...il.com>,
	ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, Sage Weil <sage@...dream.net>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] Ceph: Fix a use-after-free bug in
 ceph_destroy_client().

On Sat, Dec 25, 2010 at 10:24:57PM +0100, Jesper Juhl wrote:
> On Sat, 25 Dec 2010, richard -rw- weinberger wrote:
> 
> > On Sat, Dec 25, 2010 at 7:17 PM, Jesper Juhl <jj@...osbits.net> wrote:
> > > Hello,
> > >
> > > In net/ceph/ceph_common.c::ceph_destroy_client() the pointer 'client' is
> > > freed by kfree() and subsequently used in a call to dout() - use after
> > > free bug.
> > 
> > Not really. %p reads only the address of "client".
> > kfree() does not alter this address.
> > 
> 
> Ok, I see your point and you are correct. But still, the patch does not 
> change behaviour and it makes it absolutely clear that there's no 
> use-after-free bug, so it might still have merit... or?
> 

I see these with Smatch as well.  This type of usage is quite common.
People do it deliberately and I guess they feel it's readable.  Don't
change them.

If it were something that a static checker couldn't figure out, then
I'd say change it, but really the static checkers should just be made
smarter.  Some day I'm going to make Smatch complain if it's a %s in
the string instead of a %p, but for now I just ignore the false
positives.

regards,
dan carpenter
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