linux-kernel - Re: [PATCH v2 1/1] uio.c: solve memory leak

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Date:	Sat, 8 Dec 2012 00:15:45 +0000
From:	Cong Ding <dinggnu@...il.com>
To:	"Hans J. Koch" <hjk@...sjkoch.de>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] uio.c: solve memory leak

On Sat, Dec 08, 2012 at 01:10:40AM +0100, Hans J. Koch wrote:
> On Fri, Dec 07, 2012 at 12:02:11AM +0100, Cong Ding wrote:
> > ping Hans, did you have any comment on this?
> 
> Sounds right what you say. Is your patch v2 your final solution, or would
> you like to come up with v3?
> 
> Thanks a lot for your patience and your thorough analysis.
If you don't have more comment or objection, it would be the final version.

Thanks

- cong
> > On Fri, Nov 30, 2012 at 12:03 PM, Cong Ding <dinggnu@...il.com> wrote:
> > > Hi Hans, I think the memory allocated with kzalloc is properly freed
> > > by calling kobject_put.
> > >
> > > I can give a simple explanation.
> > >
> > > 1)  when we call kobject_init, the parameter portio_attr_type is
> > > passed in. portio_attr_type includes a function pointer to
> > > portio_release, which releases the memory of portio.
> > >
> > > 2) when we call kobject_put, kref_put is called with the pointer of
> > > function kobject_release.
> > > 3) kref_put calls kref_sub, with the same pointer of function kobject_release.
> > > 4) and kref_put calls the function kboject_release if
> > > atomic_sub_and_test returns true
> > >
> > > 5) let's look at what kobject_release is. it calls kobject_cleanup,
> > > and kobject_cleanup calls t->release(kobj) where t->release is exactly
> > > the function we passed in through portio_init at step (1). so function
> > > portio_release is called, and the memory allocated with kzalloc is
> > > freed.
> > >
> > > If there are anything wrong in my analysis, please feel free to let me know.
> > >
> > > Personally, I suggest to add a function to create and release
> > > uio_portio, which is similar as kobject_create and kobject_put in file
> > > lib/kobject.c. In this way, it avoid other readers thinking the memory
> > > is not freed (and we should add some comments here). For example,
> > > uio_portio_create call kzalloc and kboject_init, and returns
> > > uio_portio, which is similar as function kobject_create; and
> > > uio_portio_release calls kobject_put to release the memory. And we do
> > > same thing for uio_map.
> > >
> > > The usage here is quite strange, but it works. If I write this
> > > function from zero, I will use a pointer to kobject in uio_portio
> > > struct instead of kobject struct itself. In this case I can call
> > > kobject_create instead of kobject_init, and then we do both
> > > kzalloc(uio_portio) and kfree(uio_portio) in the file uio.c.
> > >
> > > Best,
> > > Cong
> > >
> > > On Fri, Nov 30, 2012 at 1:13 AM, Hans J. Koch <hjk@...sjkoch.de> wrote:
> > >> There's still another bug: The memory allocated with kzalloc is
> > >> never freed.
> > 
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