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

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Date:	Fri, 30 Nov 2012 12:03:45 +0100
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

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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