linux-kernel - RE: [PATCH 1/8] vfio/mdev: Fix to not do put_device on device

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Date:   Mon, 25 Mar 2019 21:11:58 +0000
From:   Parav Pandit <parav@...lanox.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        Kirti Wankhede <kwankhede@...dia.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/8] vfio/mdev: Fix to not do put_device on
 device_register failure



> -----Original Message-----
> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Monday, March 25, 2019 2:21 PM
> To: Kirti Wankhede <kwankhede@...dia.com>
> Cc: Parav Pandit <parav@...lanox.com>; kvm@...r.kernel.org; linux-
> kernel@...r.kernel.org
> Subject: Re: [PATCH 1/8] vfio/mdev: Fix to not do put_device on
> device_register failure
> 
> On Mon, 25 Mar 2019 23:47:30 +0530
> Kirti Wankhede <kwankhede@...dia.com> wrote:
> 
> > On 3/23/2019 4:50 AM, Parav Pandit wrote:
> > > device_register() performs put_device() if device_add() fails.
> > > This balances with device_initialize().
> > >
> > > mdev core performing put_device() when device_register() fails, is
> > > an error that puts already released device again.
> > > Therefore, don't put the device on error.
> > >
> >
> > device_add() on all errors doesn't call put_device(dev). It releases
> > reference to its parent, put_device(parent), but not the device
> > itself, put_device(dev).
> 
> Sort of, device_initialize() initializes the reference count to 1,
> device_add() increments the reference count to 2 via the get_device() and
> then drops it back to 1 on all exit paths.  The oddity is the failure path of
> get_device() itself, but that can only happen if passed a NULL device, where
> put_device() is a no-op and not relevant here.  So in all cases
> device_register() returns with a reference count of 1 and we need to call
> put_device() to free the allocated object.  The below change would leak the
> mdev on error.  Thanks,
> 
Yes.
I missed the NOTE at the starting of device_add() comment block.
I will drop this patch in series.

> Alex
> 
> > > Fixes: 7b96953bc640 ("vfio: Mediated device Core driver")
> > > Signed-off-by: Parav Pandit <parav@...lanox.com>
> > > ---
> > >  drivers/vfio/mdev/mdev_core.c | 4 +---
> > >  1 file changed, 1 insertion(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/vfio/mdev/mdev_core.c
> > > b/drivers/vfio/mdev/mdev_core.c index 0212f0e..3e5880a 100644
> > > --- a/drivers/vfio/mdev/mdev_core.c
> > > +++ b/drivers/vfio/mdev/mdev_core.c
> > > @@ -318,10 +318,8 @@ int mdev_device_create(struct kobject *kobj,
> struct device *dev, uuid_le uuid)
> > >  	dev_set_name(&mdev->dev, "%pUl", uuid.b);
> > >
> > >  	ret = device_register(&mdev->dev);
> > > -	if (ret) {
> > > -		put_device(&mdev->dev);
> > > +	if (ret)
> > >  		goto mdev_fail;
> > > -	}
> > >
> > >  	ret = mdev_device_create_ops(kobj, mdev);
> > >  	if (ret)
> > >