RE: [RFC PATCH] misc: xilinx-sdfec: Check if file->private_data is NULL

[Dragan Cvetic <draganc@xilinx.com>]

> -----Original Message-----
> From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Sent: Friday 21 May 2021 19:40
> To: Guenter Roeck <linux@roeck-us.net>
> Cc: Derek Kiernan <dkiernan@xilinx.com>; Dragan Cvetic <draganc@xilinx.com>; Arnd Bergmann <arnd@arndb.de>; Michal Simek
> <michals@xilinx.com>; linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org
> Subject: Re: [RFC PATCH] misc: xilinx-sdfec: Check if file->private_data is NULL
> 
> On Thu, May 20, 2021 at 10:01:50AM -0700, Guenter Roeck wrote:
> > container_of() only returns NULL if the passed pointer is NULL _and_
> > the embedded element is the first element of the structure. Even if that
> > is the case, testing against it is misleading and possibly dangerous
> > because the position of the embedded element may change. Explicitly
> > check if the parameter is NULL and bail out if so instead of checking
> > the result of container_of().
> >
> > Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> > ---
> > RFC:
> >
> > The NULL check in the poll function is likely unnecessary. Interestingly,
> > there is no NULL check in the ioctl function, even though there is a
> > similar container_of() in that function. However, I do not feel
> > comfortable enough to change the functionality of this code and drop
> > the check entirely.
> >
> >  drivers/misc/xilinx_sdfec.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/misc/xilinx_sdfec.c b/drivers/misc/xilinx_sdfec.c
> > index 23c8448a9c3b..0a3721d31dea 100644
> > --- a/drivers/misc/xilinx_sdfec.c
> > +++ b/drivers/misc/xilinx_sdfec.c
> > @@ -1011,11 +1011,11 @@ static __poll_t xsdfec_poll(struct file *file, poll_table *wait)
> >  	__poll_t mask = 0;
> >  	struct xsdfec_dev *xsdfec;
> >
> > -	xsdfec = container_of(file->private_data, struct xsdfec_dev, miscdev);
> > -
> > -	if (!xsdfec)
> > +	if (!file->private_data)
> >  		return EPOLLNVAL | EPOLLHUP;
> >
> > +	xsdfec = container_of(file->private_data, struct xsdfec_dev, miscdev);
> > +
> >  	poll_wait(file, &xsdfec->waitq, wait);
> >
> >  	/* XSDFEC ISR detected an error */
> > --
> > 2.25.1
> >
> 
> 
> It should be safe not to check this for NULL as the misc device sets the
> pointer in the open, and removes it in release.  poll or ioctl can not
> be called if release has already happened.
> 
> So feel free to drop the check here, xsdfec_dev_ioctl() looks correct.
> 
> thanks,
> 
> greg k-h

Acked-by: Dragan Cvetic <dragan.cvetic@xilinx.com>

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