linux-kernel - Re: [PATCH] i2c: tegra: fix a possible NULL dereference

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Date:	Thu, 12 Nov 2015 14:45:20 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	LABBE Corentin <montjoie.mailing@...il.com>,
	LABBE Corentin <clabbe.montjoie@...il.com>, gnurou@...il.com,
	ldewangan@...dia.com, swarren@...dotorg.org, wsa@...-dreams.de,
	linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-tegra@...r.kernel.org
Subject: Re: [PATCH] i2c: tegra: fix a possible NULL dereference

On Thu, Nov 12, 2015 at 02:28:37PM +0100, Thierry Reding wrote:
> On Thu, Nov 12, 2015 at 01:54:22PM +0100, LABBE Corentin wrote:
> > On Thu, Nov 12, 2015 at 01:29:23PM +0100, Thierry Reding wrote:
> > > On Thu, Nov 12, 2015 at 08:26:03AM +0100, LABBE Corentin wrote:
> > > > of_match_device could return NULL, and so cause a NULL pointer
> > > 
> > > No. There is no way that of_match_device() can ever fail. The driver
> > > core uses the same table to match the OF device to the driver, so the
> > > only case where of_match_device() would return NULL is if no match was
> > > found, in which case the tegra_i2c_probe() function would never have
> > > been called in the first place.
> > > 
> > > Thierry
> > > 
> > 
> > In a parallel thread for i2c-rcar, the conclusion was different.
> > https://lkml.org/lkml/2015/11/12/83
> 
> The conclusion was the same: there should be no case where this happens.
> The example that Uwe gave is hypothetical and not valid DT in the first
> place. So instead of chickening out I think it'd be better to just crash
> to make sure people fix the DT.

It depends in your trust in the DT. Just because it's not advisable to
do something that is not documented usually isn't a good excuse to not
handle broken input. That't the case for webserver requests, arguments
to system calls and several more. I admit DT is a bit special because
you have to assume it's trusted, but still handling errors in a sane way
is IMHO nice.

> On a side-note I think that platform_match() should be stricter and do
> something like this instead:
> 
> 	if (dev->of_node) {
> 		if (of_driver_match_device(dev, drv))
> 			return 1;
> 
> 		return 0;
> 	}
That's equivalent to

	if (dev->of_node)
		return of_driver_match_device(dev, drv);

and was already suggested in the thread referenced from my reply to
http://article.gmane.org/gmane.linux.kernel/2083641 :-)

Best regards
Uwe 

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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