linux-kernel - Re: [PATCH] NFC: Driver for Inside Secure MicroRead NFC chip

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Date:	Fri, 18 Mar 2011 15:20:14 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Cc:	Waldemar Rymarkiewicz <waldemar.rymarkiewicz@...to.com>,
	linux-i2c@...r.kernel.org, sameo@...ux.intel.com,
	linux-kernel@...r.kernel.org, hthebaud@...idefr.com,
	matti.j.aaltonen@...ia.com, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] NFC: Driver for Inside Secure MicroRead NFC chip

On Friday 18 March 2011, Mark Brown wrote:
> On Fri, Mar 18, 2011 at 01:19:53PM +0100, Arnd Bergmann wrote:
> > On Friday 18 March 2011, Waldemar Rymarkiewicz wrote:
> 
> > > +
> > > +   mutex_lock(&info->rx_mutex);
> > > +   info->irq_state = 1;
> > > +   mutex_unlock(&info->rx_mutex);
> > > +
> > > +   wake_up_interruptible(&info->rx_waitq);
> > > +
> > > +   return IRQ_HANDLED;
> > > +}
> 
> > You cannot take a mutex from interrupt context, that may
> > cause deadlocks.
> 
> This is a threaded IRQ handler so mutexes are fine.

Ah, right. I've never seen one of these used in the field,
so I didn't think of this.

Looking at the mutexes though: The read function does
not hold the rx_mutex when reading the irq_state
variable, so that is potentially racy.

The read function seems to have another problem regarding
the user space buffer: it bails out if the provided buffer
is larger than the available data, which is pointless,
but it does not check if the user buffer is too short.

	Arnd
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