linux-kernel - Re: [PATCH] spi/pxa2xx: Clear cur_chip pointer before starting next message

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Date:	Fri, 5 Dec 2014 10:24:29 +0200
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	Robert Jarzmik <robert.jarzmik@...e.fr>
Cc:	linux-spi@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	Daniel Mack <daniel@...que.org>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	Martin Oldfield <m@...ldfield.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] spi/pxa2xx: Clear cur_chip pointer before starting next
 message

On Thu, Dec 04, 2014 at 10:01:06PM +0100, Robert Jarzmik wrote:
> Mika Westerberg <mika.westerberg@...ux.intel.com> writes:
> 
> > Once the current message is finished, the driver notifies SPI core about
> > this by calling spi_finalize_current_message(). This function queues next
> > message to be transferred. If there are more messages in the queue, it is
> > possible that the driver is asked to transfer the next message at this
> > point.
> >
> > When spi_finalize_current_message() returns the driver clears the
> > drv_data->cur_chip pointer to NULL. The problem is that if the driver
> > already started the next message clearing drv_data->cur_chip will cause
> > NULL pointer dereference which crashes the kernel like:
> ..zip..
> > Fix this by clearing drv_data->cur_chip before we call
> > spi_finalize_current_message().
> 
> So with your change, we have :
>   	drv_data->cur_chip = NULL;
> 	spi_finalize_current_message(drv_data->master);
> 
> In that case, if spi_finalize_current_message() queues another message, upon
> this next message completion, won't giveback() be called, and dereference
> cur_chip as well ?

When the next message is started pxa2xx_spi_transfer_one_message() gets
called and that will set cur_chip again.
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