Date: Thu, 30 Apr 2009 13:18:18 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Simon Arlott <simon@fire.lp0.eu>
Cc: linux-kernel@vger.kernel.org, mchehab@infradead.org, linux-dvb@linuxtv.org
Subject: Re: [PATCH] dvb-core: Fix potential mutex_unlock without mutex_lock
 in dvb_dvr_read
Message-Id: <20090430131818.d8aded42.akpm@linux-foundation.org>
In-Reply-To: <49F0A61D.1010002@simon.arlott.org.uk>
References: <49F0A61D.1010002@simon.arlott.org.uk>
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On Thu, 23 Apr 2009 18:32:13 +0100
Simon Arlott <simon@fire.lp0.eu> wrote:

> dvb_dvr_read may unlock the dmxdev mutex and return -ENODEV,
> except this function is a file op and will never be called
> with the mutex held.
> 
> There's existing mutex_lock and mutex_unlock around the actual
> read but it's commented out. These should probably be uncommented
> but the read blocks and this could block another non-blocking
> reader on the mutex instead.
> 
> This change comments out the extra mutex_unlock.
> 
> Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
> ---
> This has been on my TODO list for far too long... I did come
> up with a mutex_trylock/mutex_lock_interruptible version but
> claiming that it'll block when it may not doesn't make sense
> (and any blocking read would cause all non-blocking reads to
> continually return -EWOULDBLOCK until there is data).
> 
>  drivers/media/dvb/dvb-core/dmxdev.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/media/dvb/dvb-core/dmxdev.c b/drivers/media/dvb/dvb-core/dmxdev.c
> index c35fbb8..d6d098a 100644
> --- a/drivers/media/dvb/dvb-core/dmxdev.c
> +++ b/drivers/media/dvb/dvb-core/dmxdev.c
> @@ -247,7 +247,7 @@ static ssize_t dvb_dvr_read(struct file *file, char __user *buf, size_t count,
>  	int ret;
>  
>  	if (dmxdev->exit) {
> -		mutex_unlock(&dmxdev->mutex);
> +		//mutex_unlock(&dmxdev->mutex);
>  		return -ENODEV;
>  	}

Is there any value in retaining all the commented-out lock operations,
or can we zap 'em?

--- a/drivers/media/dvb/dvb-core/dmxdev.c~dvb-core-fix-potential-mutex_unlock-without-mutex_lock-in-dvb_dvr_read-fix
+++ a/drivers/media/dvb/dvb-core/dmxdev.c
@@ -244,19 +244,13 @@ static ssize_t dvb_dvr_read(struct file 
 {
 	struct dvb_device *dvbdev = file->private_data;
 	struct dmxdev *dmxdev = dvbdev->priv;
-	int ret;
 
-	if (dmxdev->exit) {
-		//mutex_unlock(&dmxdev->mutex);
+	if (dmxdev->exit)
 		return -ENODEV;
-	}
 
-	//mutex_lock(&dmxdev->mutex);
-	ret = dvb_dmxdev_buffer_read(&dmxdev->dvr_buffer,
-				     file->f_flags & O_NONBLOCK,
-				     buf, count, ppos);
-	//mutex_unlock(&dmxdev->mutex);
-	return ret;
+	return dvb_dmxdev_buffer_read(&dmxdev->dvr_buffer,
+				      file->f_flags & O_NONBLOCK,
+				      buf, count, ppos);
 }
 
 static int dvb_dvr_set_buffer_size(struct dmxdev *dmxdev,
_

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