Re: [PATCH v2 1/3] virtio-rng cleanup: move some code out of mutex protection

From: Amos Kong <akong@redhat.com>
To: "Michael Büsch" <m@bues.ch>
Cc: herbert@gondor.apana.org.au, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org, mpm@selenic.com,
	amit.shah@redhat.com
Subject: Re: [PATCH v2 1/3] virtio-rng cleanup: move some code out of mutex protection
Date: Tue, 16 Sep 2014 08:30:31 +0800	[thread overview]
Message-ID: <20140916003031.GB5671@zen> (raw)
In-Reply-To: <20140915181320.28f642d6@wiggum>

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On Mon, Sep 15, 2014 at 06:13:20PM +0200, Michael Büsch wrote:
> On Tue, 16 Sep 2014 00:02:27 +0800
> Amos Kong <akong@redhat.com> wrote:
> 
> > It doesn't save too much cpu time as expected, just a cleanup.
> > 
> > Signed-off-by: Amos Kong <akong@redhat.com>
> > ---
> >  drivers/char/hw_random/core.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> > index aa30a25..c591d7e 100644
> > --- a/drivers/char/hw_random/core.c
> > +++ b/drivers/char/hw_random/core.c
> > @@ -270,8 +270,8 @@ static ssize_t hwrng_attr_current_show(struct device *dev,
> >  		return -ERESTARTSYS;
> >  	if (current_rng)
> >  		name = current_rng->name;
> > -	ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
> >  	mutex_unlock(&rng_mutex);
> > +	ret = snprintf(buf, PAGE_SIZE, "%s\n", name);
> 
> I'm not sure this is safe.
> Name is just a pointer.
> What if the hwrng gets unregistered after unlock and just before the snprintf?

Oh, it points to protected current_rng->name, I will drop this
cleanup. Thanks.

> >  	return ret;
> >  }
> > @@ -284,19 +284,19 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
> >  	ssize_t ret = 0;
> >  	struct hwrng *rng;
> >  
> > +	buf[0] = '\0';
> >  	err = mutex_lock_interruptible(&rng_mutex);
> >  	if (err)
> >  		return -ERESTARTSYS;
> > -	buf[0] = '\0';
> >  	list_for_each_entry(rng, &rng_list, list) {
> >  		strncat(buf, rng->name, PAGE_SIZE - ret - 1);
> >  		ret += strlen(rng->name);
> >  		strncat(buf, " ", PAGE_SIZE - ret - 1);
> >  		ret++;
> >  	}
> > +	mutex_unlock(&rng_mutex);
> >  	strncat(buf, "\n", PAGE_SIZE - ret - 1);
> >  	ret++;
> > -	mutex_unlock(&rng_mutex);
> >  
> >  	return ret;
> >  }
> 
> This looks ok.
> 
> -- 
> Michael

-- 
			Amos.

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