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From:   Xiaomeng Tong <xiam0nd.tong@gmail.com>
To:     viresh.kumar@linaro.org
Cc:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, nm@ti.com,
        rafael.j.wysocki@intel.com, sboyd@kernel.org,
        stable@vger.kernel.org, vireshk@kernel.org, xiam0nd.tong@gmail.com
Subject: Re: [PATCH] opp: fix a missing check on list iterator
Date:   Mon, 28 Mar 2022 15:43:22 +0800
Message-Id: <20220328074322.25349-1-xiam0nd.tong@gmail.com>
In-Reply-To: <20220328031739.72togwws2u2rlluo@vireshk-i7>
References: <20220328031739.72togwws2u2rlluo@vireshk-i7>
Precedence: bulk

On Mon, 28 Mar 2022 08:47:39 +0530, Viresh Kumar wrote:
> > diff --git a/drivers/opp/debugfs.c b/drivers/opp/debugfs.c
> > index 596c185b5dda..a4476985e4ce 100644
> > --- a/drivers/opp/debugfs.c
> > +++ b/drivers/opp/debugfs.c
> > @@ -187,14 +187,19 @@ void opp_debug_register(struct opp_device *opp_dev, struct opp_table *opp_table)
> >  static void opp_migrate_dentry(struct opp_device *opp_dev,
> >  			       struct opp_table *opp_table)
> >  {
> > -	struct opp_device *new_dev;
> > +	struct opp_device *new_dev = NULL, *iter;
> >  	const struct device *dev;
> >  	struct dentry *dentry;
> >  
> >  	/* Look for next opp-dev */
> > -	list_for_each_entry(new_dev, &opp_table->dev_list, node)
> > -		if (new_dev != opp_dev)
> > +	list_for_each_entry(iter, &opp_table->dev_list, node)
> > +		if (iter != opp_dev) {
> > +			new_dev = iter;
> >  			break;
> > +		}
> > +
> > +	if (!new_dev)
> > +		return;
> 
> I think you missed this check in the parent function ?
> 
> 		if (!list_is_singular(&opp_table->dev_list)) {
> 
> 
> i.e. this bug can never happen.
>

No. the conditon to call opp_migrate_dentry(opp_dev, opp_table); is:
if (!list_is_singular(&opp_table->dev_list)), 

while list_is_singlular is: !list_empty(head) && (head->next == head->prev);

so the condition is: list_empty(head) || (head->next != head->prev)

if the list is empty, the bug can be triggered.

--
Xiaomeng Tong