linux-kernel - Re: [PATCH] opp: fix a missing check on list iterator

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Date:   Mon, 28 Mar 2022 15:43:22 +0800
From:   Xiaomeng Tong <xiam0nd.tong@...il.com>
To:     viresh.kumar@...aro.org
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org, nm@...com,
        rafael.j.wysocki@...el.com, sboyd@...nel.org,
        stable@...r.kernel.org, vireshk@...nel.org, xiam0nd.tong@...il.com
Subject: Re: [PATCH] opp: fix a missing check on list iterator

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