netdev - Re: [PATCH RESEND net] bonding: fix missed rcu protection

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Date:   Tue, 17 May 2022 09:24:00 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Hangbin Liu <liuhangbin@...il.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, Jay Vosburgh <j.vosburgh@...il.com>,
        Veaceslav Falico <vfalico@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>,
        "David S . Miller" <davem@...emloft.net>,
        David Ahern <dsahern@...il.com>,
        Jonathan Toppins <jtoppins@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        syzbot+92beb3d46aab498710fa@...kaller.appspotmail.com,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH RESEND net] bonding: fix missed rcu protection

On Tue, 2022-05-17 at 11:42 +0800, Hangbin Liu wrote:
> On Mon, May 16, 2022 at 06:10:28PM -0700, Jakub Kicinski wrote:
> > Can't ->get_ts_info sleep now? It'd be a little sad to force it 
> > to be atomic just because of one upper dev trying to be fancy.
> > Maybe all we need to do is to take a ref on the real_dev?
> 
> Do you mean
> 
> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> index 38e152548126..b60450211579 100644
> --- a/drivers/net/bonding/bond_main.c
> +++ b/drivers/net/bonding/bond_main.c
> @@ -5591,16 +5591,20 @@ static int bond_ethtool_get_ts_info(struct net_device *bond_dev,
>  	const struct ethtool_ops *ops;
>  	struct net_device *real_dev;
>  	struct phy_device *phydev;
> +	int ret = 0;
>  

You additionally need something alike the following:

	rcu_read_lock();
>  	real_dev = bond_option_active_slave_get_rcu(bond);
>  	if (real_dev) {
> +		dev_hold(real_dev)
		rcu_read_unlock();

>  		ops = real_dev->ethtool_ops;
>  		phydev = real_dev->phydev;
>  
>  		if (phy_has_tsinfo(phydev)) {
> -			return phy_ts_info(phydev, info);
> +			ret = phy_ts_info(phydev, info);
> +			goto out;
>  		} else if (ops->get_ts_info) {
> -			return ops->get_ts_info(real_dev, info);
> +			ret = ops->get_ts_info(real_dev, info);
> +			goto out;
>  		}
	} else {
		rcu_read_unlock();
>  	}

... or you will hit the initial RCU splat. Overall this will not put
atomicy constraint on get_ts_info.

Cheers,

Paol

>  
> @@ -5608,7 +5612,10 @@ static int bond_ethtool_get_ts_info(struct net_device *bond_dev,
>  				SOF_TIMESTAMPING_SOFTWARE;
>  	info->phc_index = -1;
>  
> -	return 0;
> +out:
> +	if (real_dev)
> +		dev_put(real_dev);
> +	return ret;
>  }
> 
> 
> This look OK to me.
> 
> Vladimir, Jay, WDYT?
> 
> > 
> > Also please add a Link: to the previous discussion, it'd have been
> > useful to get the context in which Vladimir suggested this.
> 
> OK, I will.
> 
> Thanks
> Hangbin
>