netdev - Re: [PATCH net-next 18/32] netfilter: egress: avoid a lockdep splat

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Message-ID: <20220228023246.GA26547@breakpoint.cc>
Date:   Mon, 28 Feb 2022 03:32:46 +0100
From:   Florian Westphal <fw@...len.de>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        netfilter-devel@...r.kernel.org, davem@...emloft.net,
        netdev@...r.kernel.org, kuba@...nel.org
Subject: Re: [PATCH net-next 18/32] netfilter: egress: avoid a lockdep splat

Eric Dumazet <eric.dumazet@...il.com> wrote:
> > index b71b57a83bb4..b4dd96e4dc8d 100644
> > --- a/include/linux/netfilter_netdev.h
> > +++ b/include/linux/netfilter_netdev.h
> > @@ -94,7 +94,7 @@ static inline struct sk_buff *nf_hook_egress(struct sk_buff *skb, int *rc,
> >   		return skb;
> >   #endif
> > -	e = rcu_dereference(dev->nf_hooks_egress);
> > +	e = rcu_dereference_check(dev->nf_hooks_egress, rcu_read_lock_bh_held());
> >   	if (!e)
> >   		return skb;
> 
> 
> It seems other rcu_dereference() uses will also trigger lockdep splat.
 
> if (genbit)
>     blob = rcu_dereference(chain->blob_gen_1);
> else
>    blob = rcu_dereference(chain->blob_gen_0);
> 
> I wonder how many other places will need a fix ?

I don't like that, all nf hooks assume rcu_read_lock is held.
nf_hook_egress() call to nf_hook_slow() needs to be wrapped
in rcu_read_(un)lock pair.