netdev - Re: [PATCH net-next 2/3] tcp: ulp: add functions to dump ulp-specific information

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Message-ID: <b765aa08456ef258615a46e7ff106703a240ddb5.camel@redhat.com>
Date:   Mon, 19 Aug 2019 15:32:09 +0200
From:   Davide Caratti <dcaratti@...hat.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Boris Pismenny <borisp@...lanox.com>,
        John Fastabend <john.fastabend@...il.com>,
        Dave Watson <davejwatson@...com>,
        Aviad Yehezkel <aviadye@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 2/3] tcp: ulp: add functions to dump
 ulp-specific information

On Thu, 2019-08-15 at 14:38 -0700, Jakub Kicinski wrote:
> On Thu, 15 Aug 2019 20:46:01 +0200, Eric Dumazet wrote:

hello Eric and Jakub, thanks a lot for looking at this.

> > On 8/15/19 6:00 PM, Davide Caratti wrote:
> > 
> > >  
> > > +	if (net_admin) {
> > > +		const struct tcp_ulp_ops *ulp_ops;
> > > +
> > > +		rcu_read_lock();
> > > +		ulp_ops = icsk->icsk_ulp_ops;
> > > +		if (ulp_ops)
> > > +			err = tcp_diag_put_ulp(skb, sk, ulp_ops);
> > > +		rcu_read_unlock();
> > > +		if (err)
> > > +			return err;
> > > +	}
> > >  	return 0;  
> > 
> > Why is rcu_read_lock() and rcu_read_unlock() used at all ?
> > 
> > icsk->icsk_ulp_ops does not seem to be rcu protected ?
> > 
> > If this was, then an rcu_dereference() would be appropriate.
> 
> Indeed it's ulp_data not ulp_ops that are protected. 

the goal is to protect execution of 'ss -tni' against concurrent removal
of tls.ko module, similarly to what was done in inet_sk_diag_fill() when
INET_DIAG_CONG is requested [1]. But after reading more carefully, the
assignment of ulp_ops needs to be:

	ulp_ops = READ_ONCE(icsk->icsk_ulp_ops);

which I lost in internal reviews, with some additional explanatory
comment. Ok if I correct the above hunk with READ_ONCE() and add a
comment?

> Davide, perhaps we could push the RCU lock into tls_get_info(), after all?

It depends on whether concurrent dump / module removal is an issue for TCP
ULPs, like it was for congestion control schemes [1]. Any advice?

> And tls_context has to use rcu_deference there, as Eric points out, 
> plus we should probably NULL-check it.

yes, it makes sense, for patch 3/3, in the assignment of 'ctx'. Instead of
calling tls_get_ctx() in tls_get_info() I will do

	ctx = rcu_dereference(inet_csk(sk)->icsk_ulp_data);

and let it return 0 in case of NULL ctx (as it doesn't look like a faulty
situation). Ok? 

-- 
davide

[1] see:
commit 521f1cf1dbb9d5ad858dca5dc75d1b45f64b6589
Author: Eric Dumazet <edumazet@...gle.com>
Date:   Thu Apr 16 18:10:35 2015 -0700

    inet_diag: fix access to tcp cc information