netdev - Re: [RFC PATCH net-next v2 10/15] bpf: Add support for changing congestion control

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Date:   Tue, 20 Jun 2017 00:35:16 +0000
From:   Lawrence Brakmo <brakmo@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>
CC:     Kernel Team <Kernel-team@...com>, Blake Matheny <bmatheny@...com>,
        "Alexei Starovoitov" <ast@...com>,
        David Ahern <dsa@...ulusnetworks.com>
Subject: Re: [RFC PATCH net-next v2 10/15] bpf: Add support for changing
 congestion control


On 6/19/17, 3:34 PM, "Daniel Borkmann" <daniel@...earbox.net> wrote:

    On 06/18/2017 04:39 AM, Lawrence Brakmo wrote:
    > On 6/16/17, 6:58 AM, "Daniel Borkmann" <daniel@...earbox.net> wrote:
    [...]
    >      >   /* Change congestion control for socket */
    >      > -int tcp_set_congestion_control(struct sock *sk, const char *name)
    >      > +int tcp_set_congestion_control(struct sock *sk, const char *name, bool load)
    >      >   {
    >      >   	struct inet_connection_sock *icsk = inet_csk(sk);
    >      >   	const struct tcp_congestion_ops *ca;
    >      > @@ -344,7 +344,10 @@ int tcp_set_congestion_control(struct sock *sk, const char *name)
    >      >   		return -EPERM;
    >      >
    >      >   	rcu_read_lock();
    >      > -	ca = __tcp_ca_find_autoload(name);
    >      > +	if (!load)
    >      > +		ca = tcp_ca_find(name);
    >      > +	else
    >      > +		ca = __tcp_ca_find_autoload(name);
    >
    >       From BPF program side, we call with !load since we're not allowed
    >      to sleep under RCU, that's correct ...
    >
    >      >   	/* No change asking for existing value */
    >      >   	if (ca == icsk->icsk_ca_ops) {
    >      >   		icsk->icsk_ca_setsockopt = 1;
    >      > @@ -352,8 +355,10 @@ int tcp_set_congestion_control(struct sock *sk, const char *name)
    >      >   	}
    >      >   	if (!ca)
    >      >   		err = -ENOENT;
    >      > +	else if (!load)
    >      > +		icsk->icsk_ca_ops = ca;
    >
    >      ... but don't we also need to hold a module ref in this case as done
    >      below?
    >
    >      Meaning, tcp_ca_find() could return a ca that was previously loaded
    >      to the tcp_cong_list as module, then resulting in ref count imbalance
    >      when set from BPF?
    >
    > As I mentioned above, this can be called before congestion has been
    > initialized (op <= BPF_SOCKET_OPS_NEEDS_ECN) in which case
    > tcp_init_congestion_control will be called later. If op > ..OPS_NEEDS_ECN
    > then bpf_setsockopt() will call the reinit_congestion_control().
    >
    > But this points to an issue where someone else could call
    > tcp_set_congestion_control() with load == false not knowing they
    > need to call either init or reinit. I will add a comment to the function
    > to make it clear.
    
    Hm, I'm not sure it answers my question. What I meant was that from BPF
    prog, you're setting tcp_set_congestion_control(..., false) so if
    tcp_ca_find() returns a ca that was loaded earlier as a from a module
    (so it becomes available in tcp_cong_list), the above...
    
       [...]
       else if (!load)
           icsk->icsk_ca_ops = ca;
       [...]
    
    ... will basically prevent the later try_module_get() on the ca. So any
    later tcp_reinit_congestion_control() or tcp_init_congestion_control()
    will still run not having the refcount held on the owner module. Meaning
    a module unload would let the machine crash due to the refcnt imbalance?
    What am I missing?

Nothing, you are correct. I was mistakenly thinking that the refcount update
was being done in tcp_init_congestion_control. Done.