Re: [PATCH v7 bpf-next 03/11] tcp: Keep TCP_CLOSE sockets in the reuseport group.

[Eric Dumazet <eric.dumazet@gmail.com>]

On 5/21/21 8:20 PM, Kuniyuki Iwashima wrote:
> When we close a listening socket, to migrate its connections to another
> listener in the same reuseport group, we have to handle two kinds of child
> sockets. One is that a listening socket has a reference to, and the other
> is not.
> 
> The former is the TCP_ESTABLISHED/TCP_SYN_RECV sockets, and they are in the
> accept queue of their listening socket. So we can pop them out and push
> them into another listener's queue at close() or shutdown() syscalls. On
> the other hand, the latter, the TCP_NEW_SYN_RECV socket is during the
> three-way handshake and not in the accept queue. Thus, we cannot access
> such sockets at close() or shutdown() syscalls. Accordingly, we have to
> migrate immature sockets after their listening socket has been closed.
> 
> Currently, if their listening socket has been closed, TCP_NEW_SYN_RECV
> sockets are freed at receiving the final ACK or retransmitting SYN+ACKs. At
> that time, if we could select a new listener from the same reuseport group,
> no connection would be aborted. However, we cannot do that because
> reuseport_detach_sock() sets NULL to sk_reuseport_cb and forbids access to
> the reuseport group from closed sockets.
> 
> This patch allows TCP_CLOSE sockets to remain in the reuseport group and
> access it while any child socket references them. The point is that
> reuseport_detach_sock() was called twice from inet_unhash() and
> sk_destruct(). This patch replaces the first reuseport_detach_sock() with
> reuseport_stop_listen_sock(), which checks if the reuseport group is
> capable of migration. If capable, it decrements num_socks, moves the socket
> backwards in socks[] and increments num_closed_socks. When all connections
> are migrated, sk_destruct() calls reuseport_detach_sock() to remove the
> socket from socks[], decrement num_closed_socks, and set NULL to
> sk_reuseport_cb.
> 
> By this change, closed or shutdowned sockets can keep sk_reuseport_cb.
> Consequently, calling listen() after shutdown() can cause EADDRINUSE or
> EBUSY in inet_csk_bind_conflict() or reuseport_add_sock() which expects
> such sockets not to have the reuseport group. Therefore, this patch also
> loosens such validation rules so that a socket can listen again if it has a
> reuseport group with num_closed_socks more than 0.
> 
> When such sockets listen again, we handle them in reuseport_resurrect(). If
> there is an existing reuseport group (reuseport_add_sock() path), we move
> the socket from the old group to the new one and free the old one if
> necessary. If there is no existing group (reuseport_alloc() path), we
> allocate a new reuseport group, detach sk from the old one, and free it if
> necessary, not to break the current shutdown behaviour:
> 
>   - we cannot carry over the eBPF prog of shutdowned sockets
>   - we cannot attach/detach an eBPF prog to/from listening sockets via
>     shutdowned sockets
> 
> Note that when the number of sockets gets over U16_MAX, we try to detach a
> closed socket randomly to make room for the new listening socket in
> reuseport_grow().
> 
> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
> Signed-off-by: Martin KaFai Lau <kafai@fb.com>
> ---
>  include/net/sock_reuseport.h    |   1 +
>  net/core/sock_reuseport.c       | 184 ++++++++++++++++++++++++++++++--
>  net/ipv4/inet_connection_sock.c |  12 ++-
>  net/ipv4/inet_hashtables.c      |   2 +-
>  4 files changed, 188 insertions(+), 11 deletions(-)
> 
> diff --git a/include/net/sock_reuseport.h b/include/net/sock_reuseport.h
> index 0e558ca7afbf..1333d0cddfbc 100644
> --- a/include/net/sock_reuseport.h
> +++ b/include/net/sock_reuseport.h
> @@ -32,6 +32,7 @@ extern int reuseport_alloc(struct sock *sk, bool bind_inany);
>  extern int reuseport_add_sock(struct sock *sk, struct sock *sk2,
>  			      bool bind_inany);
>  extern void reuseport_detach_sock(struct sock *sk);
> +void reuseport_stop_listen_sock(struct sock *sk);
>  extern struct sock *reuseport_select_sock(struct sock *sk,
>  					  u32 hash,
>  					  struct sk_buff *skb,
> diff --git a/net/core/sock_reuseport.c b/net/core/sock_reuseport.c
> index 079bd1aca0e7..ea0e900d3e97 100644
> --- a/net/core/sock_reuseport.c
> +++ b/net/core/sock_reuseport.c
> @@ -17,6 +17,8 @@
>  DEFINE_SPINLOCK(reuseport_lock);
>  
>  static DEFINE_IDA(reuseport_ida);
> +static int reuseport_resurrect(struct sock *sk, struct sock_reuseport *old_reuse,
> +			       struct sock_reuseport *reuse, bool bind_inany);
>  
>  static int reuseport_sock_index(struct sock *sk,
>  				struct sock_reuseport *reuse,
> @@ -61,6 +63,29 @@ static bool __reuseport_detach_sock(struct sock *sk,
>  	return true;
>  }
>  
> +static void __reuseport_add_closed_sock(struct sock *sk,
> +					struct sock_reuseport *reuse)
> +{
> +	reuse->socks[reuse->max_socks - reuse->num_closed_socks - 1] = sk;
> +	/* paired with READ_ONCE() in inet_csk_bind_conflict() */
> +	WRITE_ONCE(reuse->num_closed_socks, reuse->num_closed_socks + 1);
> +}
> +
> +static bool __reuseport_detach_closed_sock(struct sock *sk,
> +					   struct sock_reuseport *reuse)
> +{
> +	int i = reuseport_sock_index(sk, reuse, true);
> +
> +	if (i == -1)
> +		return false;
> +
> +	reuse->socks[i] = reuse->socks[reuse->max_socks - reuse->num_closed_socks];
> +	/* paired with READ_ONCE() in inet_csk_bind_conflict() */
> +	WRITE_ONCE(reuse->num_closed_socks, reuse->num_closed_socks - 1);
> +
> +	return true;
> +}
> +
>  static struct sock_reuseport *__reuseport_alloc(unsigned int max_socks)
>  {
>  	unsigned int size = sizeof(struct sock_reuseport) +
> @@ -92,6 +117,14 @@ int reuseport_alloc(struct sock *sk, bool bind_inany)
>  	reuse = rcu_dereference_protected(sk->sk_reuseport_cb,
>  					  lockdep_is_held(&reuseport_lock));
>  	if (reuse) {
> +		if (reuse->num_closed_socks) {
> +			/* sk was shutdown()ed before */
> +			int err = reuseport_resurrect(sk, reuse, NULL, bind_inany);
> +
> +			spin_unlock_bh(&reuseport_lock);
> +			return err;

It seems coding style in this function would rather do
			ret = reuseport_resurrect(sk, reuse, NULL, bind_inany);
			goto out;

Overall, changes in this commit are a bit scarry.