Re: [PATCH v7 bpf-next 00/11] Socket migration for SO_REUSEPORT.

[Kuniyuki Iwashima <kuniyu@amazon.co.jp>]

From:   Yuchung Cheng <ycheng@google.com>
Date:   Tue, 8 Jun 2021 10:48:06 -0700
> On Tue, May 25, 2021 at 11:42 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
> >
> > On 5/21/21 8:20 PM, Kuniyuki Iwashima wrote:
> > > The SO_REUSEPORT option allows sockets to listen on the same port and to
> > > accept connections evenly. However, there is a defect in the current
> > > implementation [1]. When a SYN packet is received, the connection is tied
> > > to a listening socket. Accordingly, when the listener is closed, in-flight
> > > requests during the three-way handshake and child sockets in the accept
> > > queue are dropped even if other listeners on the same port could accept
> > > such connections.
> > >
> > > This situation can happen when various server management tools restart
> > > server (such as nginx) processes. For instance, when we change nginx
> > > configurations and restart it, it spins up new workers that respect the new
> > > configuration and closes all listeners on the old workers, resulting in the
> > > in-flight ACK of 3WHS is responded by RST.
> > >
> > > To avoid such a situation, users have to know deeply how the kernel handles
> > > SYN packets and implement connection draining by eBPF [2]:
> > >
> > >    1. Stop routing SYN packets to the listener by eBPF.
> > >    2. Wait for all timers to expire to complete requests
> > >    3. Accept connections until EAGAIN, then close the listener.
> > >
> > >    or
> > >
> > >    1. Start counting SYN packets and accept syscalls using the eBPF map.
> > >    2. Stop routing SYN packets.
> > >    3. Accept connections up to the count, then close the listener.
> > >
> > > In either way, we cannot close a listener immediately. However, ideally,
> > > the application need not drain the not yet accepted sockets because 3WHS
> > > and tying a connection to a listener are just the kernel behaviour. The
> > > root cause is within the kernel, so the issue should be addressed in kernel
> > > space and should not be visible to user space. This patchset fixes it so
> > > that users need not take care of kernel implementation and connection
> > > draining. With this patchset, the kernel redistributes requests and
> > > connections from a listener to the others in the same reuseport group
> > > at/after close or shutdown syscalls.
> > >
> > > Although some software does connection draining, there are still merits in
> > > migration. For some security reasons, such as replacing TLS certificates,
> > > we may want to apply new settings as soon as possible and/or we may not be
> > > able to wait for connection draining. The sockets in the accept queue have
> > > not started application sessions yet. So, if we do not drain such sockets,
> > > they can be handled by the newer listeners and could have a longer
> > > lifetime. It is difficult to drain all connections in every case, but we
> > > can decrease such aborted connections by migration. In that sense,
> > > migration is always better than draining.
> > >
> > > Moreover, auto-migration simplifies user space logic and also works well in
> > > a case where we cannot modify and build a server program to implement the
> > > workaround.
> > >
> > > Note that the source and destination listeners MUST have the same settings
> > > at the socket API level; otherwise, applications may face inconsistency and
> > > cause errors. In such a case, we have to use the eBPF program to select a
> > > specific listener or to cancel migration.
> This looks to be a useful feature. What happens to migrating a
> passively fast-opened socket in the old listener but it has not yet
> been accepted (TFO is both a mini-socket and a full-socket)?
> It gets tricky when the old and new listener have different TFO key

The tricky situation can happen without this patch set. We can change
the listener's TFO key when TCP_SYN_RECV sockets are still in the accept
queue. The change is already handled properly, so it does not crash
applications.

In the normal 3WHS case, a full-socket is created after 3WHS. In the TFO
case, a full-socket is created after validating the TFO cookie in the
initial SYN packet.

After that, the connection is basically handled via the full-socket, except
for accept() syscall. So in the both cases, the mini-socket is poped out of
old listener's queue, cloned, and put into the new listner's queue. Then we
can accept() its full-socket via the cloned mini-socket.