On Mon, May 17, 2021 at 09:22:47AM +0900, 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. > > Special thanks to Martin KaFai Lau for bouncing ideas and exchanging code > snippets along the way. > > > Link: > [1] The SO_REUSEPORT socket option > https://lwn.net/Articles/542629/ > > [2] Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN_OFF socket option as drain mode > https://lore.kernel.org/netdev/1458828813.10868.65.camel@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > > > Changelog: > v6: > * Change description in ip-sysctl.rst > * Test IPPROTO_TCP before reading tfo_listener > * Move reqsk_clone() to inet_connection_sock.c and rename to > inet_reqsk_clone() > * Pass req->rsk_listener to inet_csk_reqsk_queue_drop() and > reqsk_queue_removed() in the migration path of receiving ACK > * s/ARG_PTR_TO_SOCKET/PTR_TO_SOCKET/ in sk_reuseport_is_valid_access() > * In selftest, use atomic ops to increment global vars, drop ACK by XDP, > enable force fastopen, use "skel->bss" instead of "skel->data" Some commit messages need to be updated: s/reqsk_clone/inet_reqsk_clone/ One thing needs to be addressed in patch 3. Others lgtm. Acked-by: Martin KaFai Lau <kafai@xxxxxx>
