Re: [PATCH v4 bpf-next 00/11] Socket migration for SO_REUSEPORT.

[Eric Dumazet]

On 4/28/21 3:27 AM, Martin KaFai Lau wrote:
> On Tue, Apr 27, 2021 at 12:38:58PM -0400, Jason Baron wrote:
>>
>>
>> On 4/26/21 11:46 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.
>>
>> Hi Kuniyuki,
>>
>> I had implemented a different approach to this that I wanted to get your
>> thoughts about. The idea is to use unix sockets and SCM_RIGHTS to pass the
>> listen fd (or any other fd) around. Currently, if you have an 'old' webserver
>> that you want to replace with a 'new' webserver, you would need a separate
>> process to receive the listen fd and then have that process send the fd to
>> the new webserver, if they are not running con-currently. So instead what
>> I'm proposing is a 'delayed close' for a unix socket. That is, one could do:
>>
>> 1) bind unix socket with path '/sockets'
>> 2) sendmsg() the listen fd via the unix socket
>> 2) setsockopt() some 'timeout' on the unix socket (maybe 10 seconds or so)
>> 3) exit/close the old webserver and the listen socket
>> 4) start the new webserver
>> 5) create new unix socket and bind to '/sockets' (if has MAY_WRITE file permissions)
>> 6) recvmsg() the listen fd
>>
>> So the idea is that we set a timeout on the unix socket. If the new process
>> does not start and bind to the unix socket, it simply closes, thus releasing
>> the listen socket. However, if it does bind it can now call recvmsg() and
>> use the listen fd as normal. It can then simply continue to use the old listen
>> fds and/or create new ones and drain the old ones.
>>
>> Thus, the old and new webservers do not have to run concurrently. This doesn't
>> involve any changes to the tcp layer and can be used to pass any type of fd.
>> not sure if it's actually useful for anything else though.
> We also used to do tcp-listen(/udp) fd transfer because the new process can not
> bind to the same IP:PORT in the old kernel without SO_REUSEPORT.  Some of the
> services listen to many different IP:PORT(s).  Transferring all of them
> was ok-ish but the old and new process do not necessary listen to the same set
> of IP:PORT(s) (e.g. the config may have changed during restart) and it further
> complicates the fd transfer logic in the userspace.
> 
> It was then moved to SO_REUSEPORT.  The new process can create its listen fds
> without depending on the old process.  It pretty much starts as if there is
> no old process.  There is no need to transfer the fds, simplified the userspace
> logic.  The old and new process can work independently.  The old and new process
> still run concurrently for a brief time period to avoid service disruption.
> 


Note that another technique is to force syncookies during the switch of old/new servers.

echo 2 >/proc/sys/net/ipv4/tcp_syncookies

If there is interest, we could add a socket option to override the sysctl on a per-socket basis.