Re: [PATCH 1/1] SUNRPC: fix handling of half-closed connection

Olga Kornievskaia
2019-02-22T15:11:18Z

On Fri, Feb 22, 2019 at 10:06 AM Trond Myklebust
<trondmy@hammerspace.com> wrote:
>
> On Fri, 2019-02-22 at 09:46 -0500, Olga Kornievskaia wrote:
> > On Fri, Feb 22, 2019 at 8:45 AM Trond Myklebust <
> > trondmy@hammerspace.com> wrote:
> > > On Fri, 2019-02-22 at 07:12 -0500, Dave Wysochanski wrote:
> > > > Hi Olga,
> > > >
> > > > Do you have a reproducer for this? A number of months ago I did
> > > > a
> > > > significant amount of testing with half-closed connections, after
> > > > we
> > > > had reports of connections stuck in FIN_WAIT2 in some older
> > > > kernels.
> > > > What I found was with kernels that had the tcp keepalives (commit
> > > > 7f260e8575bf53b93b77978c1e39f8e67612759c), I could only reproduce
> > > > a
> > > > hang of a few minutes, after which time the tcp keepalive code
> > > > would
> > > > reset the connection.
> > > >
> > > > That said it was a while ago and something subtle may have
> > > > changed.
> > > > Also I'm not not sure if your header implies an indefinite hang
> > > > or
> > > > just
> > > > a few minutes.
> > > >
> > > > Thanks.
> > > >
> > > >
> > > > On Wed, 2019-02-20 at 09:56 -0500, Olga Kornievskaia wrote:
> > > > > From: Olga Kornievskaia <kolga@netapp.com>
> > > > >
> > > > > When server replies with an ACK to client's FIN/ACK, client
> > > > > ends
> > > > > up stuck in a TCP_FIN_WAIT2 state and client's mount hangs.
> > > > > Instead, make sure to close and reset client's socket and
> > > > > transport
> > > > > when transitioned into that state.
> >
> > Hi Trond,
> >
> > > So, please do note that we do not want to ignore the FIN_WAIT2
> > > state
> >
> > But we do ignore the FIN_WAIT2 state.
>
> We do not. We wait for the server to send a FIN, which is precisely the
> reason for which FIN_WAIT2 exists.
>
> >
> > > because it implies that the server has not closed the socket on its
> > > side.
> >
> > That's correct.
> >
> > > That again means that we cannot re-establish a connection using
> > > the same source IP+port to the server, which is problematic for
> > > protocols such as NFSv3 which rely on standard duplicate reply
> > > cache
> > > for correct replay semantics.
> >
> > that's exactly what's happening that a client is unable to establish
> > a
> > new connection to the server. With the patch, the client does an RST
> > and it re-uses the port and all is well for NFSv3.
>
> RST is not guaranteed to be delivered to the recipient. That's why the
> TCP protocol defines FIN: it is a guaranteed to be delivered because it
> is ACKed.
>
> > > This is why we don't just set the TCP_LINGER2 socket option and
> > > call
> > > sock_release(). The choice to try to wait it out is deliberate
> > > because
> > > the alternative is that we end up with busy-waiting re-connection
> > > attempts.
> >
> > Why would it busy-wait? In my testing, RST happens and new connection
> > is established?
>
> Only if the server has dropped the connection without notifying the
> client.

Yes the server dropped the connection without notifying the client (or
perhaps something in the middle did it as an attack). Again, I raise
this concern for the sake of dealing with this as an attack. I have no
intentions of catering to broken servers. If this is not a possible
attack, then we don't have to deal with it.