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[PATCH 0/3] nbd: Try for cleaner TLS shutdown
From: |
Eric Blake |
Subject: |
[PATCH 0/3] nbd: Try for cleaner TLS shutdown |
Date: |
Fri, 27 Mar 2020 11:19:33 -0500 |
I'm currently trying to debug a hang in 'nbdkit nbd' as a client when
TLS is in use, when dealing with three processes:
nbdkit example1[1] <=tls=> nbdkit nbd[2] <=plain=> qemu-nbd --list[3]
In theory, when process [3] exits, the server side of process [2] is
supposed to disconnect as client from process [1]. But we were
hitting sporadic hangs where process [2] was stuck in a poll() while
in gnutls cleanup code, which appears to be because it did not
recognize a clean shutdown from process [3].
I do not yet have a strong reproducible test case (which might include
adding strategic sleep()s in one-off compilations) to prove that the
problem is an unclean TLS shutdown, but at the same time, the gnutls
documentation is clear that without proper use of gnutls_bye(), a
client cannot distinguish between clean end of communications and a
malicious interruption in service. Shutting down the write side of a
bi-directional socket as soon as possible makes it easier for a client
to begin its own shutdown actions, and can help in avoiding deadlocks
where both sides of a connection are waiting for the other side to
close() the socket first. If I can come up with a reliable test case,
this series may be a candidate for 5.0; but for now, I'm only
proposing it for 5.1 (we've had more than one release where qemu was
not exploiting the full power of gnutls_bye(), and even if that
triggers poor interaction with other endpoints such as nbdkit, going
one more release with the same behavior isn't making things worse).
Eric Blake (3):
crypto: Add qcrypto_tls_shutdown()
io: Support shutdown of TLS channel
nbd: Use shutdown(SHUT_WR) after last item sent
qapi/crypto.json | 15 +++++++++++++++
include/crypto/tlssession.h | 24 ++++++++++++++++++++++++
block/nbd.c | 1 +
crypto/tlssession.c | 27 +++++++++++++++++++++++++++
io/channel-tls.c | 27 ++++++++++++++++++++++++++-
nbd/client.c | 3 ++-
nbd/server.c | 4 ++++
7 files changed, 99 insertions(+), 2 deletions(-)
--
2.26.0.rc2
[PATCH 3/3] nbd: Use shutdown(SHUT_WR) after last item sent, Eric Blake, 2020/03/27