io: return 0 for EOF in TLS session read after shutdown

GNUTLS takes a paranoid approach when seeing 0 bytes returned by the
underlying OS read() function. It will consider this an error and
return GNUTLS_E_PREMATURE_TERMINATION instead of propagating the 0
return value. It expects apps to arrange for clean termination at
the protocol level and not rely on seeing EOF from a read call to
detect shutdown. This is to harden apps against a malicious 3rd party
causing termination of the sockets layer.

This is unhelpful for the QEMU NBD code which does have a clean
protocol level shutdown, but still relies on seeing 0 from the I/O
channel read in the coroutine handling incoming replies.

The upshot is that when using a plain NBD connection shutdown is
silent, but when using TLS, the client spams the console with

  Cannot read from TLS channel: Broken pipe

The NBD connection has, however, called qio_channel_shutdown()
at this point to indicate that it is done with I/O. This gives
the opportunity to optimize the code such that when the channel
has been shutdown in the read direction, the error code
GNUTLS_E_PREMATURE_TERMINATION gets turned into a '0' return
instead of an error.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20181119134228.11031-1-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/crypto/tlssession.c b/crypto/tlssession.c
index 2f28fa7f71..0dedd4af52 100644
--- a/crypto/tlssession.c
+++ b/crypto/tlssession.c
@@ -473,6 +473,9 @@ qcrypto_tls_session_read(QCryptoTLSSession *session,
         case GNUTLS_E_INTERRUPTED:
             errno = EINTR;
             break;
+        case GNUTLS_E_PREMATURE_TERMINATION:
+            errno = ECONNABORTED;
+            break;
         default:
             errno = EIO;
             break;