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>

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;

include/io/channel-tls.h

@@ -48,6 +48,7 @@ struct QIOChannelTLS {
     QIOChannel parent;
     QIOChannel *master;
     QCryptoTLSSession *session;
+    QIOChannelShutdown shutdown;
 };
 
 /**

include/io/channel.h

@@ -51,9 +51,9 @@ enum QIOChannelFeature {
 typedef enum QIOChannelShutdown QIOChannelShutdown;
 
 enum QIOChannelShutdown {
-    QIO_CHANNEL_SHUTDOWN_BOTH,
+    QIO_CHANNEL_SHUTDOWN_READ = 1,
-    QIO_CHANNEL_SHUTDOWN_READ,
+    QIO_CHANNEL_SHUTDOWN_WRITE = 2,
-    QIO_CHANNEL_SHUTDOWN_WRITE,
+    QIO_CHANNEL_SHUTDOWN_BOTH = 3,
 };
 
 typedef gboolean (*QIOChannelFunc)(QIOChannel *ioc,

io/channel-tls.c

@@ -275,6 +275,9 @@ static ssize_t qio_channel_tls_readv(QIOChannel *ioc,
                 } else {
                     return QIO_CHANNEL_ERR_BLOCK;
                 }
+            } else if (errno == ECONNABORTED &&
+                       (tioc->shutdown & QIO_CHANNEL_SHUTDOWN_READ)) {
+                return 0;
             }
 
             error_setg_errno(errp, errno,
@@ -357,6 +360,8 @@ static int qio_channel_tls_shutdown(QIOChannel *ioc,
 {
     QIOChannelTLS *tioc = QIO_CHANNEL_TLS(ioc);
 
+    tioc->shutdown |= how;
+
     return qio_channel_shutdown(tioc->master, how, errp);
 }