[v5,1/2] nbd/server: do not poll within a coroutine context

Commit Message

Eric Blake April 8, 2024, 4 p.m. UTC

From: Zhu Yangyang <[e-mail]>

Coroutines are not supposed to block. Instead, they should yield.

The client performs TLS upgrade outside of an AIOContext, during
synchronous handshake; this still requires g_main_loop.  But the
server responds to TLS upgrade inside a coroutine, so a nested
g_main_loop is wrong.  Since the two callbacks no longer share more
than the setting of data.complete and data.error, it's just as easy to
use static helpers instead of trying to share a common code path.  It
is also possible to add assertions that no other code is interfering
with the eventual path to qio reaching the callback, whether or not it
required a yield or main loop.

Fixes: f95910f ("nbd: implement TLS support in the protocol negotiation")
Signed-off-by: Zhu Yangyang <[e-mail]>
[eblake: move callbacks to their use point, add assertions]
Signed-off-by: Eric Blake <[e-mail]>
---
 nbd/nbd-internal.h | 10 ----------
 nbd/client.c       | 28 ++++++++++++++++++++++++----
 nbd/common.c       | 11 -----------
 nbd/server.c       | 28 +++++++++++++++++++++++-----
 4 files changed, 47 insertions(+), 30 deletions(-)

Comments

Vladimir Sementsov-Ogievskiy April 9, 2024, 6:13 a.m. UTC | #1

On 08.04.24 19:00, Eric Blake wrote:
> From: Zhu Yangyang<[e-mail]>
> 
> Coroutines are not supposed to block. Instead, they should yield.
> 
> The client performs TLS upgrade outside of an AIOContext, during
> synchronous handshake; this still requires g_main_loop.  But the
> server responds to TLS upgrade inside a coroutine, so a nested
> g_main_loop is wrong.  Since the two callbacks no longer share more
> than the setting of data.complete and data.error, it's just as easy to
> use static helpers instead of trying to share a common code path.  It
> is also possible to add assertions that no other code is interfering
> with the eventual path to qio reaching the callback, whether or not it
> required a yield or main loop.
> 
> Fixes: f95910f ("nbd: implement TLS support in the protocol negotiation")
> Signed-off-by: Zhu Yangyang<[e-mail]>
> [eblake: move callbacks to their use point, add assertions]
> Signed-off-by: Eric Blake<[e-mail]>

Reviewed-by: Vladimir Sementsov-Ogievskiy <[e-mail]>

Patch

diff --git a/nbd/nbd-internal.h b/nbd/nbd-internal.h
index dfa02f77ee4..91895106a95 100644
--- a/nbd/nbd-internal.h
+++ b/nbd/nbd-internal.h
@@ -72,16 +72,6 @@  static inline int nbd_write(QIOChannel *ioc, const void *buffer, size_t size,
     return qio_channel_write_all(ioc, buffer, size, errp) < 0 ? -EIO : 0;
 }

-struct NBDTLSHandshakeData {
-    GMainLoop *loop;
-    bool complete;
-    Error *error;
-};
-
-
-void nbd_tls_handshake(QIOTask *task,
-                       void *opaque);
-
 int nbd_drop(QIOChannel *ioc, size_t size, Error **errp);

 #endif
diff --git a/nbd/client.c b/nbd/client.c
index 29ffc609a4b..c89c7504673 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -596,13 +596,31 @@  static int nbd_request_simple_option(QIOChannel *ioc, int opt, bool strict,
     return 1;
 }

+/* Callback to learn when QIO TLS upgrade is complete */
+struct NBDTLSClientHandshakeData {
+    bool complete;
+    Error *error;
+    GMainLoop *loop;
+};
+
+static void nbd_client_tls_handshake(QIOTask *task, void *opaque)
+{
+    struct NBDTLSClientHandshakeData *data = opaque;
+
+    qio_task_propagate_error(task, &data->error);
+    data->complete = true;
+    if (data->loop) {
+        g_main_loop_quit(data->loop);
+    }
+}
+
 static QIOChannel *nbd_receive_starttls(QIOChannel *ioc,
                                         QCryptoTLSCreds *tlscreds,
                                         const char *hostname, Error **errp)
 {
     int ret;
     QIOChannelTLS *tioc;
-    struct NBDTLSHandshakeData data = { 0 };
+    struct NBDTLSClientHandshakeData data = { 0 };

     ret = nbd_request_simple_option(ioc, NBD_OPT_STARTTLS, true, errp);
     if (ret <= 0) {
@@ -619,18 +637,20 @@  static QIOChannel *nbd_receive_starttls(QIOChannel *ioc,
         return NULL;
     }
     qio_channel_set_name(QIO_CHANNEL(tioc), "nbd-client-tls");
-    data.loop = g_main_loop_new(g_main_context_default(), FALSE);
     trace_nbd_receive_starttls_tls_handshake();
     qio_channel_tls_handshake(tioc,
-                              nbd_tls_handshake,
+                              nbd_client_tls_handshake,
                               &data,
                               NULL,
                               NULL);

     if (!data.complete) {
+        data.loop = g_main_loop_new(g_main_context_default(), FALSE);
         g_main_loop_run(data.loop);
+        assert(data.complete);
+        g_main_loop_unref(data.loop);
     }
-    g_main_loop_unref(data.loop);
+
     if (data.error) {
         error_propagate(errp, data.error);
         object_unref(OBJECT(tioc));
diff --git a/nbd/common.c b/nbd/common.c
index 3247c1d618a..589a748cfe6 100644
--- a/nbd/common.c
+++ b/nbd/common.c
@@ -47,17 +47,6 @@  int nbd_drop(QIOChannel *ioc, size_t size, Error **errp)
 }


-void nbd_tls_handshake(QIOTask *task,
-                       void *opaque)
-{
-    struct NBDTLSHandshakeData *data = opaque;
-
-    qio_task_propagate_error(task, &data->error);
-    data->complete = true;
-    g_main_loop_quit(data->loop);
-}
-
-
 const char *nbd_opt_lookup(uint32_t opt)
 {
     switch (opt) {
diff --git a/nbd/server.c b/nbd/server.c
index c3484cc1ebc..98ae0e16326 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -748,6 +748,23 @@  static int nbd_negotiate_handle_info(NBDClient *client, Error **errp)
     return rc;
 }

+/* Callback to learn when QIO TLS upgrade is complete */
+struct NBDTLSServerHandshakeData {
+    bool complete;
+    Error *error;
+    Coroutine *co;
+};
+
+static void nbd_server_tls_handshake(QIOTask *task, void *opaque)
+{
+    struct NBDTLSServerHandshakeData *data = opaque;
+
+    qio_task_propagate_error(task, &data->error);
+    data->complete = true;
+    if (!qemu_coroutine_entered(data->co)) {
+        aio_co_wake(data->co);
+    }
+}

 /* Handle NBD_OPT_STARTTLS. Return NULL to drop connection, or else the
  * new channel for all further (now-encrypted) communication. */
@@ -756,7 +773,7 @@  static QIOChannel *nbd_negotiate_handle_starttls(NBDClient *client,
 {
     QIOChannel *ioc;
     QIOChannelTLS *tioc;
-    struct NBDTLSHandshakeData data = { 0 };
+    struct NBDTLSServerHandshakeData data = { 0 };

     assert(client->opt == NBD_OPT_STARTTLS);

@@ -777,17 +794,18 @@  static QIOChannel *nbd_negotiate_handle_starttls(NBDClient *client,

     qio_channel_set_name(QIO_CHANNEL(tioc), "nbd-server-tls");
     trace_nbd_negotiate_handle_starttls_handshake();
-    data.loop = g_main_loop_new(g_main_context_default(), FALSE);
+    data.co = qemu_coroutine_self();
     qio_channel_tls_handshake(tioc,
-                              nbd_tls_handshake,
+                              nbd_server_tls_handshake,
                               &data,
                               NULL,
                               NULL);

     if (!data.complete) {
-        g_main_loop_run(data.loop);
+        qemu_coroutine_yield();
+        assert(data.complete);
     }
-    g_main_loop_unref(data.loop);
+
     if (data.error) {
         object_unref(OBJECT(tioc));
         error_propagate(errp, data.error);