[PATCH v4] monitor/qmp: cleanup SocketChardev listener sources early to avoid fd handling race

[Jie Song <mail@jiesong.me>]

From: Jie Song <songjie_yewu@cmss.chinamobile.com>

When starting a dummy QEMU process with virsh version, monitor_init_qmp()
enables IOThread monitoring of the QMP fd by default. However, a race
condition exists during the initialization phase: the IOThread only removes
the main thread's fd watch when it reaches qio_net_listener_set_client_func_full(),
which may be delayed under high system load.

This creates a window between monitor_qmp_setup_handlers_bh() and
qio_net_listener_set_client_func_full() where both the main thread and
IOThread are simultaneously monitoring the same fd and processing events.
This race can cause either the main thread or the IOThread to hang and
become unresponsive.

Fix this by proactively cleaning up the listener's IO sources in
monitor_init_qmp() before the IOThread initializes QMP monitoring,
ensuring exclusive fd ownership and eliminating the race condition.

Signed-off-by: Jie Song <songjie_yewu@cmss.chinamobile.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
Changes in v4:
- Correct typos and add braces, reviewed by Marc-André Lureau.
- Link to v3:
  https://lists.nongnu.org/archive/html/qemu-devel/2025-11/msg03504.html
- Link to v2:
  https://lists.nongnu.org/archive/html/qemu-devel/2025-11/msg02443.html
- Link to v1:
  https://lists.nongnu.org/archive/html/qemu-devel/2025-11/msg01621.html
---
 chardev/char-io.c         |  8 ++++++++
 chardev/char-socket.c     | 10 ++++++++++
 include/chardev/char-io.h |  2 ++
 include/chardev/char.h    |  2 ++
 monitor/qmp.c             |  5 +++++
 5 files changed, 27 insertions(+)

diff --git a/chardev/char-io.c b/chardev/char-io.c
index 3be17b51ca..beac5cd245 100644
--- a/chardev/char-io.c
+++ b/chardev/char-io.c
@@ -182,3 +182,11 @@ int io_channel_send(QIOChannel *ioc, const void *buf, size_t len)
 {
     return io_channel_send_full(ioc, buf, len, NULL, 0);
 }
+
+void remove_listener_fd_in_watch(Chardev *chr)
+{
+    ChardevClass *cc = CHARDEV_GET_CLASS(chr);
+    if (cc->chr_listener_cleanup) {
+        cc->chr_listener_cleanup(chr);
+    }
+}
diff --git a/chardev/char-socket.c b/chardev/char-socket.c
index 26d2f11202..3f45dd2ecd 100644
--- a/chardev/char-socket.c
+++ b/chardev/char-socket.c
@@ -1570,6 +1570,15 @@ char_socket_get_connected(Object *obj, Error **errp)
     return s->state == TCP_CHARDEV_STATE_CONNECTED;
 }
 
+static void tcp_chr_listener_cleanup(Chardev *chr)
+{
+    SocketChardev *s = SOCKET_CHARDEV(chr);
+    if (s->listener) {
+        qio_net_listener_set_client_func_full(s->listener, NULL, NULL,
+                                              NULL, chr->gcontext);
+    }
+}
+
 static void char_socket_class_init(ObjectClass *oc, const void *data)
 {
     ChardevClass *cc = CHARDEV_CLASS(oc);
@@ -1587,6 +1596,7 @@ static void char_socket_class_init(ObjectClass *oc, const void *data)
     cc->chr_add_client = tcp_chr_add_client;
     cc->chr_add_watch = tcp_chr_add_watch;
     cc->chr_update_read_handler = tcp_chr_update_read_handler;
+    cc->chr_listener_cleanup = tcp_chr_listener_cleanup;
 
     object_class_property_add(oc, "addr", "SocketAddress",
                               char_socket_get_addr, NULL,
diff --git a/include/chardev/char-io.h b/include/chardev/char-io.h
index ac379ea70e..540131346d 100644
--- a/include/chardev/char-io.h
+++ b/include/chardev/char-io.h
@@ -43,4 +43,6 @@ int io_channel_send(QIOChannel *ioc, const void *buf, size_t len);
 int io_channel_send_full(QIOChannel *ioc, const void *buf, size_t len,
                          int *fds, size_t nfds);
 
+void remove_listener_fd_in_watch(Chardev *chr);
+
 #endif /* CHAR_IO_H */
diff --git a/include/chardev/char.h b/include/chardev/char.h
index b65e9981c1..192cad67d4 100644
--- a/include/chardev/char.h
+++ b/include/chardev/char.h
@@ -307,6 +307,8 @@ struct ChardevClass {
 
     /* handle various events */
     void (*chr_be_event)(Chardev *s, QEMUChrEvent event);
+
+    void (*chr_listener_cleanup)(Chardev *chr);
 };
 
 Chardev *qemu_chardev_new(const char *id, const char *typename,
diff --git a/monitor/qmp.c b/monitor/qmp.c
index cb99a12d94..7ae070dc8d 100644
--- a/monitor/qmp.c
+++ b/monitor/qmp.c
@@ -537,6 +537,11 @@ void monitor_init_qmp(Chardev *chr, bool pretty, Error **errp)
          * e.g. the chardev is in client mode, with wait=on.
          */
         remove_fd_in_watch(chr);
+        /*
+         * Clean up listener IO sources early to prevent racy fd
+         * handling between the main thread and the I/O thread.
+         */
+        remove_listener_fd_in_watch(chr);
         /*
          * We can't call qemu_chr_fe_set_handlers() directly here
          * since chardev might be running in the monitor I/O
-- 
2.43.0