[PULL 04/13] io/channel-socket: do not treat a zero length write as an error

["Daniel P. Berrangé" <berrange@redhat.com>]

From: Denis V. Lunev <den@openvz.org>

qio_channel_socket_writev() checks "ret <= 0" after sendmsg(). A zero
length iovec is written successfully and returns 0, so the success
falls into the errno switch, which acts on whatever the last failing
syscall left in errno. A stale EAGAIN turns it into
QIO_CHANNEL_ERR_BLOCK with errp untouched, and a caller which treats
every negative return as fatal then passes a NULL Error to
error_get_pretty(). The websocket handshake does exactly that, so an
unauthenticated client crashes QEMU during the greeting.

Returning 0 is safe for callers which loop until everything is
written. qio_channel_writev_full_all() has no zero progress guard, but
iov_copy() yields no entries for a zero length write, so that loop is
never entered. A connected stream socket returns 0 only when there is
nothing to send.

The WIN32 implementation in the same file uses "ret < 0".

Fixes: CVE-2026-84788
Fixes: 559607ea173a ("io: add QIOChannelSocket class")
Cc: qemu-stable@nongnu.org
Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 io/channel-socket.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/io/channel-socket.c b/io/channel-socket.c
index 12773b832c..7920cee639 100644
--- a/io/channel-socket.c
+++ b/io/channel-socket.c
@@ -667,7 +667,7 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
 
  retry:
     ret = sendmsg(sioc->fd, &msg, sflags);
-    if (ret <= 0) {
+    if (ret < 0) {
         switch (errno) {
         case EAGAIN:
             return QIO_CHANNEL_ERR_BLOCK;
-- 
2.55.0