[Qemu-devel] [PATCH v2 4/4] chardev: clear O_NONBLOCK on SCM_RIGHTS file descriptors

[Stefan Hajnoczi <stefanha@redhat.com>]

When we receive a file descriptor over a UNIX domain socket the
O_NONBLOCK flag is preserved.  Clear the O_NONBLOCK flag and rely on
QEMU file descriptor users like migration, SPICE, VNC, block layer, and
others to set non-blocking only when necessary.

This change ensures we don't accidentally expose O_NONBLOCK in the QMP
API.  QMP clients should not need to get the non-blocking state
"correct".

A recent real-world example was when libvirt passed a non-blocking TCP
socket for migration where we expected a blocking socket.  The source
QEMU produced a corrupted migration stream since its code did not cope
with non-blocking sockets.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 qemu-char.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/qemu-char.c b/qemu-char.c
index 500a582..091f2dc 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -2434,6 +2434,9 @@ static void unix_process_msgfd(CharDriverState *chr, struct msghdr *msg)
         if (fd < 0)
             continue;
 
+        /* O_NONBLOCK is preserved across SCM_RIGHTS so reset it */
+        qemu_set_block(fd);
+
 #ifndef MSG_CMSG_CLOEXEC
         qemu_set_cloexec(fd);
 #endif
-- 
1.8.1.4