Re: [Qemu-devel] [PATCH v2] virtio-serial: Fix segfault on guest boot

[Amit Shah <amit.shah@redhat.com>]

On (Fri) 17 Jun 2011 [15:58:08], Luiz Capitulino wrote:
> If I start qemu with:
> 
>   # qemu -hda disks/test.img -enable-kvm -m 1G -snapshot \
>          -device virtio-serial \
>          -chardev socket,host=localhost,port=1234,server,nowait,id=foo \
>          -device virtserialport,chardev=foo,name=org.qemu.guest_agent
> 
> I get a segfault when booting a Fedora 14 guest. The backtrace says:
> 
>   Program terminated with signal 11, Segmentation fault.
>     #0  0x0000000000420850 in handle_control_message (vser=0x3732bd0, buf=0x2c173e0, len=8) at /home/lcapitulino/src/qmp-unstable/hw/virtio-serial-bus.c:335
>     335     info = DO_UPCAST(VirtIOSerialPortInfo, qdev, port->dev.info);
> 
> What's happening is VIRTIO_CONSOLE_DEVICE_READY is a message for the
> whole device, not for an individual port. So port is NULL. This bug was
> introduced by commit a15bb0d6a981de749452a5180fc8084d625671da.
> 
> This commit fixes that by making the port returned by find_port_by_id()
> be used only by the VIRTIO_CONSOLE_PORT_READY and
> VIRTIO_CONSOLE_PORT_OPEN messages.
> 
> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> ---
>  hw/virtio-serial-bus.c |   24 ++++++++++++++----------
>  1 files changed, 14 insertions(+), 10 deletions(-)
> 
> diff --git a/hw/virtio-serial-bus.c b/hw/virtio-serial-bus.c
> index 9a12104..33a6f61 100644
> --- a/hw/virtio-serial-bus.c
> +++ b/hw/virtio-serial-bus.c
> @@ -328,18 +328,11 @@ static void handle_control_message(VirtIOSerial *vser, void *buf, size_t len)
>      cpkt.event = lduw_p(&gcpkt->event);
>      cpkt.value = lduw_p(&gcpkt->value);
>  
> -    port = find_port_by_id(vser, ldl_p(&gcpkt->id));
> -    if (!port && cpkt.event != VIRTIO_CONSOLE_DEVICE_READY)
> -        return;
> -
> -    info = DO_UPCAST(VirtIOSerialPortInfo, qdev, port->dev.info);
> -
> -    switch(cpkt.event) {
> -    case VIRTIO_CONSOLE_DEVICE_READY:
> +    if (cpkt.event == VIRTIO_CONSOLE_DEVICE_READY) {

What we lose after this re-arrangement is the check that port is NULL
when this message is received.  i.e., a guest bug where port is set to
a valid value when this message arrives.  (I think I pointed this out
in a previous mail?)

>          if (!cpkt.value) {
>              error_report("virtio-serial-bus: Guest failure in adding device %s\n",
>                           vser->bus.qbus.name);
> -            break;
> +            return;
>          }
>          /*
>           * The device is up, we can now tell the device about all the
> @@ -348,8 +341,19 @@ static void handle_control_message(VirtIOSerial *vser, void *buf, size_t len)
>          QTAILQ_FOREACH(port, &vser->ports, next) {
>              send_control_event(port, VIRTIO_CONSOLE_PORT_ADD, 1);
>          }
> -        break;
> +        return;
> +    }
>  
> +    port = find_port_by_id(vser, ldl_p(&gcpkt->id));
> +    if (!port) {
> +        error_report("virtio-serial-bus: Expected port id %d for device %s\n",
> +                     ldl_p(&gcpkt->id), vser->bus.qbus.name);

It's actually 'unexpected port id...' :-)

Also, port id is %u, not %d.

Fixed these two and committed to virtio-serial tree.

Thanks,
		Amit