Re: [Qemu-devel] [PATCH] pty: Fix byte loss bug when connecting to pty

[Paolo Bonzini <pbonzini@redhat.com>]

Il 28/07/2014 13:39, Sebastian Tanase ha scritto:
> When trying to print data to the pty, we first check if it is connected.
> If not, we try to reconnect, but we drop the pending data even if we
> have successfully reconnected; this makes us lose the first byte of the very
> first transmission.
> This small fix addresses the issue by checking once more if the pty is connected
> after having tried to reconnect.
> 
> Signed-off-by: Sebastian Tanase <sebastian.tanase@openwide.fr>
> ---
> 
> To reproduce the bug, launch a qemu image that has a parallel port (say lp0)
> and redirect it to a pty (-parallel pty). After the VM is launched,
> open the corresponding pty on your host (cat /dev/pts/X) and send some
> data from the VM to the host: echo "abcd" > /dev/lp0
> The first time, the received string will be "bcd" instead of "abcd".
> This bug can have important consequences if you try, for example,
> to send a postscript file from a printer within the VM. Losing the
> first character will render the .ps file unusable.
> ---
>  qemu-char.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/qemu-char.c b/qemu-char.c
> index 7acc03f..ce52d0f 100644
> --- a/qemu-char.c
> +++ b/qemu-char.c
> @@ -1160,7 +1160,9 @@ static int pty_chr_write(CharDriverState *chr, const uint8_t *buf, int len)
>      if (!s->connected) {
>          /* guest sends data, check for (re-)connect */
>          pty_chr_update_read_handler_locked(chr);
> -        return 0;
> +        if (!s->connected) {
> +            return 0;
> +        }
>      }
>      return io_channel_send(s->fd, buf, len);
>  }
> 

Looks ok, though only for 2.2 and 2.1.1.  Gerd, can you take care of
this patch?

Paolo