Re: [Qemu-devel] [PATCH] qemu-char: fix infinite recursion connecting to monitor pty

[Michael Roth]

Quoting James Hogan (2013-08-08 06:09:38)
> Since commit bd5c51e (qemu-char: don't issue CHR_EVENT_OPEN in a BH), an
> infinite recursion occurs when putting the monitor on a pty (-monitor
> pty) and connecting a terminal to the slave port.
> 
> This is because of the qemu_chr_be_event(s, CHR_EVENT_OPENED) added to
> qemu_chr_be_generic_open(). This event is captured by monitor_event()
> which prints a welcome message to the character device. The flush of
> that welcome message retriggers another open event in pty_chr_state()
> because it checks s->connected, but only sets it to 1 after calling
> qemu_chr_be_generic_open().
> 
> I've fixed this by setting s->connected = 1 before the call to
> qemu_chr_be_generic_open() instead of after, so that the recursive
> pty_chr_state() doesn't call it again.
> 
> An example snippet of repeating backtrace:
>  ...
>  #107486 0x007aec58 in monitor_flush (mon=0xf418b0) at qemu/monitor.c:288
>  #107487 0x007aee7c in monitor_puts (mon=0xf418b0, str=0x1176d07 "") at 
> qemu/monitor.c:322
>  #107488 0x007aef20 in monitor_vprintf (mon=0xf418b0, fmt=0x8d4820 "QEMU %s 
> monitor - type 'help' for more information\n",
>      ap=0x7f432be0) at qemu/monitor.c:339
>  #107489 0x007aefac in monitor_printf (mon=0xf418b0, fmt=0x8d4820 "QEMU %s 
> monitor - type 'help' for more information\n")
>      at qemu/monitor.c:347
>  #107490 0x007ba4bc in monitor_event (opaque=0xf418b0, event=2) at 
> qemu/monitor.c:4699
>  #107491 0x00684c28 in qemu_chr_be_event (s=0xf37788, event=2) at 
> qemu/qemu-char.c:108
>  #107492 0x00684c70 in qemu_chr_be_generic_open (s=0xf37788) at 
> qemu/qemu-char.c:113
>  #107493 0x006880a4 in pty_chr_state (chr=0xf37788, connected=1) at 
> qemu/qemu-char.c:1145
>  #107494 0x00687fa4 in pty_chr_update_read_handler (chr=0xf37788) at 
> qemu/qemu-char.c:1121
>  #107495 0x00687c9c in pty_chr_write (chr=0xf37788, buf=0x70b3c008 <Address 
> 0x70b3c008 out of bounds>, len=538720)
>      at qemu/qemu-char.c:1063
>  #107496 0x00684cc4 in qemu_chr_fe_write (s=0xf37788, buf=0x70b3c008 <Address 
> 0x70b3c008 out of bounds>, len=538720)
>      at qemu/qemu-char.c:118
>  ...
> 
> Signed-off-by: James Hogan <address@hidden>
> Cc: Michael Roth <address@hidden>
> Cc: Anthony Liguori <address@hidden>

Thanks for the catch!

I reproduced the bug and can confirm the this fixes it.

Prior to bd5c51e, the code relied on deferring CHR_EVENT_OPENED via BH to
ensure setting s->connected = 1 was visible by the time the event was
processed, so this patch simply makes that ordering explicit.

Tested-by: Michael Roth <address@hidden>

> ---
> Note the commit I mentioned Cc's address@hidden Assuming this
> patch is acceptable it probably makes sense for it to follow the other
> patch into stable too.
> 
>  qemu-char.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/qemu-char.c b/qemu-char.c
> index 16f3ad7..1be1cf6 100644
> --- a/qemu-char.c
> +++ b/qemu-char.c
> @@ -1142,8 +1142,8 @@ static void pty_chr_state(CharDriverState *chr, int 
> connected)
>              s->timer_tag = 0;
>          }
>          if (!s->connected) {
> -            qemu_chr_be_generic_open(chr);
>              s->connected = 1;
> +            qemu_chr_be_generic_open(chr);
>              s->fd_tag = io_add_watch_poll(s->fd, pty_chr_read_poll, 
> pty_chr_read, chr);
>          }
>      }
> -- 
> 1.8.1.2