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

[James Hogan]

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>
---
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