qemu-char: BUGFIX, don't call FD_ISSET with negative fd

tcp_chr_connect(), unlike for example udp_chr_update_read_handler() does
not check if the fd it is using is valid (>= 0) before passing it to
qemu_set_fd_handler2().  If using e.g. a TCP serial port, which is not
initially connected, this can result in -1 being passed to FD_ISSET, which
has undefined behaviour.  On x86 it seems to harmlessly return 0, but on
PowerPC, it causes a fortify buffer overflow error to be thrown.

This patch fixes this by putting an extra test in tcp_chr_connect(), and
also adds an assert qemu_set_fd_handler2() to catch other such errors on
all platforms, rather than just some.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

iohandler.c

@@ -56,6 +56,8 @@ int qemu_set_fd_handler2(int fd,
 {
     IOHandlerRecord *ioh;
 
+    assert(fd >= 0);
+
     if (!fd_read && !fd_write) {
         QLIST_FOREACH(ioh, &io_handlers, next) {
             if (ioh->fd == fd) {

qemu-char.c

@@ -2329,8 +2329,10 @@ static void tcp_chr_connect(void *opaque)
     TCPCharDriver *s = chr->opaque;
 
     s->connected = 1;
-    qemu_set_fd_handler2(s->fd, tcp_chr_read_poll,
+    if (s->fd >= 0) {
-                         tcp_chr_read, NULL, chr);
+        qemu_set_fd_handler2(s->fd, tcp_chr_read_poll,
+                             tcp_chr_read, NULL, chr);
+    }
     qemu_chr_generic_open(chr);
 }