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[PATCH] qom: Fix default values in help

From: Markus Armbruster
Subject: [PATCH] qom: Fix default values in help
Date: Wed, 24 Mar 2021 09:41:30 +0100

Output of default values in device help is broken:

    $ ./qemu-system-x86_64 -S -display none -monitor stdio
    QEMU 5.2.50 monitor - type 'help' for more information
    (qemu) device_add pvpanic,help
    pvpanic options:
      events=<uint8>         -  (default: (null))
      ioport=<uint16>        -  (default: (null))

The "(null)" is glibc printing a null pointer.  Other systems crash
instead.  Having a help request crash a running VM can really spoil
your day.

Root cause is a botched replacement of qstring_free() by
g_string_free(): to get the string back, we need to pass true to the
former, but false to the latter.  Fix the argument.

Fixes: eab3a4678b07267c39e7290a6e9e7690b1d2a521
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
 qom/object_interfaces.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/qom/object_interfaces.c b/qom/object_interfaces.c
index c3324b0f86..bd8a947a63 100644
--- a/qom/object_interfaces.c
+++ b/qom/object_interfaces.c
@@ -159,7 +159,7 @@ char *object_property_help(const char *name, const char 
     if (defval) {
         g_autofree char *def_json = g_string_free(qobject_to_json(defval),
-                                                  true);
+                                                  false);
         g_string_append_printf(str, " (default: %s)", def_json);

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