[PATCH] hw/i386/pc: Fix crash that occurs when introspecting TYPE_PC_MACHINE machines

[Thomas Huth]

QEMU currently crashes when you try to inspect the machines based on
TYPE_PC_MACHINE for their properties:

 $ echo '{ "execute": "qmp_capabilities" }
         { "execute": "qom-list-properties","arguments":
                      { "typename": "pc-q35-10.0-machine"}}' \
   | ./qemu-system-x86_64 -M pc -qmp stdio
 {"QMP": {"version": {"qemu": {"micro": 50, "minor": 2, "major": 9},
  "package": "v9.2.0-1070-g87e115c122-dirty"}, "capabilities": ["oob"]}}
 {"return": {}}
 Segmentation fault (core dumped)

This happens because TYPE_PC_MACHINE machines add a machine_init-
done_notifier in their instance_init function - but instance_init
of machines are not only called for machines that are realized,
but also for machines that are introspected, so in this case the
listener is added for a q35 machine that is never realized. But
since there is already a running pc machine, the listener function
is triggered immediately, causing a crash since it was not for the
right machine it was meant for.

Such listener functions must never be installed from an instance_init
function. Let's do it from pc_basic_device_init() instead - this
function is called from the MachineClass->init() function instead,
i.e. guaranteed to be only called once in the lifetime of a QEMU
process.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2779
Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 hw/i386/pc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index b46975c8a4..85b8a76455 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -1241,6 +1241,9 @@ void pc_basic_device_init(struct PCMachineState *pcms,
     /* Super I/O */
     pc_superio_init(isa_bus, create_fdctrl, pcms->i8042_enabled,
                     pcms->vmport != ON_OFF_AUTO_ON, &error_fatal);
+
+    pcms->machine_done.notify = pc_machine_done;
+    qemu_add_machine_init_done_notifier(&pcms->machine_done);
 }
 
 void pc_nic_init(PCMachineClass *pcmc, ISABus *isa_bus, PCIBus *pci_bus)
@@ -1714,9 +1717,6 @@ static void pc_machine_initfn(Object *obj)
     if (pcmc->pci_enabled) {
         cxl_machine_init(obj, &pcms->cxl_devices_state);
     }
-
-    pcms->machine_done.notify = pc_machine_done;
-    qemu_add_machine_init_done_notifier(&pcms->machine_done);
 }
 
 static void pc_machine_reset(MachineState *machine, ResetType type)
-- 
2.48.1