qdev: Fix set_pci_devfn() to visit option only once

pci_devfn properties accept either a string or an integer as input. To
implement this, set_pci_devfn() first tries to visit the option as a
string, and if that fails, it visits it as an integer instead. While the
QemuOpts visitor happens to accept this, it is invalid according to the
visitor interface. QObject input visitors run into an assertion failure
when this is done.

QObject input visitors are used with the JSON syntax version of -device
on the command line:

$ ./qemu-system-x86_64 -enable-kvm -M q35 -device pcie-pci-bridge,id=pci.1,bus=pcie.0 -blockdev null-co,node-name=disk -device '{ "driver": "virtio-blk-pci", "drive": "disk", "id": "virtio-disk0", "bus": "pci.1", "addr": 1 }'
qemu-system-x86_64: ../qapi/qobject-input-visitor.c:143: QObject *qobject_input_try_get_object(QObjectInputVisitor *, const char *, _Bool): Assertion `removed' failed.

The proper way to accept both strings and integers is using the
alternate mechanism, which tells us the type of the input before it's
visited. With this information, we can directly visit it as the right
type.

This fixes set_pci_devfn() by using the alternate mechanism.

Cc: qemu-stable@nongnu.org
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20241119120353.57812-1-kwolf@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/core/qdev-properties-system.c

@@ -816,39 +816,57 @@ static void set_pci_devfn(Object *obj, Visitor *v, const char *name,
                           void *opaque, Error **errp)
 {
     Property *prop = opaque;
+    g_autofree GenericAlternate *alt;
     int32_t value, *ptr = object_field_prop_ptr(obj, prop);
     unsigned int slot, fn, n;
-    char *str;
+    g_autofree char *str = NULL;
+
+    if (!visit_start_alternate(v, name, &alt, sizeof(*alt), errp)) {
+        return;
+    }
+
+    switch (alt->type) {
+    case QTYPE_QSTRING:
+        if (!visit_type_str(v, name, &str, errp)) {
+            goto out;
+        }
 
-    if (!visit_type_str(v, name, &str, NULL)) {
+        if (sscanf(str, "%x.%x%n", &slot, &fn, &n) != 2) {
+            fn = 0;
+            if (sscanf(str, "%x%n", &slot, &n) != 1) {
+                goto invalid;
+            }
+        }
+        if (str[n] != '\0' || fn > 7 || slot > 31) {
+            goto invalid;
+        }
+        *ptr = slot << 3 | fn;
+        break;
+
+    case QTYPE_QNUM:
         if (!visit_type_int32(v, name, &value, errp)) {
-            return;
+            goto out;
         }
         if (value < -1 || value > 255) {
             error_setg(errp, QERR_INVALID_PARAMETER_VALUE,
                        name ? name : "null", "a value between -1 and 255");
-            return;
+            goto out;
         }
         *ptr = value;
-        return;
-    }
+        break;
 
-    if (sscanf(str, "%x.%x%n", &slot, &fn, &n) != 2) {
-        fn = 0;
-        if (sscanf(str, "%x%n", &slot, &n) != 1) {
-            goto invalid;
-        }
-    }
-    if (str[n] != '\0' || fn > 7 || slot > 31) {
-        goto invalid;
+    default:
+        error_setg(errp, "Invalid parameter type for '%s', expected int or str",
+                   name ? name : "null");
+        goto out;
     }
-    *ptr = slot << 3 | fn;
-    g_free(str);
-    return;
+
+    goto out;
 
 invalid:
     error_set_from_qdev_prop_error(errp, EINVAL, obj, name, str);
-    g_free(str);
+out:
+    visit_end_alternate(v, (void **) &alt);
 }
 
 static int print_pci_devfn(Object *obj, Property *prop, char *dest,