[Qemu-block] [PATCH v4 1/6] block/qdev: Allow node name for drive properties

[Kevin Wolf]

If a node name instead of a BlockBackend name is specified as the driver
for a guest device, an anonymous BlockBackend is created now.

The order of operations in release_drive() must be reversed in order to
avoid a use-after-free bug because now blk_detach_dev() frees the last
reference if an anonymous BlockBackend is used.

usb-storage uses a hack where it forwards its BlockBackend as a property
to another device that it internally creates. This hack must be updated
so that it doesn't drop its original BB before it can be passed to the
other device. This used to work because we always had the monitor
reference around, but with node-names the device reference is the only
one now.

Signed-off-by: Kevin Wolf <address@hidden>
Reviewed-by: Max Reitz <address@hidden>
---

v4:
- Fix use-after-free bug in release_drive()

 hw/core/qdev-properties-system.c | 39 +++++++++++++++++++++++++++++++++------
 hw/usb/dev-storage.c             |  5 ++++-
 2 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/hw/core/qdev-properties-system.c b/hw/core/qdev-properties-system.c
index 65d9fa9..ab1bc5e 100644
--- a/hw/core/qdev-properties-system.c
+++ b/hw/core/qdev-properties-system.c
@@ -72,12 +72,21 @@ static void parse_drive(DeviceState *dev, const char *str, 
void **ptr,
                         const char *propname, Error **errp)
 {
     BlockBackend *blk;
+    bool blk_created = false;
 
     blk = blk_by_name(str);
     if (!blk) {
+        BlockDriverState *bs = bdrv_lookup_bs(NULL, str, NULL);
+        if (bs) {
+            blk = blk_new();
+            blk_insert_bs(blk, bs);
+            blk_created = true;
+        }
+    }
+    if (!blk) {
         error_setg(errp, "Property '%s.%s' can't find value '%s'",
                    object_get_typename(OBJECT(dev)), propname, str);
-        return;
+        goto fail;
     }
     if (blk_attach_dev(blk, dev) < 0) {
         DriveInfo *dinfo = blk_legacy_dinfo(blk);
@@ -91,9 +100,16 @@ static void parse_drive(DeviceState *dev, const char *str, 
void **ptr,
             error_setg(errp, "Drive '%s' is already in use by another device",
                        str);
         }
-        return;
+        goto fail;
     }
+
     *ptr = blk;
+
+fail:
+    if (blk_created) {
+        /* If we need to keep a reference, blk_attach_dev() took it */
+        blk_unref(blk);
+    }
 }
 
 static void release_drive(Object *obj, const char *name, void *opaque)
@@ -103,8 +119,8 @@ static void release_drive(Object *obj, const char *name, 
void *opaque)
     BlockBackend **ptr = qdev_get_prop_ptr(dev, prop);
 
     if (*ptr) {
-        blk_detach_dev(*ptr, dev);
         blockdev_auto_del(*ptr);
+        blk_detach_dev(*ptr, dev);
     }
 }
 
@@ -127,7 +143,7 @@ static void set_drive(Object *obj, Visitor *v, const char 
*name, void *opaque,
 
 PropertyInfo qdev_prop_drive = {
     .name  = "str",
-    .description = "ID of a drive to use as a backend",
+    .description = "Node name or ID of a block device to use as a backend",
     .get   = get_drive,
     .set   = set_drive,
     .release = release_drive,
@@ -362,8 +378,19 @@ PropertyInfo qdev_prop_vlan = {
 void qdev_prop_set_drive(DeviceState *dev, const char *name,
                          BlockBackend *value, Error **errp)
 {
-    object_property_set_str(OBJECT(dev), value ? blk_name(value) : "",
-                            name, errp);
+    const char *ref = "";
+
+    if (value) {
+        ref = blk_name(value);
+        if (!*ref) {
+            BlockDriverState *bs = blk_bs(value);
+            if (bs) {
+                ref = bdrv_get_node_name(bs);
+            }
+        }
+    }
+
+    object_property_set_str(OBJECT(dev), ref, name, errp);
 }
 
 void qdev_prop_set_chr(DeviceState *dev, const char *name,
diff --git a/hw/usb/dev-storage.c b/hw/usb/dev-storage.c
index 4d605b8..78038a2 100644
--- a/hw/usb/dev-storage.c
+++ b/hw/usb/dev-storage.c
@@ -609,10 +609,12 @@ static void usb_msd_realize_storage(USBDevice *dev, Error 
**errp)
      * a SCSI bus that can serve only a single device, which it
      * creates automatically.  But first it needs to detach from its
      * blockdev, or else scsi_bus_legacy_add_drive() dies when it
-     * attaches again.
+     * attaches again. We also need to take another reference so that
+     * blk_detach_dev() doesn't free blk while we still need it.
      *
      * The hack is probably a bad idea.
      */
+    blk_ref(blk);
     blk_detach_dev(blk, &s->dev.qdev);
     s->conf.blk = NULL;
 
@@ -623,6 +625,7 @@ static void usb_msd_realize_storage(USBDevice *dev, Error 
**errp)
     scsi_dev = scsi_bus_legacy_add_drive(&s->bus, blk, 0, !!s->removable,
                                          s->conf.bootindex, dev->serial,
                                          &err);
+    blk_unref(blk);
     if (!scsi_dev) {
         error_propagate(errp, err);
         return;
-- 
1.8.3.1