savevm: Really verify if a drive supports snapshots

Both bdrv_can_snapshot() and bdrv_has_snapshot() does not work as advertized.

First issue: Their names implies different porpouses, but they do the same thing
and have exactly the same code. Maybe copied and pasted and forgotten?
bdrv_has_snapshot() is called in various places for actually checking if there
is snapshots or not.

Second issue: the way bdrv_can_snapshot() verifies if a block driver supports or
not snapshots does not catch all cases. E.g.: a raw image.

So when do_savevm() is called, first thing it does is to set a global
BlockDriverState to save the VM memory state calling get_bs_snapshots().

static BlockDriverState *get_bs_snapshots(void)
{
    BlockDriverState *bs;
    DriveInfo *dinfo;

    if (bs_snapshots)
        return bs_snapshots;
    QTAILQ_FOREACH(dinfo, &drives, next) {
        bs = dinfo->bdrv;
        if (bdrv_can_snapshot(bs))
            goto ok;
    }
    return NULL;
 ok:
    bs_snapshots = bs;
    return bs;
}

bdrv_can_snapshot() may return a BlockDriverState that does not support
snapshots and do_savevm() goes on.

Later on in do_savevm(), we find:

    QTAILQ_FOREACH(dinfo, &drives, next) {
        bs1 = dinfo->bdrv;
        if (bdrv_has_snapshot(bs1)) {
            /* Write VM state size only to the image that contains the state */
            sn->vm_state_size = (bs == bs1 ? vm_state_size : 0);
            ret = bdrv_snapshot_create(bs1, sn);
            if (ret < 0) {
                monitor_printf(mon, "Error while creating snapshot on '%s'\n",
                               bdrv_get_device_name(bs1));
            }
        }
    }

bdrv_has_snapshot(bs1) is not checking if the device does support or has
snapshots as explained above. Only in bdrv_snapshot_create() the device is
actually checked for snapshot support.

So, in cases where the first device supports snapshots, and the second does not,
the snapshot on the first will happen anyways. I believe this is not a good
behavior. It should be an all or nothing process.

This patch addresses these issues by making bdrv_can_snapshot() actually do
what it must do and enforces better tests to avoid errors in the middle of
do_savevm(). bdrv_has_snapshot() is removed and replaced by bdrv_can_snapshot()
where appropriate.

bdrv_can_snapshot() was moved from savevm.c to block.c. It makes more sense to me.

The loadvm_state() function was updated too to enforce that when loading a VM at
least all writable devices must support snapshots too.

Signed-off-by: Miguel Di Ciurcio Filho <miguel.filho@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -1666,6 +1666,23 @@ void bdrv_debug_event(BlockDriverState *bs, BlkDebugEvent event)
 /**************************************************************/
 /* handling of snapshots */
 
+int bdrv_can_snapshot(BlockDriverState *bs)
+{
+    BlockDriver *drv = bs->drv;
+    if (!drv || bdrv_is_removable(bs) || bdrv_is_read_only(bs)) {
+        return 0;
+    }
+
+    if (!drv->bdrv_snapshot_create) {
+        if (bs->file != NULL) {
+            return bdrv_can_snapshot(bs->file);
+        }
+        return 0;
+    }
+
+    return 1;
+}
+
 int bdrv_snapshot_create(BlockDriverState *bs,
                          QEMUSnapshotInfo *sn_info)
 {

block.h

@@ -175,6 +175,7 @@ int bdrv_get_info(BlockDriverState *bs, BlockDriverInfo *bdi);
 const char *bdrv_get_encrypted_filename(BlockDriverState *bs);
 void bdrv_get_backing_filename(BlockDriverState *bs,
                                char *filename, int filename_size);
+int bdrv_can_snapshot(BlockDriverState *bs);
 int bdrv_snapshot_create(BlockDriverState *bs,
                          QEMUSnapshotInfo *sn_info);
 int bdrv_snapshot_goto(BlockDriverState *bs,

savevm.c

@@ -1575,22 +1575,6 @@ out:
     return ret;
 }
 
-/* device can contain snapshots */
-static int bdrv_can_snapshot(BlockDriverState *bs)
-{
-    return (bs &&
-            !bdrv_is_removable(bs) &&
-            !bdrv_is_read_only(bs));
-}
-
-/* device must be snapshots in order to have a reliable snapshot */
-static int bdrv_has_snapshot(BlockDriverState *bs)
-{
-    return (bs &&
-            !bdrv_is_removable(bs) &&
-            !bdrv_is_read_only(bs));
-}
-
 static BlockDriverState *get_bs_snapshots(void)
 {
     BlockDriverState *bs;
@@ -1600,8 +1584,9 @@ static BlockDriverState *get_bs_snapshots(void)
         return bs_snapshots;
     QTAILQ_FOREACH(dinfo, &drives, next) {
         bs = dinfo->bdrv;
-        if (bdrv_can_snapshot(bs))
+        if (bdrv_can_snapshot(bs)) {
             goto ok;
+        }
     }
     return NULL;
  ok:
@@ -1675,12 +1660,26 @@ void do_savevm(Monitor *mon, const QDict *qdict)
 #endif
     const char *name = qdict_get_try_str(qdict, "name");
 
+    /* Verify if there is a device that doesn't support snapshots and is writable */
+    QTAILQ_FOREACH(dinfo, &drives, next) {
+        bs = dinfo->bdrv;
+
+        if (bdrv_is_removable(bs) || bdrv_is_read_only(bs)) {
+            continue;
+        }
+
+        if (!bdrv_can_snapshot(bs)) {
+            monitor_printf(mon, "Device '%s' is writable but does not support snapshots.\n",
+                               bdrv_get_device_name(bs));
+            return;
+        }
+    }
+
     bs = get_bs_snapshots();
     if (!bs) {
         monitor_printf(mon, "No block device can accept snapshots\n");
         return;
     }
-
     /* ??? Should this occur after vm_stop?  */
     qemu_aio_flush();
 
@@ -1733,7 +1732,7 @@ void do_savevm(Monitor *mon, const QDict *qdict)
 
     QTAILQ_FOREACH(dinfo, &drives, next) {
         bs1 = dinfo->bdrv;
-        if (bdrv_has_snapshot(bs1)) {
+        if (bdrv_can_snapshot(bs1)) {
             /* Write VM state size only to the image that contains the state */
             sn->vm_state_size = (bs == bs1 ? vm_state_size : 0);
             ret = bdrv_snapshot_create(bs1, sn);
@@ -1757,6 +1756,21 @@ int load_vmstate(const char *name)
     QEMUFile *f;
     int ret;
 
+    /* Verify if there is a device that doesn't support snapshots and is writable */
+    QTAILQ_FOREACH(dinfo, &drives, next) {
+        bs = dinfo->bdrv;
+
+        if (bdrv_is_removable(bs) || bdrv_is_read_only(bs)) {
+            continue;
+        }
+
+        if (!bdrv_can_snapshot(bs)) {
+            error_report("Device '%s' is writable but does not support snapshots.",
+                               bdrv_get_device_name(bs));
+            return -ENOTSUP;
+        }
+    }
+
     bs = get_bs_snapshots();
     if (!bs) {
         error_report("No block device supports snapshots");
@@ -1768,7 +1782,7 @@ int load_vmstate(const char *name)
 
     QTAILQ_FOREACH(dinfo, &drives, next) {
         bs1 = dinfo->bdrv;
-        if (bdrv_has_snapshot(bs1)) {
+        if (bdrv_can_snapshot(bs1)) {
             ret = bdrv_snapshot_goto(bs1, name);
             if (ret < 0) {
                 switch(ret) {
@@ -1830,7 +1844,7 @@ void do_delvm(Monitor *mon, const QDict *qdict)
 
     QTAILQ_FOREACH(dinfo, &drives, next) {
         bs1 = dinfo->bdrv;
-        if (bdrv_has_snapshot(bs1)) {
+        if (bdrv_can_snapshot(bs1)) {
             ret = bdrv_snapshot_delete(bs1, name);
             if (ret < 0) {
                 if (ret == -ENOTSUP)
@@ -1861,7 +1875,7 @@ void do_info_snapshots(Monitor *mon)
     monitor_printf(mon, "Snapshot devices:");
     QTAILQ_FOREACH(dinfo, &drives, next) {
         bs1 = dinfo->bdrv;
-        if (bdrv_has_snapshot(bs1)) {
+        if (bdrv_can_snapshot(bs1)) {
             if (bs == bs1)
                 monitor_printf(mon, " %s", bdrv_get_device_name(bs1));
         }