block: Add bdrv_make_empty()

Right now, all users of bdrv_make_empty() call the BlockDriver method
directly.  That is not only bad style, it is also wrong, unless the
caller has a BdrvChild with a WRITE or WRITE_UNCHANGED permission.
(WRITE_UNCHANGED suffices, because callers generally use this function
to clear a node with a backing file after a commit operation.)

Introduce bdrv_make_empty() that verifies that it does.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200429141126.85159-2-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

2 files changed, 24 insertions, 0 deletions

diff --git a/block.c b/block.c
index 0653ccb913..c6a5c144b7 100644
--- a/block.c
+++ b/block.c
@@ -6764,3 +6764,26 @@ void bdrv_del_child(BlockDriverState *parent_bs, BdrvChild *child, Error **errp)
 
     parent_bs->drv->bdrv_del_child(parent_bs, child, errp);
 }
+
+int bdrv_make_empty(BdrvChild *c, Error **errp)
+{
+    BlockDriver *drv = c->bs->drv;
+    int ret;
+
+    assert(c->perm & (BLK_PERM_WRITE | BLK_PERM_WRITE_UNCHANGED));
+
+    if (!drv->bdrv_make_empty) {
+        error_setg(errp, "%s does not support emptying nodes",
+                   drv->format_name);
+        return -ENOTSUP;
+    }
+
+    ret = drv->bdrv_make_empty(c->bs);
+    if (ret < 0) {
+        error_setg_errno(errp, -ret, "Failed to empty %s",
+                         c->bs->filename);
+        return ret;
+    }
+
+    return 0;
+}
diff --git a/include/block/block.h b/include/block/block.h
index 4de8d8f8a6..8f3eb70df4 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -352,6 +352,7 @@ BlockMeasureInfo *bdrv_measure(BlockDriver *drv, QemuOpts *opts,
 void bdrv_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr);
 void bdrv_refresh_limits(BlockDriverState *bs, Error **errp);
 int bdrv_commit(BlockDriverState *bs);
+int bdrv_make_empty(BdrvChild *c, Error **errp);
 int bdrv_change_backing_file(BlockDriverState *bs,
     const char *backing_file, const char *backing_fmt);
 void bdrv_register(BlockDriver *bdrv);