block: Catch backing files assigned to non-COW drivers

Since we parse backing.* options to add a backing file from the command
line when the driver didn't assign one, it has been possible to have a
backing file for e.g. raw images (it just was never accessed).

This is obvious nonsense and should be rejected.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

block.c

@@ -1192,6 +1192,13 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
         bdrv_get_full_backing_filename(bs, backing_filename, PATH_MAX);
     }
 
+    if (!bs->drv || !bs->drv->supports_backing) {
+        ret = -EINVAL;
+        error_setg(errp, "Driver doesn't support backing files");
+        QDECREF(options);
+        goto free_exit;
+    }
+
     backing_hd = bdrv_new("", errp);
 
     if (bs->backing_format[0] != '\0') {

block/cow.c

@@ -414,6 +414,7 @@ static BlockDriver bdrv_cow = {
     .bdrv_close     = cow_close,
     .bdrv_create    = cow_create,
     .bdrv_has_zero_init     = bdrv_has_zero_init_1,
+    .supports_backing       = true,
 
     .bdrv_read              = cow_co_read,
     .bdrv_write             = cow_co_write,

block/qcow.c

@@ -941,6 +941,7 @@ static BlockDriver bdrv_qcow = {
     .bdrv_reopen_prepare    = qcow_reopen_prepare,
     .bdrv_create            = qcow_create,
     .bdrv_has_zero_init     = bdrv_has_zero_init_1,
+    .supports_backing       = true,
 
     .bdrv_co_readv          = qcow_co_readv,
     .bdrv_co_writev         = qcow_co_writev,

block/qcow2.c

@@ -2418,6 +2418,7 @@ static BlockDriver bdrv_qcow2 = {
     .bdrv_save_vmstate    = qcow2_save_vmstate,
     .bdrv_load_vmstate    = qcow2_load_vmstate,
 
+    .supports_backing           = true,
     .bdrv_change_backing_file   = qcow2_change_backing_file,
 
     .bdrv_refresh_limits        = qcow2_refresh_limits,

block/qed.c

@@ -1652,6 +1652,7 @@ static BlockDriver bdrv_qed = {
     .format_name              = "qed",
     .instance_size            = sizeof(BDRVQEDState),
     .create_opts              = &qed_create_opts,
+    .supports_backing         = true,
 
     .bdrv_probe               = bdrv_qed_probe,
     .bdrv_rebind              = bdrv_qed_rebind,

block/vmdk.c

@@ -2180,6 +2180,7 @@ static BlockDriver bdrv_vmdk = {
     .bdrv_detach_aio_context      = vmdk_detach_aio_context,
     .bdrv_attach_aio_context      = vmdk_attach_aio_context,
 
+    .supports_backing             = true,
     .create_opts                  = &vmdk_create_opts,
 };
 

include/block/block_int.h

@@ -100,6 +100,9 @@ struct BlockDriver {
      */
     bool bdrv_needs_filename;
 
+    /* Set if a driver can support backing files */
+    bool supports_backing;
+
     /* For handling image reopen for split or non-split files */
     int (*bdrv_reopen_prepare)(BDRVReopenState *reopen_state,
                                BlockReopenQueue *queue, Error **errp);

tests/qemu-iotests/051

@@ -100,6 +100,11 @@ echo
 
 echo "info block" | run_qemu -drive file="$TEST_IMG",driver=qcow2,backing.file.filename="$TEST_IMG.orig" -nodefaults
 
+# Drivers that don't support backing files
+run_qemu -drive file="$TEST_IMG",driver=raw,backing.file.filename="$TEST_IMG.orig"
+run_qemu -drive file="$TEST_IMG",file.backing.driver=file,file.backing.filename="$TEST_IMG.orig"
+run_qemu -drive file="$TEST_IMG",file.backing.driver=qcow2,file.backing.file.filename="$TEST_IMG.orig"
+
 echo
 echo === Enable and disable lazy refcounting on the command line, plus some invalid values ===
 echo

tests/qemu-iotests/051.out

@@ -53,6 +53,15 @@ ide0-hd0: TEST_DIR/t.qcow2 (qcow2)
     Backing file:     TEST_DIR/t.qcow2.orig (chain depth: 1)
 (qemu) qququiquit
 
+Testing: -drive file=TEST_DIR/t.qcow2,driver=raw,backing.file.filename=TEST_DIR/t.qcow2.orig
+QEMU_PROG: -drive file=TEST_DIR/t.qcow2,driver=raw,backing.file.filename=TEST_DIR/t.qcow2.orig: could not open disk image TEST_DIR/t.qcow2: Driver doesn't support backing files
+
+Testing: -drive file=TEST_DIR/t.qcow2,file.backing.driver=file,file.backing.filename=TEST_DIR/t.qcow2.orig
+QEMU_PROG: -drive file=TEST_DIR/t.qcow2,file.backing.driver=file,file.backing.filename=TEST_DIR/t.qcow2.orig: could not open disk image TEST_DIR/t.qcow2: Driver doesn't support backing files
+
+Testing: -drive file=TEST_DIR/t.qcow2,file.backing.driver=qcow2,file.backing.file.filename=TEST_DIR/t.qcow2.orig
+QEMU_PROG: -drive file=TEST_DIR/t.qcow2,file.backing.driver=qcow2,file.backing.file.filename=TEST_DIR/t.qcow2.orig: could not open disk image TEST_DIR/t.qcow2: Driver doesn't support backing files
+
 
 === Enable and disable lazy refcounting on the command line, plus some invalid values ===