commit: Allow users to request only format driver names in backing file format

Introduce a new flag 'backing-mask-protocol' for the block-commit QMP
command which instructs the internals to use 'raw' instead of the
protocol driver in case when a image is used without a dummy 'raw'
wrapper.

The flag is designed such that it can be always asserted by management
tools even when there isn't any update to backing files.

The flag will be used by libvirt so that the backing images still
reference the proper format even when libvirt will stop using the dummy
raw driver (raw driver with no other config). Libvirt needs this so that
the images stay compatible with older libvirt versions which didn't
expect that a protocol driver name can appear in the backing file format
field.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <2cb46e37093ce793ea1604abc8bbb90f4c8e434b.1701796348.git.pkrempa@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

8 files changed, 58 insertions, 12 deletions

diff --git a/block.c b/block.c
index a097772..30afdcb 100644
--- a/block.c
+++ b/block.c
@@ -1309,11 +1309,14 @@ static void bdrv_backing_detach(BdrvChild *c)
 }
 
 static int bdrv_backing_update_filename(BdrvChild *c, BlockDriverState *base,
-                                        const char *filename, Error **errp)
+                                        const char *filename,
+                                        bool backing_mask_protocol,
+                                        Error **errp)
 {
     BlockDriverState *parent = c->opaque;
     bool read_only = bdrv_is_read_only(parent);
     int ret;
+    const char *format_name;
     GLOBAL_STATE_CODE();
 
     if (read_only) {
@@ -1323,9 +1326,23 @@ static int bdrv_backing_update_filename(BdrvChild *c, BlockDriverState *base,
         }
     }
 
-    ret = bdrv_change_backing_file(parent, filename,
-                                   base->drv ? base->drv->format_name : "",
-                                   false);
+    if (base->drv) {
+        /*
+         * If the new base image doesn't have a format driver layer, which we
+         * detect by the fact that @base is a protocol driver, we record
+         * 'raw' as the format instead of putting the protocol name as the
+         * backing format
+         */
+        if (backing_mask_protocol && base->drv->protocol_name) {
+            format_name = "raw";
+        } else {
+            format_name = base->drv->format_name;
+        }
+    } else {
+        format_name = "";
+    }
+
+    ret = bdrv_change_backing_file(parent, filename, format_name, false);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Could not update backing file link");
     }
@@ -1479,10 +1496,14 @@ static void GRAPH_WRLOCK bdrv_child_cb_detach(BdrvChild *child)
 }
 
 static int bdrv_child_cb_update_filename(BdrvChild *c, BlockDriverState *base,
-                                         const char *filename, Error **errp)
+                                         const char *filename,
+                                         bool backing_mask_protocol,
+                                         Error **errp)
 {
     if (c->role & BDRV_CHILD_COW) {
-        return bdrv_backing_update_filename(c, base, filename, errp);
+        return bdrv_backing_update_filename(c, base, filename,
+                                            backing_mask_protocol,
+                                            errp);
     }
     return 0;
 }
@@ -5803,7 +5824,8 @@ void bdrv_unfreeze_backing_chain(BlockDriverState *bs, BlockDriverState *base)
  *
  */
 int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
-                           const char *backing_file_str)
+                           const char *backing_file_str,
+                           bool backing_mask_protocol)
 {
     BlockDriverState *explicit_top = top;
     bool update_inherits_from;
@@ -5869,6 +5891,7 @@ int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
 
         if (c->klass->update_filename) {
             ret = c->klass->update_filename(c, base, backing_file_str,
+                                            backing_mask_protocol,
                                             &local_err);
             if (ret < 0) {
                 /*
diff --git a/block/commit.c b/block/commit.c
index 1dd7a65..7c3fdcb 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -42,6 +42,7 @@ typedef struct CommitBlockJob {
     bool base_read_only;
     bool chain_frozen;
     char *backing_file_str;
+    bool backing_mask_protocol;
 } CommitBlockJob;
 
 static int commit_prepare(Job *job)
@@ -61,7 +62,8 @@ static int commit_prepare(Job *job)
     /* FIXME: bdrv_drop_intermediate treats total failures and partial failures
      * identically. Further work is needed to disambiguate these cases. */
     return bdrv_drop_intermediate(s->commit_top_bs, s->base_bs,
-                                  s->backing_file_str);
+                                  s->backing_file_str,
+                                  s->backing_mask_protocol);
 }
 
 static void commit_abort(Job *job)
@@ -254,6 +256,7 @@ void commit_start(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *base, BlockDriverState *top,
                   int creation_flags, int64_t speed,
                   BlockdevOnError on_error, const char *backing_file_str,
+                  bool backing_mask_protocol,
                   const char *filter_node_name, Error **errp)
 {
     CommitBlockJob *s;
@@ -408,6 +411,7 @@ void commit_start(const char *job_id, BlockDriverState *bs,
     blk_set_disable_request_queuing(s->top, true);
 
     s->backing_file_str = g_strdup(backing_file_str);
+    s->backing_mask_protocol = backing_mask_protocol;
     s->on_error = on_error;
 
     trace_commit_start(bs, base, top, s);
diff --git a/blockdev.c b/blockdev.c
index 3a5e722..292c8af 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2420,6 +2420,8 @@ void qmp_block_commit(const char *job_id, const char *device,
                       const char *top_node,
                       const char *top,
                       const char *backing_file,
+                      bool has_backing_mask_protocol,
+                      bool backing_mask_protocol,
                       bool has_speed, int64_t speed,
                       bool has_on_error, BlockdevOnError on_error,
                       const char *filter_node_name,
@@ -2450,6 +2452,9 @@ void qmp_block_commit(const char *job_id, const char *device,
     if (has_auto_dismiss && !auto_dismiss) {
         job_flags |= JOB_MANUAL_DISMISS;
     }
+    if (!has_backing_mask_protocol) {
+        backing_mask_protocol = false;
+    }
 
     /* Important Note:
      *  libvirt relies on the DeviceNotFound error class in order to probe for
@@ -2591,6 +2596,7 @@ void qmp_block_commit(const char *job_id, const char *device,
         }
         commit_start(job_id, bs, base_bs, top_bs, job_flags,
                      speed, on_error, backing_file,
+                     backing_mask_protocol,
                      filter_node_name, &local_err);
     }
     if (local_err != NULL) {
diff --git a/include/block/block-global-state.h b/include/block/block-global-state.h
index 4ec0b21..bd7cecd 100644
--- a/include/block/block-global-state.h
+++ b/include/block/block-global-state.h
@@ -144,7 +144,8 @@ int GRAPH_RDLOCK bdrv_make_empty(BdrvChild *c, Error **errp);
 
 void bdrv_register(BlockDriver *bdrv);
 int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
-                           const char *backing_file_str);
+                           const char *backing_file_str,
+                           bool backing_mask_protocol);
 
 BlockDriverState * GRAPH_RDLOCK
 bdrv_find_overlay(BlockDriverState *active, BlockDriverState *bs);
diff --git a/include/block/block_int-common.h b/include/block/block_int-common.h
index 151279d..7612761 100644
--- a/include/block/block_int-common.h
+++ b/include/block/block_int-common.h
@@ -985,7 +985,9 @@ struct BdrvChildClass {
      * can update its reference.
      */
     int (*update_filename)(BdrvChild *child, BlockDriverState *new_base,
-                           const char *filename, Error **errp);
+                           const char *filename,
+                           bool backing_mask_protocol,
+                           Error **errp);
 
     bool (*change_aio_ctx)(BdrvChild *child, AioContext *ctx,
                            GHashTable *visited, Transaction *tran,
diff --git a/include/block/block_int-global-state.h b/include/block/block_int-global-state.h
index ef31c58..2162269 100644
--- a/include/block/block_int-global-state.h
+++ b/include/block/block_int-global-state.h
@@ -82,6 +82,8 @@ void stream_start(const char *job_id, BlockDriverState *bs,
  * @speed: The maximum speed, in bytes per second, or 0 for unlimited.
  * @on_error: The action to take upon error.
  * @backing_file_str: String to use as the backing file in @top's overlay
+ * @backing_mask_protocol: Replace potential protocol name with 'raw' in
+ *                         'backing file format' header
  * @filter_node_name: The node name that should be assigned to the filter
  * driver that the commit job inserts into the graph above @top. NULL means
  * that a node name should be autogenerated.
@@ -92,6 +94,7 @@ void commit_start(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *base, BlockDriverState *top,
                   int creation_flags, int64_t speed,
                   BlockdevOnError on_error, const char *backing_file_str,
+                  bool backing_mask_protocol,
                   const char *filter_node_name, Error **errp);
 /**
  * commit_active_start:
diff --git a/qapi/block-core.json b/qapi/block-core.json
index ca390c5..726145e 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -1810,6 +1810,11 @@
 #     Care should be taken when specifying the string, to specify a
 #     valid filename or protocol.  (Since 2.1)
 #
+# @backing-mask-protocol: If true, replace any protocol mentioned in the
+#     'backing file format' with 'raw', rather than storing the protocol
+#     name as the backing format.  Can be used even when no image header
+#     will be updated (default false; since 9.0).
+#
 # @speed: the maximum speed, in bytes per second
 #
 # @on-error: the action to take on an error.  'ignore' means that the
@@ -1856,7 +1861,8 @@
             '*base': { 'type': 'str', 'features': [ 'deprecated' ] },
             '*top-node': 'str',
             '*top': { 'type': 'str', 'features': [ 'deprecated' ] },
-            '*backing-file': 'str', '*speed': 'int',
+            '*backing-file': 'str', '*backing-mask-protocol': 'bool',
+            '*speed': 'int',
             '*on-error': 'BlockdevOnError',
             '*filter-node-name': 'str',
             '*auto-finalize': 'bool', '*auto-dismiss': 'bool' },
diff --git a/tests/unit/test-bdrv-drain.c b/tests/unit/test-bdrv-drain.c
index 17830a6..6668804 100644
--- a/tests/unit/test-bdrv-drain.c
+++ b/tests/unit/test-bdrv-drain.c
@@ -1591,6 +1591,7 @@ static const BlockJobDriver test_simple_job_driver = {
 static int drop_intermediate_poll_update_filename(BdrvChild *child,
                                                   BlockDriverState *new_base,
                                                   const char *filename,
+                                                  bool backing_mask_protocol,
                                                   Error **errp)
 {
     /*
@@ -1702,7 +1703,7 @@ static void test_drop_intermediate_poll(void)
     job->should_complete = true;
 
     g_assert(!job_has_completed);
-    ret = bdrv_drop_intermediate(chain[1], chain[0], NULL);
+    ret = bdrv_drop_intermediate(chain[1], chain[0], NULL, false);
     aio_poll(qemu_get_aio_context(), false);
     g_assert(ret == 0);
     g_assert(job_has_completed);