block: JSON filenames and relative backing files

When using a relative backing file name, qemu needs to know the
directory of the top image file. For JSON filenames, such a directory
cannot be easily determined (e.g. how do you determine the directory of
a qcow2 BDS directly on top of a quorum BDS?). Therefore, do not allow
relative filenames for the backing file of BDSs only having a JSON
filename.

Furthermore, BDS::exact_filename should be used whenever possible. If
BDS::filename is not equal to BDS::exact_filename, the former will
always be a JSON object.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -305,19 +305,28 @@ void path_combine(char *dest, int dest_size,
 
 void bdrv_get_full_backing_filename_from_filename(const char *backed,
                                                   const char *backing,
-                                                  char *dest, size_t sz)
+                                                  char *dest, size_t sz,
+                                                  Error **errp)
 {
-    if (backing[0] == '\0' || path_has_protocol(backing)) {
+    if (backing[0] == '\0' || path_has_protocol(backing) ||
+        path_is_absolute(backing))
+    {
         pstrcpy(dest, sz, backing);
+    } else if (backed[0] == '\0' || strstart(backed, "json:", NULL)) {
+        error_setg(errp, "Cannot use relative backing file names for '%s'",
+                   backed);
     } else {
         path_combine(dest, sz, backed, backing);
     }
 }
 
-void bdrv_get_full_backing_filename(BlockDriverState *bs, char *dest, size_t sz)
+void bdrv_get_full_backing_filename(BlockDriverState *bs, char *dest, size_t sz,
+                                    Error **errp)
 {
-    bdrv_get_full_backing_filename_from_filename(bs->filename, bs->backing_file,
-                                                 dest, sz);
+    char *backed = bs->exact_filename[0] ? bs->exact_filename : bs->filename;
+
+    bdrv_get_full_backing_filename_from_filename(backed, bs->backing_file,
+                                                 dest, sz, errp);
 }
 
 void bdrv_register(BlockDriver *bdrv)
@@ -1225,7 +1234,14 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
         QDECREF(options);
         goto free_exit;
     } else {
-        bdrv_get_full_backing_filename(bs, backing_filename, PATH_MAX);
+        bdrv_get_full_backing_filename(bs, backing_filename, PATH_MAX,
+                                       &local_err);
+        if (local_err) {
+            ret = -EINVAL;
+            error_propagate(errp, local_err);
+            QDECREF(options);
+            goto free_exit;
+        }
     }
 
     if (!bs->drv || !bs->drv->supports_backing) {

block/qapi.c

@@ -214,7 +214,12 @@ void bdrv_query_image_info(BlockDriverState *bs,
         info->backing_filename = g_strdup(backing_filename);
         info->has_backing_filename = true;
         bdrv_get_full_backing_filename(bs, backing_filename2,
-                                       sizeof(backing_filename2));
+                                       sizeof(backing_filename2), &err);
+        if (err) {
+            error_propagate(errp, err);
+            qapi_free_ImageInfo(info);
+            return;
+        }
 
         if (strcmp(backing_filename, backing_filename2) != 0) {
             info->full_backing_filename =

include/block/block.h

@@ -396,10 +396,11 @@ const char *bdrv_get_encrypted_filename(BlockDriverState *bs);
 void bdrv_get_backing_filename(BlockDriverState *bs,
                                char *filename, int filename_size);
 void bdrv_get_full_backing_filename(BlockDriverState *bs,
-                                    char *dest, size_t sz);
+                                    char *dest, size_t sz, Error **errp);
 void bdrv_get_full_backing_filename_from_filename(const char *backed,
                                                   const char *backing,
-                                                  char *dest, size_t sz);
+                                                  char *dest, size_t sz,
+                                                  Error **errp);
 int bdrv_is_snapshot(BlockDriverState *bs);
 
 int path_has_protocol(const char *path);