1 files changed, 55 insertions, 30 deletions

diff --git a/qemu-img.c b/qemu-img.c
index 28fba1e7a7..b0535919b1 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3164,7 +3164,7 @@ static int img_rebase(int argc, char **argv)
     BlockBackend *blk = NULL, *blk_old_backing = NULL, *blk_new_backing = NULL;
     uint8_t *buf_old = NULL;
     uint8_t *buf_new = NULL;
-    BlockDriverState *bs = NULL;
+    BlockDriverState *bs = NULL, *prefix_chain_bs = NULL;
     char *filename;
     const char *fmt, *cache, *src_cache, *out_basefmt, *out_baseimg;
     int c, flags, src_flags, ret;
@@ -3309,29 +3309,18 @@ static int img_rebase(int argc, char **argv)
 
     /* For safe rebasing we need to compare old and new backing file */
     if (!unsafe) {
-        char backing_name[PATH_MAX];
         QDict *options = NULL;
+        BlockDriverState *base_bs = backing_bs(bs);
 
-        if (bs->backing) {
-            if (bs->backing_format[0] != '\0') {
-                options = qdict_new();
-                qdict_put_str(options, "driver", bs->backing_format);
-            }
-
-            if (force_share) {
-                if (!options) {
-                    options = qdict_new();
-                }
-                qdict_put_bool(options, BDRV_OPT_FORCE_SHARE, true);
-            }
-            bdrv_get_backing_filename(bs, backing_name, sizeof(backing_name));
-            blk_old_backing = blk_new_open(backing_name, NULL,
-                                           options, src_flags, &local_err);
-            if (!blk_old_backing) {
+        if (base_bs) {
+            blk_old_backing = blk_new(BLK_PERM_CONSISTENT_READ,
+                                      BLK_PERM_ALL);
+            ret = blk_insert_bs(blk_old_backing, base_bs,
+                                &local_err);
+            if (ret < 0) {
                 error_reportf_err(local_err,
-                                  "Could not open old backing file '%s': ",
-                                  backing_name);
-                ret = -1;
+                                  "Could not reuse old backing file '%s': ",
+                                  base_bs->filename);
                 goto out;
             }
         } else {
@@ -3364,15 +3353,34 @@ static int img_rebase(int argc, char **argv)
                 goto out;
             }
 
-            blk_new_backing = blk_new_open(out_real_path, NULL,
-                                           options, src_flags, &local_err);
-            g_free(out_real_path);
-            if (!blk_new_backing) {
-                error_reportf_err(local_err,
-                                  "Could not open new backing file '%s': ",
-                                  out_baseimg);
-                ret = -1;
-                goto out;
+            /*
+             * Find out whether we rebase an image on top of a previous image
+             * in its chain.
+             */
+            prefix_chain_bs = bdrv_find_backing_image(bs, out_real_path);
+            if (prefix_chain_bs) {
+                g_free(out_real_path);
+                blk_new_backing = blk_new(BLK_PERM_CONSISTENT_READ,
+                                          BLK_PERM_ALL);
+                ret = blk_insert_bs(blk_new_backing, prefix_chain_bs,
+                                    &local_err);
+                if (ret < 0) {
+                    error_reportf_err(local_err,
+                                      "Could not reuse backing file '%s': ",
+                                      out_baseimg);
+                    goto out;
+                }
+            } else {
+                blk_new_backing = blk_new_open(out_real_path, NULL,
+                                               options, src_flags, &local_err);
+                g_free(out_real_path);
+                if (!blk_new_backing) {
+                    error_reportf_err(local_err,
+                                      "Could not open new backing file '%s': ",
+                                      out_baseimg);
+                    ret = -1;
+                    goto out;
+                }
             }
         }
     }
@@ -3448,6 +3456,23 @@ static int img_rebase(int argc, char **argv)
                 continue;
             }
 
+            if (prefix_chain_bs) {
+                /*
+                 * If cluster wasn't changed since prefix_chain, we don't need
+                 * to take action
+                 */
+                ret = bdrv_is_allocated_above(backing_bs(bs), prefix_chain_bs,
+                                              offset, n, &n);
+                if (ret < 0) {
+                    error_report("error while reading image metadata: %s",
+                                 strerror(-ret));
+                    goto out;
+                }
+                if (!ret) {
+                    continue;
+                }
+            }
+
             /*
              * Read old and new backing file and take into consideration that
              * backing files may be smaller than the COW image.