[Qemu-stable] [PATCH v2 1/2] qemu-img: Resolve relative backing paths in rebase

[Max Reitz]

Currently, rebase interprets a relative path for the new backing image
as follows:
(1) Open the new backing image with the given relative path (thus relative to
    qemu-img's working directory).
(2) Write it directly into the overlay's backing path field (thus
    relative to the overlay).

If the overlay is not in qemu-img's working directory, both will be
different interpretations, which may either lead to an error somewhere
(either rebase fails because it cannot open the new backing image, or
 your overlay becomes unusable because its backing path does not point
 to a file), or, even worse, it may result in your rebase being
performed for a different backing file than what your overlay will point
to after the rebase.

Fix this by interpreting the target backing path as relative to the
overlay, like qemu-img does everywhere else.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1569835
Cc: address@hidden
Signed-off-by: Max Reitz <address@hidden>
---
 qemu-img.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/qemu-img.c b/qemu-img.c
index ea62d2d61e..55b146aa49 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3191,6 +3191,9 @@ static int img_rebase(int argc, char **argv)
         }
 
         if (out_baseimg[0]) {
+            const char *overlay_filename;
+            char *out_real_path;
+
             options = qdict_new();
             if (out_basefmt) {
                 qdict_put_str(options, "driver", out_basefmt);
@@ -3199,8 +3202,26 @@ static int img_rebase(int argc, char **argv)
                 qdict_put_bool(options, BDRV_OPT_FORCE_SHARE, true);
             }
 
-            blk_new_backing = blk_new_open(out_baseimg, NULL,
+            overlay_filename = bs->exact_filename[0] ? bs->exact_filename
+                                                     : bs->filename;
+            out_real_path = g_malloc(PATH_MAX);
+
+            bdrv_get_full_backing_filename_from_filename(overlay_filename,
+                                                         out_baseimg,
+                                                         out_real_path,
+                                                         PATH_MAX,
+                                                         &local_err);
+            if (local_err) {
+                error_reportf_err(local_err,
+                                  "Could not resolve backing filename: ");
+                ret = -1;
+                g_free(out_real_path);
+                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': ",
-- 
2.14.3