[Qemu-devel] [PATCH 1/2] block: JSON filenames and relative backing files

[Max Reitz]

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.

Signed-off-by: Max Reitz <address@hidden>
---
Just by the way, the reason for using bs->exact_filename in
bdrv_get_full_backing_filename() instead of just testing whether
bs->filename is prefixed by "json:" is that in the future we might have
cases where bs->filename contains a JSON filename, but
bs->exact_filename is set to a non-JSON filename (because there are some
rather vital options, which radically change performance or something
like that, so we want them to be included in bs->filename if they were
specified; but we can still generate a plain filename which results in
the same data read and written, but just in some very different way or
something like that).
Actually, I might write a follow-up patch which makes
bdrv_refresh_filename() always generate an exact_filename if somehow
possible, even if unknown options were specified. This would then be
very useful for this function, but on the other hand, it would no longer
fit the definition of exact_filename (in order to follow that
definition, we have to be certain that we don't omit any vital options
which really change the data read and written).
---
 block.c               | 19 +++++++++++++++----
 block/qapi.c          |  7 ++++++-
 include/block/block.h |  2 +-
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/block.c b/block.c
index 88f6d9b..6ccc94b 100644
--- a/block.c
+++ b/block.c
@@ -303,12 +303,17 @@ void path_combine(char *dest, int dest_size,
     }
 }
 
-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)
 {
-    if (bs->backing_file[0] == '\0' || path_has_protocol(bs->backing_file)) {
+    if (bs->backing_file[0] == '\0' || path_has_protocol(bs->backing_file) ||
+        path_is_absolute(bs->backing_file)) {
         pstrcpy(dest, sz, bs->backing_file);
+    } else if (!bs->exact_filename[0] ||
+               strstart(bs->exact_filename, "json:", NULL)) {
+        error_setg(errp, "Cannot use relative backing file names for '%s'",
+                   bs->filename);
     } else {
-        path_combine(dest, sz, bs->filename, bs->backing_file);
+        path_combine(dest, sz, bs->exact_filename, bs->backing_file);
     }
 }
 
@@ -1197,7 +1202,13 @@ 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) {
diff --git a/block/qapi.c b/block/qapi.c
index 1301144..77baa36 100644
--- a/block/qapi.c
+++ b/block/qapi.c
@@ -229,7 +229,12 @@ 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 =
diff --git a/include/block/block.h b/include/block/block.h
index 341054d..3abc7df 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -387,7 +387,7 @@ 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);
 int bdrv_is_snapshot(BlockDriverState *bs);
 
 int path_is_absolute(const char *path);
-- 
1.9.3