[Qemu-devel] [PATCH 1/2] block: make bdrv_find_backing_image compare canonical filenames

[Jeff Cody]

Currently, bdrv_find_backing_image compares bs->backing_file with
what is passed in as a backing_file name.  Mismatches may occur,
however, when bs->backing_file and backing_file are both not
absolute or relative.

Use path_combine() to make sure any relative backing filenames are
relative to the current image filename being searched, and then use
realpath() to make all comparisons based on absolute filenames.

This also changes bdrv_find_backing_image to no longer be recursive,
but iterative.

Signed-off-by: Jeff Cody <address@hidden>
---
 block.c | 38 ++++++++++++++++++++++++++++++++------
 1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/block.c b/block.c
index e95f613..641b8fa 100644
--- a/block.c
+++ b/block.c
@@ -3123,18 +3123,44 @@ int bdrv_snapshot_load_tmp(BlockDriverState *bs,
     return -ENOTSUP;
 }
 
+/* backing_file can either be relative, or absolute.  If it is
+ * relative, it must be relative to the chain.  So, passing in
+ * bs->filename from a BDS as backing_file should not be done,
+ * as that may be relative to the CWD rather than the chain. */
 BlockDriverState *bdrv_find_backing_image(BlockDriverState *bs,
         const char *backing_file)
 {
-    if (!bs->drv) {
+    char filename_full[PATH_MAX];
+    char backing_file_full[PATH_MAX];
+    char filename_tmp[PATH_MAX];
+    BlockDriverState *curr_bs = NULL;
+
+    if (!bs || !bs->drv) {
         return NULL;
     }
 
-    if (bs->backing_hd) {
-        if (strcmp(bs->backing_file, backing_file) == 0) {
-            return bs->backing_hd;
-        } else {
-            return bdrv_find_backing_image(bs->backing_hd, backing_file);
+    for (curr_bs = bs; curr_bs->backing_hd; curr_bs = curr_bs->backing_hd) {
+        /* If not an absolute filename path, make it relative to the current
+         * image's filename path */
+        path_combine(filename_tmp, sizeof(filename_tmp),
+                     curr_bs->filename, backing_file);
+
+        /* We are going to compare absolute pathnames */
+        if (!realpath(filename_tmp, filename_full)) {
+            continue;
+        }
+
+        /* We need to make sure the backing filename we are comparing against
+         * is relative to the current image filename (or absolute) */
+        path_combine(filename_tmp, sizeof(filename_tmp),
+                     curr_bs->filename, curr_bs->backing_file);
+
+        if (!realpath(filename_tmp, backing_file_full)) {
+            continue;
+        }
+
+        if (strcmp(backing_file_full, filename_full) == 0) {
+            return curr_bs->backing_hd;
         }
     }
 
-- 
1.7.11.4