[Qemu-devel] [PATCH] block: Fix relative backing file path checking

[Xu Wang <gesaint@linux.vnet.ibm.com>]

This patch is made for Bug #1257334 (diffuse handling of image
creation from another path). The cause of it is user could create
image even though the backing file doesn't exist. Becase backing
file checking in the bdrv_img_create() is from the user's current
path instead of relative path to the image to be created. This patch
makes qemu check backing file according to the relative path to the
image to be created. Hence if relative backing file path doesn't
exist, the backing file checking will fail now.

Test case:
  Reproduce process (from bug page):
    1. $mkdir a/
    2. $qemu-img create -f qcow2 a/blob.img 10G
    3. $qemu-img create -f qcow2 -b a/blob.img a/ovl.img
       Here the actual backing file of ovl.img is a/a/blob.img. But
       the backing file checking will check from the user's current
       path and find a/blob.img successfully. But the path saved in
       the ovl.img is a/a/blob.img. Bug occurred.

  After patched:
    The step 3 above an error message will be thrown because backing
    file checking started after got the full path of backing file
    intead of relative path.

Signed-off-by: Xu Wang <gesaint@linux.vnet.ibm.com>
---
 block.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/block.c b/block.c
index 13f001a..20d2b66 100644
--- a/block.c
+++ b/block.c
@@ -4790,18 +4790,31 @@ void bdrv_img_create(const char *filename, const char *fmt,
             uint64_t size;
             char buf[32];
             int back_flags;
+            char backing_filename_full[PATH_MAX];
 
+            /* check if relative backing file path could be accessed */
+            if (backing_file->value.s[0] == '\0' ||
+                path_has_protocol(backing_file->value.s)) {
+                pstrcpy(backing_filename_full, sizeof(backing_filename_full),
+                        backing_file->value.s);
+            } else {
+                path_combine(backing_filename_full,
+                             sizeof(backing_filename_full),
+                             filename, backing_file->value.s);
+            }
             /* backing files always opened read-only */
             back_flags =
                 flags & ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
 
             bs = bdrv_new("");
 
-            ret = bdrv_open(bs, backing_file->value.s, NULL, back_flags,
+            ret = bdrv_open(bs, backing_filename_full, NULL, back_flags,
                             backing_drv, &local_err);
             if (ret < 0) {
-                error_setg_errno(errp, -ret, "Could not open '%s': %s",
+                error_setg_errno(errp, -ret, "Backing file '%s'"
+                                 "(actual path is '%s') error: %s",
                                  backing_file->value.s,
+                                 backing_filename_full,
                                  error_get_pretty(local_err));
                 error_free(local_err);
                 local_err = NULL;
-- 
1.8.1.4