[Qemu-devel] [PATCH 13/26] block: open backing file as read-only when probing for size

[Paolo Bonzini]

bdrv_img_create will temporarily open the backing file to probe its size.
However, this could be done with a read-write open if the wrong flags are
passed to bdrv_img_create.  Since there is really no documentation on
what flags can be passed, assume that bdrv_img_create receives the flags
with which the new image will be opened; sanitize them when opening
the backing file.

Reported-by: Eric Blake <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
---
 block.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/block.c b/block.c
index 3f5bc92..e84a0a9 100644
--- a/block.c
+++ b/block.c
@@ -4064,10 +4064,15 @@ int bdrv_img_create(const char *filename, const char 
*fmt,
         if (backing_file && backing_file->value.s) {
             uint64_t size;
             char buf[32];
+            int back_flags;
+
+            /* 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, flags, backing_drv);
+            ret = bdrv_open(bs, backing_file->value.s, back_flags, 
backing_drv);
             if (ret < 0) {
                 error_report("Could not open '%s'", backing_file->value.s);
                 goto out;
-- 
1.7.9.3