[Qemu-devel] [PATCH] block: Enable fall-back to read-only for backing file

[Naphtali Sprei <nsprei@redhat.com>]

There's a problem when trying to use an image file based on a read-only image file.
Before this patch, qemu fails to open the base image and stop.
With this patch, qemu tries to open the backing file with same permissions as the "top" file,
but if it fails, qemu tries to open it with read-only permissions. If succeeded it goes on.

This fall-back works both for an image file based on a read-only file
and also for a read-only file opened with the snapshot attribute/mode (where the real file is the backing file
for the snapshot file).

Is it better to always open the backing file with read-only mode ? this will be more consistent/predictable ?
Or is it better not to fall-back to read-only ? Will a warning message help ?

 TIA,

   Naphtali
   

>From 4a10750f5c91b1383118e4421f6b8d3ff3e79b2f Mon Sep 17 00:00:00 2001
From: Naphtali Sprei <nsprei@redhat.com>
Date: Sun, 31 Jan 2010 18:23:44 +0200
Subject: [PATCH] block: Enable fall-back to read-only for backing file

In order to use an image file backed by a read-only file, allow opening
the backing file with read-only permission.

Signed-off-by: Naphtali Sprei <nsprei@redhat.com>
---
 block.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/block.c b/block.c
index 1919d19..d1b0f3d 100644
--- a/block.c
+++ b/block.c
@@ -483,6 +483,11 @@ int bdrv_open2(BlockDriverState *bs, const char *filename, int flags,
             back_drv = bdrv_find_format(bs->backing_format);
         ret = bdrv_open2(bs->backing_hd, backing_filename, open_flags,
                          back_drv);
+        if (ret < 0) {
+            open_flags &= ~BDRV_O_RDWR;  /* Fall-back to read-only for the backing file */
+            ret = bdrv_open2(bs->backing_hd, backing_filename, open_flags,
+                             back_drv);
+        }
         bs->backing_hd->read_only =  (open_flags & BDRV_O_RDWR) == 0;
         if (ret < 0) {
             bdrv_close(bs);
-- 
1.6.3.3