block: Separate blk_is_writable() and blk_supports_write_perm()

Currently, blk_is_read_only() tells whether a given BlockBackend can
only be used in read-only mode because its root node is read-only. Some
callers actually try to answer a slightly different question: Is the
BlockBackend configured to be writable, by taking write permissions on
the root node?

This can differ, for example, for CD-ROM devices which don't take write
permissions, but may be backed by a writable image file. scsi-cd allows
write requests to the drive if blk_is_read_only() returns false.
However, the write request will immediately run into an assertion
failure because the write permission is missing.

This patch introduces separate functions for both questions.
blk_supports_write_perm() answers the question whether the block
node/image file can support writable devices, whereas blk_is_writable()
tells whether the BlockBackend is currently configured to be writable.

All calls of blk_is_read_only() are converted to one of the two new
functions.

Fixes: https://bugs.launchpad.net/bugs/1906693
Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210118123448.307825-2-kwolf@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/hw/block/xen-block.c b/hw/block/xen-block.c
index 718d886..0e7d66c 100644
--- a/hw/block/xen-block.c
+++ b/hw/block/xen-block.c
@@ -567,7 +567,7 @@ static void xen_disk_realize(XenBlockDevice *blockdev, Error **errp)
         return;
     }
 
-    blockdev->info = blk_is_read_only(conf->blk) ? VDISK_READONLY : 0;
+    blockdev->info = blk_supports_write_perm(conf->blk) ? 0 : VDISK_READONLY;
 }
 
 static void xen_disk_class_init(ObjectClass *class, void *data)