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, 16 insertions, 3 deletions

diff --git a/block/block-backend.c b/block/block-backend.c
index ce78d30794..e493f17515 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1826,17 +1826,30 @@ void blk_error_action(BlockBackend *blk, BlockErrorAction action,
     }
 }
 
-bool blk_is_read_only(BlockBackend *blk)
+/*
+ * Returns true if the BlockBackend can support taking write permissions
+ * (because its root node is not read-only).
+ */
+bool blk_supports_write_perm(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
 
     if (bs) {
-        return bdrv_is_read_only(bs);
+        return !bdrv_is_read_only(bs);
     } else {
-        return blk->root_state.read_only;
+        return !blk->root_state.read_only;
     }
 }
 
+/*
+ * Returns true if the BlockBackend can be written to in its current
+ * configuration (i.e. if write permission have been requested)
+ */
+bool blk_is_writable(BlockBackend *blk)
+{
+    return blk->perm & BLK_PERM_WRITE;
+}
+
 bool blk_is_sg(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);