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, 3 insertions, 3 deletions

diff --git a/hw/block/swim.c b/hw/block/swim.c
index 20133a814c..509c2f4900 100644
--- a/hw/block/swim.c
+++ b/hw/block/swim.c
@@ -137,8 +137,8 @@ static void swim_change_cb(void *opaque, bool load, Error **errp)
         blk_set_perm(drive->blk, 0, BLK_PERM_ALL, &error_abort);
     } else {
         if (!blkconf_apply_backend_options(drive->conf,
-                                           blk_is_read_only(drive->blk), false,
-                                           errp)) {
+                                           !blk_supports_write_perm(drive->blk),
+                                           false, errp)) {
             return;
         }
     }
@@ -210,7 +210,7 @@ static void swim_drive_realize(DeviceState *qdev, Error **errp)
     dev->conf.werror = BLOCKDEV_ON_ERROR_AUTO;
 
     if (!blkconf_apply_backend_options(&dev->conf,
-                                       blk_is_read_only(dev->conf.blk),
+                                       !blk_supports_write_perm(dev->conf.blk),
                                        false, errp)) {
         return;
     }