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/virtio-blk.c b/hw/block/virtio-blk.c
index bac2d6fa2b..e8600b069d 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -1021,7 +1021,7 @@ static uint64_t virtio_blk_get_features(VirtIODevice *vdev, uint64_t features,
          virtio_has_feature(features, VIRTIO_BLK_F_CONFIG_WCE))) {
         virtio_add_feature(&features, VIRTIO_BLK_F_WCE);
     }
-    if (blk_is_read_only(s->blk)) {
+    if (!blk_is_writable(s->blk)) {
         virtio_add_feature(&features, VIRTIO_BLK_F_RO);
     }
     if (s->conf.num_queues > 1) {
@@ -1175,8 +1175,8 @@ static void virtio_blk_device_realize(DeviceState *dev, Error **errp)
     }
 
     if (!blkconf_apply_backend_options(&conf->conf,
-                                       blk_is_read_only(conf->conf.blk), true,
-                                       errp)) {
+                                       !blk_supports_write_perm(conf->conf.blk),
+                                       true, errp)) {
         return;
     }
     s->original_wce = blk_enable_write_cache(conf->conf.blk);