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, 5 insertions, 4 deletions

diff --git a/hw/block/fdc.c b/hw/block/fdc.c
index 3636874..292ea87 100644
--- a/hw/block/fdc.c
+++ b/hw/block/fdc.c
@@ -444,7 +444,7 @@ static void fd_revalidate(FDrive *drv)
 
     FLOPPY_DPRINTF("revalidate\n");
     if (drv->blk != NULL) {
-        drv->ro = blk_is_read_only(drv->blk);
+        drv->ro = !blk_is_writable(drv->blk);
         if (!blk_is_inserted(drv->blk)) {
             FLOPPY_DPRINTF("No disk in drive\n");
             drv->disk = FLOPPY_DRIVE_TYPE_NONE;
@@ -479,8 +479,8 @@ static void fd_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;
         }
     }
@@ -553,7 +553,8 @@ static void floppy_drive_realize(DeviceState *qdev, Error **errp)
          * read-only node later */
         read_only = true;
     } else {
-        read_only = !blk_bs(dev->conf.blk) || blk_is_read_only(dev->conf.blk);
+        read_only = !blk_bs(dev->conf.blk) ||
+                    !blk_supports_write_perm(dev->conf.blk);
     }
 
     if (!blkconf_blocksizes(&dev->conf, errp)) {