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/sd/sd.c b/hw/sd/sd.c
index b3952514fe..8517dbce8b 100644
--- a/hw/sd/sd.c
+++ b/hw/sd/sd.c
@@ -567,7 +567,7 @@ static void sd_reset(DeviceState *dev)
     sd_set_sdstatus(sd);
 
     g_free(sd->wp_groups);
-    sd->wp_switch = sd->blk ? blk_is_read_only(sd->blk) : false;
+    sd->wp_switch = sd->blk ? !blk_is_writable(sd->blk) : false;
     sd->wpgrps_size = sect;
     sd->wp_groups = bitmap_new(sd->wpgrps_size);
     memset(sd->function_group, 0, sizeof(sd->function_group));
@@ -735,7 +735,7 @@ void sd_set_cb(SDState *sd, qemu_irq readonly, qemu_irq insert)
 {
     sd->readonly_cb = readonly;
     sd->inserted_cb = insert;
-    qemu_set_irq(readonly, sd->blk ? blk_is_read_only(sd->blk) : 0);
+    qemu_set_irq(readonly, sd->blk ? !blk_is_writable(sd->blk) : 0);
     qemu_set_irq(insert, sd->blk ? blk_is_inserted(sd->blk) : 0);
 }
 
@@ -2131,7 +2131,7 @@ static void sd_realize(DeviceState *dev, Error **errp)
     if (sd->blk) {
         int64_t blk_size;
 
-        if (blk_is_read_only(sd->blk)) {
+        if (!blk_supports_write_perm(sd->blk)) {
             error_setg(errp, "Cannot use read-only drive as SD card");
             return;
         }