block: do not set BDS read_only if copy_on_read enabled

A few block drivers will set the BDS read_only flag from their
.bdrv_open() function.  This means the bs->read_only flag could
be set after we enable copy_on_read, as the BDRV_O_COPY_ON_READ
flag check occurs prior to the call to bdrv->bdrv_open().

This adds an error return to bdrv_set_read_only(), and an error will be
return if we try to set the BDS to read_only while copy_on_read is
enabled.

This patch also changes the behavior of vvfat.  Before, vvfat could
override the drive 'readonly' flag with its own, internal 'rw' flag.

For instance, this -drive parameter would result in a writable image:

"-drive format=vvfat,dir=/tmp/vvfat,rw,if=virtio,readonly=on"

This is not correct.  Now, attempting to use the above -drive parameter
will result in an error (i.e., 'rw' is incompatible with 'readonly=on').

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 0c5b4c1cc2c651471b131f21376dfd5ea24d2196.1491597120.git.jcody@redhat.com

1 files changed, 15 insertions, 4 deletions

diff --git a/block/vvfat.c b/block/vvfat.c
index d4ce6d7092..b509d55642 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -1156,8 +1156,6 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
 
     s->current_cluster=0xffffffff;
 
-    /* read only is the default for safety */
-    bdrv_set_read_only(bs, true);
     s->qcow = NULL;
     s->qcow_filename = NULL;
     s->fat2 = NULL;
@@ -1169,11 +1167,24 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
     s->sector_count = cyls * heads * secs - (s->first_sectors_number - 1);
 
     if (qemu_opt_get_bool(opts, "rw", false)) {
-        ret = enable_write_target(bs, errp);
+        if (!bdrv_is_read_only(bs)) {
+            ret = enable_write_target(bs, errp);
+            if (ret < 0) {
+                goto fail;
+            }
+        } else {
+            ret = -EPERM;
+            error_setg(errp,
+                       "Unable to set VVFAT to 'rw' when drive is read-only");
+            goto fail;
+        }
+    } else  {
+        /* read only is the default for safety */
+        ret = bdrv_set_read_only(bs, true, &local_err);
         if (ret < 0) {
+            error_propagate(errp, local_err);
             goto fail;
         }
-        bdrv_set_read_only(bs, false);
     }
 
     bs->total_sectors = cyls * heads * secs;