block: Deprecate bdrv_set_read_only() and users

bdrv_set_read_only() is used by some block drivers to override the
read-only option given by the user. This is not how read-only images
generally work in QEMU: Instead of second guessing what the user really
meant (which currently includes making an image read-only even if the
user didn't only use the default, but explicitly said read-only=off), we
should error out if we can't provide what the user requested.

This adds deprecation warnings to all callers of bdrv_set_read_only() so
that the behaviour can be corrected after the usual deprecation period.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -261,6 +261,11 @@ int bdrv_can_set_read_only(BlockDriverState *bs, bool read_only,
     return 0;
 }
 
+/* TODO Remove (deprecated since 2.11)
+ * Block drivers are not supposed to automatically change bs->read_only.
+ * Instead, they should just check whether they can provide what the user
+ * explicitly requested and error out if read-write is requested, but they can
+ * only provide read-only access. */
 int bdrv_set_read_only(BlockDriverState *bs, bool read_only, Error **errp)
 {
     int ret = 0;

block/bochs.c

@@ -28,6 +28,7 @@
 #include "block/block_int.h"
 #include "qemu/module.h"
 #include "qemu/bswap.h"
+#include "qemu/error-report.h"
 
 /**************************************************************/
 
@@ -110,10 +111,16 @@ static int bochs_open(BlockDriverState *bs, QDict *options, int flags,
         return -EINVAL;
     }
 
+    if (!bdrv_is_read_only(bs)) {
+        error_report("Opening bochs images without an explicit read-only=on "
+                     "option is deprecated. Future versions will refuse to "
+                     "open the image instead of automatically marking the "
+                     "image read-only.");
         ret = bdrv_set_read_only(bs, true, errp); /* no write support yet */
         if (ret < 0) {
             return ret;
         }
+    }
 
     ret = bdrv_pread(bs->file, 0, &bochs, sizeof(bochs));
     if (ret < 0) {

block/cloop.c

@@ -23,6 +23,7 @@
  */
 #include "qemu/osdep.h"
 #include "qapi/error.h"
+#include "qemu/error-report.h"
 #include "qemu-common.h"
 #include "block/block_int.h"
 #include "qemu/module.h"
@@ -72,10 +73,16 @@ static int cloop_open(BlockDriverState *bs, QDict *options, int flags,
         return -EINVAL;
     }
 
+    if (!bdrv_is_read_only(bs)) {
+        error_report("Opening cloop images without an explicit read-only=on "
+                     "option is deprecated. Future versions will refuse to "
+                     "open the image instead of automatically marking the "
+                     "image read-only.");
         ret = bdrv_set_read_only(bs, true, errp);
         if (ret < 0) {
             return ret;
         }
+    }
 
     /* read header */
     ret = bdrv_pread(bs->file, 128, &s->block_size, 4);

block/dmg.c

@@ -419,10 +419,16 @@ static int dmg_open(BlockDriverState *bs, QDict *options, int flags,
         return -EINVAL;
     }
 
+    if (!bdrv_is_read_only(bs)) {
+        error_report("Opening dmg images without an explicit read-only=on "
+                     "option is deprecated. Future versions will refuse to "
+                     "open the image instead of automatically marking the "
+                     "image read-only.");
         ret = bdrv_set_read_only(bs, true, errp);
         if (ret < 0) {
             return ret;
         }
+    }
 
     block_module_load_one("dmg-bz2");
 

block/rbd.c

@@ -665,12 +665,18 @@ static int qemu_rbd_open(BlockDriverState *bs, QDict *options, int flags,
     /* If we are using an rbd snapshot, we must be r/o, otherwise
      * leave as-is */
     if (s->snap != NULL) {
+        if (!bdrv_is_read_only(bs)) {
+            error_report("Opening rbd snapshots without an explicit "
+                         "read-only=on option is deprecated. Future versions "
+                         "will refuse to open the image instead of "
+                         "automatically marking the image read-only.");
             r = bdrv_set_read_only(bs, true, &local_err);
             if (r < 0) {
                 error_propagate(errp, local_err);
                 goto failed_open;
             }
         }
+    }
 
     qemu_opts_del(opts);
     return 0;

block/vvfat.c

@@ -1259,7 +1259,11 @@ static int vvfat_open(BlockDriverState *bs, QDict *options, int flags,
                        "Unable to set VVFAT to 'rw' when drive is read-only");
             goto fail;
         }
-    } else  {
+    } else  if (!bdrv_is_read_only(bs)) {
+        error_report("Opening non-rw vvfat images without an explicit "
+                     "read-only=on option is deprecated. Future versions "
+                     "will refuse to open the image instead of "
+                     "automatically marking the image read-only.");
         /* read only is the default for safety */
         ret = bdrv_set_read_only(bs, true, &local_err);
         if (ret < 0) {

qapi/block-core.json

@@ -3134,8 +3134,11 @@
 #                 This option is required on the top level of blockdev-add.
 # @discard:       discard-related options (default: ignore)
 # @cache:         cache-related options
-# @read-only:     whether the block device should be read-only
-#                 (default: false)
+# @read-only:     whether the block device should be read-only (default: false).
+#                 Note that some block drivers support only read-only access,
+#                 either generally or in certain configurations. In this case,
+#                 the default value does not work and the option must be
+#                 specified explicitly.
 # @detect-zeroes: detect and optimize zero writes (Since 2.1)
 #                 (default: off)
 # @force-share:   force share all permission on added nodes.