block: Add bdrv_qapi_perm_to_blk_perm()

We need some way to correlate QAPI BlockPermission values with
BLK_PERM_* flags.  We could:

(1) have the same order in the QAPI definition as the the BLK_PERM_*
    flags are in LSb-first order.  However, then there is no guarantee
    that they actually match (e.g. when someone modifies the QAPI schema
    without thinking of the BLK_PERM_* definitions).
    We could add static assertions, but these would break what’s good
    about this solution, namely its simplicity.

(2) define the BLK_PERM_* flags based on the BlockPermission values.
    But this way whenever someone were to modify the QAPI order
    (perfectly sensible in theory), the BLK_PERM_* values would change.
    Because these values are used for file locking, this might break
    file locking between different qemu versions.

Therefore, go the slightly more cumbersome way: Add a function to
translate from the QAPI constants to the BLK_PERM_* flags.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20191108123455.39445-2-mreitz@redhat.com
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -2227,6 +2227,24 @@ void bdrv_format_default_perms(BlockDriverState *bs, BdrvChild *c,
     *nshared = shared;
 }
 
+uint64_t bdrv_qapi_perm_to_blk_perm(BlockPermission qapi_perm)
+{
+    static const uint64_t permissions[] = {
+        [BLOCK_PERMISSION_CONSISTENT_READ]  = BLK_PERM_CONSISTENT_READ,
+        [BLOCK_PERMISSION_WRITE]            = BLK_PERM_WRITE,
+        [BLOCK_PERMISSION_WRITE_UNCHANGED]  = BLK_PERM_WRITE_UNCHANGED,
+        [BLOCK_PERMISSION_RESIZE]           = BLK_PERM_RESIZE,
+        [BLOCK_PERMISSION_GRAPH_MOD]        = BLK_PERM_GRAPH_MOD,
+    };
+
+    QEMU_BUILD_BUG_ON(ARRAY_SIZE(permissions) != BLOCK_PERMISSION__MAX);
+    QEMU_BUILD_BUG_ON(1UL << ARRAY_SIZE(permissions) != BLK_PERM_ALL + 1);
+
+    assert(qapi_perm < BLOCK_PERMISSION__MAX);
+
+    return permissions[qapi_perm];
+}
+
 static void bdrv_replace_child_noperm(BdrvChild *child,
                                       BlockDriverState *new_bs)
 {

include/block/block.h

@@ -280,6 +280,7 @@ enum {
 };
 
 char *bdrv_perm_names(uint64_t perm);
+uint64_t bdrv_qapi_perm_to_blk_perm(BlockPermission qapi_perm);
 
 /* disk I/O throttling */
 void bdrv_init(void);