block/export/fuse.c: allow writable exports to take RESIZE permission

Allow writable exports to get BLK_PERM_RESIZE permission
from creation, in fuse_export_create().
In this way, there is no need to give the permission in
fuse_do_truncate(), which might be run in an iothread.

Permissions should be set only in the main thread, so
in any case if an iothread tries to set RESIZE, it will
be blocked.

Also assert in fuse_do_truncate that if we give the
RESIZE permission we can then restore the original ones.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220303151616.325444-7-eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/export/fuse.c

@@ -86,8 +86,8 @@ static int fuse_export_create(BlockExport *blk_exp,
 
     assert(blk_exp_args->type == BLOCK_EXPORT_TYPE_FUSE);
 
-    /* For growable exports, take the RESIZE permission */
-    if (args->growable) {
+    /* For growable and writable exports, take the RESIZE permission */
+    if (args->growable || blk_exp_args->writable) {
         uint64_t blk_perm, blk_shared_perm;
 
         blk_get_perm(exp->common.blk, &blk_perm, &blk_shared_perm);
@@ -392,14 +392,23 @@ static int fuse_do_truncate(const FuseExport *exp, int64_t size,
 {
     uint64_t blk_perm, blk_shared_perm;
     BdrvRequestFlags truncate_flags = 0;
-    int ret;
+    bool add_resize_perm;
+    int ret, ret_check;
+
+    /* Growable and writable exports have a permanent RESIZE permission */
+    add_resize_perm = !exp->growable && !exp->writable;
 
     if (req_zero_write) {
         truncate_flags |= BDRV_REQ_ZERO_WRITE;
     }
 
-    /* Growable exports have a permanent RESIZE permission */
-    if (!exp->growable) {
+    if (add_resize_perm) {
+
+        if (!qemu_in_main_thread()) {
+            /* Changing permissions like below only works in the main thread */
+            return -EPERM;
+        }
+
         blk_get_perm(exp->common.blk, &blk_perm, &blk_shared_perm);
 
         ret = blk_set_perm(exp->common.blk, blk_perm | BLK_PERM_RESIZE,
@@ -412,9 +421,11 @@ static int fuse_do_truncate(const FuseExport *exp, int64_t size,
     ret = blk_truncate(exp->common.blk, size, true, prealloc,
                        truncate_flags, NULL);
 
-    if (!exp->growable) {
+    if (add_resize_perm) {
         /* Must succeed, because we are only giving up the RESIZE permission */
-        blk_set_perm(exp->common.blk, blk_perm, blk_shared_perm, &error_abort);
+        ret_check = blk_set_perm(exp->common.blk, blk_perm,
+                                 blk_shared_perm, &error_abort);
+        assert(ret_check == 0);
     }
 
     return ret;