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  1. Jan 14, 2022
    • Vladimir Sementsov-Ogievskiy's avatar
      block: drop BLK_PERM_GRAPH_MOD · 64631f36
      Vladimir Sementsov-Ogievskiy authored
      
      First, this permission never protected a node from being changed, as
      generic child-replacing functions don't check it.
      
      Second, it's a strange thing: it presents a permission of parent node
      to change its child. But generally, children are replaced by different
      mechanisms, like jobs or qmp commands, not by nodes.
      
      Graph-mod permission is hard to understand. All other permissions
      describe operations which done by parent node on its child: read,
      write, resize. Graph modification operations are something completely
      different.
      
      The only place where BLK_PERM_GRAPH_MOD is used as "perm" (not shared
      perm) is mirror_start_job, for s->target. Still modern code should use
      bdrv_freeze_backing_chain() to protect from graph modification, if we
      don't do it somewhere it may be considered as a bug. So, it's a bit
      risky to drop GRAPH_MOD, and analyzing of possible loss of protection
      is hard. But one day we should do it, let's do it now.
      
      One more bit of information is that locking the corresponding byte in
      file-posix doesn't make sense at all.
      
      Signed-off-by: default avatarVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
      Message-Id: <20210902093754.2352-1-vsementsov@virtuozzo.com>
      Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
      64631f36
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