Re: [PATCH] block: drop BLK_PERM_GRAPH_MOD

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Forget to mention in subject: for-6.2

20.07.2021 17:22, Vladimir Sementsov-Ogievskiy wrote:
> First, this permission never protected 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 it 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 corresponding byte in
> file-posix doesn't make sense at all.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy<vsementsov@virtuozzo.com>


-- 
Best regards,
Vladimir