Re: [PATCH v2] block: drop BLK_PERM_GRAPH_MOD

[Kevin Wolf <kwolf@redhat.com>]

Am 02.09.2021 um 11:37 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 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: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Thanks, applied to the block branch.

Kevin