On 02/25/2019 01:27 PM, Dmitry Safonov wrote:
> rtnl_unregister() unsets handler from table, which is protected
> by rtnl_lock or RCU. At this moment only dump handlers access the table
> with rcu_lock(). Every other user accesses under rtnl.
>
> Callers may expect that rtnl_unregister() prevents any further handlers
> calls, alike rtnl_unregister_all(). And they do expect it.
>
> I've looked on in-tree caller uses:
> br_mdb: safe, but in err-path br_netlink_init()
> fib_rules: safe - err-path is very early in __init
> ip6mr: safe - following unregister_pernet_subsys() calls internally
> rcu_barrier()
> qrtr: safe - following sock_unregister() calls internally synchronize_rcu()
If rcu_barrier() was needed, then all callers should use it.
If synchronize_rcu() was needed, then all callers should use it.
But mixing is probably wrong.
>
> While it's possible to document that rtnl_unregister() requires
> synchronize_net() afterwards - unlike rtnl_unregister_all(), I believe
> the module exit is very much slow-path.
rtnl_unregister_all() needs the sychronize_rcu() at this moment
because of the kfree(tab), not because of the kfree_rcu(link, rcu);
>
> Issue seems to be very theoretical and unlikely, so I'm not Cc'ing
> stable tree.
>
> Fixes: 6853dd488119 ("rtnetlink: protect handler table with rcu")
> Cc: "David S. Miller" <da...@davemloft.net>
> Cc: Florian Westphal <f...@strlen.de>
> Cc: "Hannes Frederic Sowa" <han...@stressinduktion.org>
> Cc: net...@vger.kernel.org
> Signed-off-by: Dmitry Safonov <d...@arista.com>
> ---
> net/core/rtnetlink.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index 5ea1bed08ede..3db70da4f951 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> @@ -308,7 +308,9 @@ int rtnl_unregister(int protocol, int msgtype)
> rcu_assign_pointer(tab[msgindex], NULL);
> rtnl_unlock();
>
> - kfree_rcu(link, rcu);
> + synchronize_net();
> +
> + kfree(link);
I really do not see a difference here (other than this being much slower of
course)
If the caller needs rcu_barrier(), then add it in the caller ?
