netdev - Re: [PATCH net-next 2/8] rtnetlink: add rtnl_register

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Date:   Tue, 7 Nov 2017 10:47:51 +0100
From:   Florian Westphal <fw@...len.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Florian Westphal <fw@...len.de>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 2/8] rtnetlink: add rtnl_register_module

Peter Zijlstra <peterz@...radead.org> wrote:
> Something like the below would go some way toward sanitizing this stuff;
> rcu_assign_pointer() is a store-release, meaning it happens after
> everything coming before.
> 
> Therefore, when you observe that tab (through rcu_dereference) you're
> guaranteed to see the thing initialized. The memory ordering on the
> consume side is through an address dependency; we need to have completed
> the load of the tab pointer before we can compute the address of its
> members and load from there, these are not things a CPU is allowed to
> reorder (lets forget about Alpha).
> 
> Quite possibly, if rtnl_unregister() is called from module unload, this
> is broken; in that case we'd need something like:
> 
> 	rcu_assign_pointer(rtnl_msg_handler[protocol], NULL);
> 	/*
> 	 * Ensure nobody can still observe our old protocol handler
> 	 * before continuing to free the module that includes the
> 	 * functions called from it.
> 	 */
> 	synchronize_rcu();
> 
> ---
> 
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index 5ace48926b19..25391c7b9c5d 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> @@ -63,6 +63,7 @@ struct rtnl_link {
>  	rtnl_doit_func		doit;
>  	rtnl_dumpit_func	dumpit;
>  	unsigned int		flags;
> +	struct rcu_head		rcu;
>  };
>  
>  static DEFINE_MUTEX(rtnl_mutex);
> @@ -172,14 +173,15 @@ int __rtnl_register(int protocol, int msgtype,
>  	BUG_ON(protocol < 0 || protocol > RTNL_FAMILY_MAX);
>  	msgindex = rtm_msgindex(msgtype);
>  
> -	tab = rcu_dereference_raw(rtnl_msg_handlers[protocol]);
> -	if (tab == NULL) {
> -		tab = kcalloc(RTM_NR_MSGTYPES, sizeof(*tab), GFP_KERNEL);
> -		if (tab == NULL)
> -			return -ENOBUFS;
> +	if (WARN_ONCE(rtnl_msg_handler[protocol],
> +		      "Double registration for protocol: %d\n", protcol))
> +		return -EEXIST;

I would expect this to trigger all the time, due to

rtnl_register(AF_INET, RTM_GETROUTE, ...
rtnl_register(AF_INET, RTM_GETADDR, ...

etc.

> @@ -227,15 +231,13 @@ int rtnl_unregister(int protocol, int msgtype)
>  	msgindex = rtm_msgindex(msgtype);
>  
>  	rtnl_lock();
> -	handlers = rtnl_dereference(rtnl_msg_handlers[protocol]);
> +	handlers = rtnl_msg_handlers[protocol];
>  	if (!handlers) {
>  		rtnl_unlock();
>  		return -ENOENT;
>  	}
> -
> -	handlers[msgindex].doit = NULL;
> -	handlers[msgindex].dumpit = NULL;
> -	handlers[msgindex].flags = 0;
> +	rcu_assign_pointer(rtnl_msg_handler[protocol], NULL);
> +	kfree_rcu(handlers, rcu);

This unregisters all handlers of "protocol" instead of
"protocol:msgtype".