Re: [RFC 1/5] net: rtnetlink: Fix incorrect RCU API usage

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Tue, Feb 19, 2019 at 11:08:23PM -0500, Joel Fernandes (Google) wrote:
> From: Joel Fernandes <joel@joelfernandes.org>
> 
> rtnl_register_internal() and rtnl_unregister_all tries to directly
> dereference an RCU protected pointed outside RCU read side section.
> While this is Ok to do since a lock is held, let us use the correct
> API to avoid programmer bugs in the future.
> 
> This also fixes sparse warnings arising from not using RCU API.
> 
> net/core/rtnetlink.c:332:13: warning: incorrect type in assignment
> (different address spaces) net/core/rtnetlink.c:332:13:    expected
> struct rtnl_link **tab net/core/rtnetlink.c:332:13:    got struct
> rtnl_link *[noderef] <asn:4>*<noident>
> 
> Signed-off-by: Joel Fernandes <joel@joelfernandes.org>

First, thank you for doing this!

I was going to complain that these were update-side accesses, but it
looks like rtnl_dereference() already handles both readers and updaters.

So looks good to me, but the maintainers of course have the final word.

							Thanx, Paul

> ---
>  net/core/rtnetlink.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index 5ea1bed08ede..98be4b4818a9 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> @@ -188,7 +188,7 @@ static int rtnl_register_internal(struct module *owner,
>  	msgindex = rtm_msgindex(msgtype);
>  
>  	rtnl_lock();
> -	tab = rtnl_msg_handlers[protocol];
> +	tab = rtnl_dereference(rtnl_msg_handlers[protocol]);
>  	if (tab == NULL) {
>  		tab = kcalloc(RTM_NR_MSGTYPES, sizeof(void *), GFP_KERNEL);
>  		if (!tab)
> @@ -329,7 +329,7 @@ void rtnl_unregister_all(int protocol)
>  	BUG_ON(protocol < 0 || protocol > RTNL_FAMILY_MAX);
>  
>  	rtnl_lock();
> -	tab = rtnl_msg_handlers[protocol];
> +	tab = rtnl_dereference(rtnl_msg_handlers[protocol]);
>  	if (!tab) {
>  		rtnl_unlock();
>  		return;
> -- 
> 2.21.0.rc0.258.g878e2cd30e-goog
>