netdev - Re: [PATCH net] bonding: Fix bonding crash

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Message-ID: <94a399f3-72b1-8895-b54f-c7d03878327e@cumulusnetworks.com>
Date:   Fri, 2 Sep 2016 08:57:14 -0600
From:   David Ahern <dsa@...ulusnetworks.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Mahesh Bandewar <mahesh@...dewar.net>,
        Jay Vosburgh <j.vosburgh@...il.com>,
        Andy Gospodarek <gospo@...ulusnetworks.com>,
        Veaceslav Falico <vfalico@...il.com>,
        David Miller <davem@...emloft.net>,
        Mahesh Bandewar <maheshb@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] bonding: Fix bonding crash

On 9/2/16 8:45 AM, Eric Dumazet wrote:
> On Fri, 2016-09-02 at 08:30 -0600, David Ahern wrote:
> 
>> This check duplicates what netdev_rx_handler_register does. Why not
>> move the call to netdev_rx_handler_register here and then call
>> unregister on failure paths?
> 
> As soon as you call netdev_rx_handler_register(), incoming packets will
> hit your driver and we'll likely crash since the enslaving is not done
> yet.
> 
> Really think about RCU, we do rcu_assign_pointer() because we want all
> prior changes being committed to memory before 'enabling' readers to see
> the updated rcu protected pointer.
> 
> There are 9 call sites where netdev_rx_handler_register() is used.
> 
> We can get rid of the extra check in netdev_rx_handler_register() once
> all of them are using netdev_is_rx_handler_busy()
> 
> Since this patch takes care of bonding only, we need to keep the
> existing check in netdev_rx_handler_register()
> 
> Anyway, we are speaking of control function, an extra check is simply
> safer, like all the ASSERT_RTNL() we do have...
> 

I hit this same problem yesterday but with the bridge. I forgot I had a macvlan device on an interface and tried to enslave it to a bridge. It failed with EBUSY without crashing the kernel so it is one example that handles the conflict, and the bridge also calls the register before the enslaving is done.