linux-kernel - Re: [PATCH net-next 3/3] tap: XDP support

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Message-ID: <597B542A.7070704@iogearbox.net>
Date:   Fri, 28 Jul 2017 17:11:38 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
CC:     Jakub Kicinski <kubakici@...pl>, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 3/3] tap: XDP support

On 07/28/2017 05:46 AM, Michael S. Tsirkin wrote:
> On Fri, Jul 28, 2017 at 11:28:54AM +0800, Jason Wang wrote:
>>>> +	old_prog = rtnl_dereference(tun->xdp_prog);
>>>> +	if (old_prog)
>>>> +		bpf_prog_put(old_prog);
>>>> +	rcu_assign_pointer(tun->xdp_prog, prog);
>>> Is this OK?  Could this lead to the program getting freed and then
>>> datapath accessing a stale pointer?  I mean in the scenario where the
>>> process gets pre-empted between the bpf_prog_put() and
>>> rcu_assign_pointer()?
>>
>> Will call bpf_prog_put() after rcu_assign_pointer().

+1, good catch Jakub.

> I suspect you need to sync RCU or something before that.

No, see for example generic_xdp_install().