On Wed, Nov 01, 2017 at 03:59:48PM +0200, Michael S. Tsirkin wrote:

On Wed, Nov 01, 2017 at 09:02:03PM +0800, Jason Wang wrote:

quoted

On 2017年11月01日 00:45, Michael S. Tsirkin wrote:

quoted

quoted

+static void __tun_set_steering_ebpf(struct tun_struct *tun,
+				    struct bpf_prog *new)
+{
+	struct bpf_prog *old;
+
+	old = rtnl_dereference(tun->steering_prog);
+	rcu_assign_pointer(tun->steering_prog, new);
+
+	if (old) {
+		synchronize_net();
+		bpf_prog_destroy(old);
+	}
+}
+

Is this really called under rtnl?

Yes it is __tun_chr_ioctl() will call rtnl_lock().

Is the call from tun_free_netdev under rtnl too?

quoted

quoted

If no then rtnl_dereference
is wrong. If yes I'm not sure you can call synchronize_net
under rtnl.

Are you worrying about the long wait? Looking at synchronize_net(), it does:

void synchronize_net(void)
{
    might_sleep();
    if (rtnl_is_locked())
        synchronize_rcu_expedited();
    else
        synchronize_rcu();
}
EXPORT_SYMBOL(synchronize_net);

Thanks

Not the wait - expedited is not a good thing to allow unpriveledged
userspace to do, it interrupts all VMs running on the same box.

We could use a callback though the docs warn userspace can use that
to cause a DOS and needs to be limited.

the whole __tun_set_steering_ebpf() looks odd to me.
There is tun_attach_filter/tun_detach_filter pattern
that works for classic BPF. Why for eBPF this strange
synchronize_net() is there?