On 5/6/19 11:23 PM, Jason Wang wrote:
> When a queue(tfile) is detached through __tun_detach(), we move the
> last enabled tfile to the position where detached one sit but don't
> NULL out last position. We expect to synchronize the datapath through
> tun->numqueues. Unfortunately, this won't work since we're lacking
> sufficient mechanism to order or synchronize the access to
> tun->numqueues.
>
> To fix this, NULL out the last position during detaching and check
> RCU protected tfile against NULL instead of checking tun->numqueues in
> datapath.
>
> Cc: YueHaibing <yuehaib...@huawei.com>
> Cc: Cong Wang <xiyou.wangc...@gmail.com>
> Cc: weiyongjun (A) <weiyongj...@huawei.com>
> Fixes: c8d68e6be1c3b ("tuntap: multiqueue support")
> Signed-off-by: Jason Wang <jasow...@redhat.com>
> ---
> drivers/net/tun.c | 12 +++++++-----
> 1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index e9ca1c0..a64c928 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -700,6 +700,8 @@ static void __tun_detach(struct tun_file *tfile, bool
> clean)
> tun->tfiles[tun->numqueues - 1]);
> ntfile = rtnl_dereference(tun->tfiles[index]);
> ntfile->queue_index = index;
> + rcu_assign_pointer(tun->tfiles[tun->numqueues - 1],
> + NULL);
>
> --tun->numqueues;
> if (clean) {
> @@ -1082,7 +1084,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb,
> struct net_device *dev)
> tfile = rcu_dereference(tun->tfiles[txq]);
>
> /* Drop packet if interface is not attached */
> - if (txq >= tun->numqueues)
> + if (!tfile)
> goto drop;
>
> if (!rcu_dereference(tun->steering_prog))
> @@ -1306,13 +1308,13 @@ static int tun_xdp_xmit(struct net_device *dev, int n,
> rcu_read_lock();
>
> numqueues = READ_ONCE(tun->numqueues);
> - if (!numqueues) {
> - rcu_read_unlock();
> - return -ENXIO; /* Caller will free/return all frames */
> - }
>
If you remove the test on (!numqueues),
the following might crash with a divide by zero...
> tfile = rcu_dereference(tun->tfiles[smp_processor_id() %
> numqueues]);
