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Date: Mon, 7 Dec 2020 11:54:12 +0800
From: Jason Wang <jasowang@...hat.com>
To: wangyunjian <wangyunjian@...wei.com>,
"mst@...hat.com" <mst@...hat.com>
Cc: "virtualization@...ts.linux-foundation.org"
<virtualization@...ts.linux-foundation.org>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"Lilijun (Jerry)" <jerry.lilijun@...wei.com>,
xudingke <xudingke@...wei.com>
Subject: Re: [PATCH net-next] tun: fix ubuf refcount incorrectly on error path
On 2020/12/4 下午6:22, wangyunjian wrote:
>> -----Original Message-----
>> From: Jason Wang [mailto:jasowang@...hat.com]
>> Sent: Friday, December 4, 2020 2:11 PM
>> To: wangyunjian <wangyunjian@...wei.com>; mst@...hat.com
>> Cc: virtualization@...ts.linux-foundation.org; netdev@...r.kernel.org; Lilijun
>> (Jerry) <jerry.lilijun@...wei.com>; xudingke <xudingke@...wei.com>
>> Subject: Re: [PATCH net-next] tun: fix ubuf refcount incorrectly on error path
>>
>>
>> On 2020/12/3 下午4:00, wangyunjian wrote:
>>> From: Yunjian Wang <wangyunjian@...wei.com>
>>>
>>> After setting callback for ubuf_info of skb, the callback
>>> (vhost_net_zerocopy_callback) will be called to decrease the refcount
>>> when freeing skb. But when an exception occurs afterwards, the error
>>> handling in vhost handle_tx() will try to decrease the same refcount
>>> again. This is wrong and fix this by clearing ubuf_info when meeting
>>> errors.
>>>
>>> Fixes: 4477138fa0ae ("tun: properly test for IFF_UP")
>>> Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP
>>> driver")
>>>
>>> Signed-off-by: Yunjian Wang <wangyunjian@...wei.com>
>>> ---
>>> drivers/net/tun.c | 11 +++++++++++
>>> 1 file changed, 11 insertions(+)
>>>
>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c index
>>> 2dc1988a8973..3614bb1b6d35 100644
>>> --- a/drivers/net/tun.c
>>> +++ b/drivers/net/tun.c
>>> @@ -1861,6 +1861,12 @@ static ssize_t tun_get_user(struct tun_struct
>> *tun, struct tun_file *tfile,
>>> if (unlikely(!(tun->dev->flags & IFF_UP))) {
>>> err = -EIO;
>>> rcu_read_unlock();
>>> + if (zerocopy) {
>>> + skb_shinfo(skb)->destructor_arg = NULL;
>>> + skb_shinfo(skb)->tx_flags &= ~SKBTX_DEV_ZEROCOPY;
>>> + skb_shinfo(skb)->tx_flags &= ~SKBTX_SHARED_FRAG;
>>> + }
>>> +
>>> goto drop;
>>> }
>>>
>>> @@ -1874,6 +1880,11 @@ static ssize_t tun_get_user(struct tun_struct
>>> *tun, struct tun_file *tfile,
>>>
>>> if (unlikely(headlen > skb_headlen(skb))) {
>>> atomic_long_inc(&tun->dev->rx_dropped);
>>> + if (zerocopy) {
>>> + skb_shinfo(skb)->destructor_arg = NULL;
>>> + skb_shinfo(skb)->tx_flags &= ~SKBTX_DEV_ZEROCOPY;
>>> + skb_shinfo(skb)->tx_flags &= ~SKBTX_SHARED_FRAG;
>>> + }
>>> napi_free_frags(&tfile->napi);
>>> rcu_read_unlock();
>>> mutex_unlock(&tfile->napi_mutex);
>>
>> It looks to me then we miss the failure feedback.
>>
>> The issues comes from the inconsistent error handling in tun.
>>
>> I wonder whether we can simply do uarg->callback(uarg, false) if necessary on
>> every failture path on tun_get_user().
> How about this?
>
> ---
> drivers/net/tun.c | 29 ++++++++++++++++++-----------
> 1 file changed, 18 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 2dc1988a8973..36a8d8eacd7b 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1637,6 +1637,19 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
> return NULL;
> }
>
> +/* copy ubuf_info for callback when skb has no error */
> +inline static tun_copy_ubuf_info(struct sk_buff *skb, bool zerocopy, void *msg_control)
> +{
> + if (zerocopy) {
> + skb_shinfo(skb)->destructor_arg = msg_control;
> + skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
> + skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
> + } else if (msg_control) {
> + struct ubuf_info *uarg = msg_control;
> + uarg->callback(uarg, false);
> + }
> +}
> +
> /* Get packet from user space buffer */
> static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> void *msg_control, struct iov_iter *from,
> @@ -1812,16 +1825,6 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> break;
> }
>
> - /* copy skb_ubuf_info for callback when skb has no error */
> - if (zerocopy) {
> - skb_shinfo(skb)->destructor_arg = msg_control;
> - skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
> - skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
> - } else if (msg_control) {
> - struct ubuf_info *uarg = msg_control;
> - uarg->callback(uarg, false);
> - }
> -
> skb_reset_network_header(skb);
> skb_probe_transport_header(skb);
> skb_record_rx_queue(skb, tfile->queue_index);
> @@ -1830,6 +1833,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> struct bpf_prog *xdp_prog;
> int ret;
>
> + tun_copy_ubuf_info(skb, zerocopy, msg_control);
If you think disabling zerocopy for XDP (which I think it makes sense).
It's better to do this in another patch.
> local_bh_disable();
> rcu_read_lock();
> xdp_prog = rcu_dereference(tun->xdp_prog);
> @@ -1880,7 +1884,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> WARN_ON(1);
> return -ENOMEM;
> }
> -
> + tun_copy_ubuf_info(skb, zerocopy, msg_control);
And for NAPI frags.
> local_bh_disable();
> napi_gro_frags(&tfile->napi);
> local_bh_enable();
> @@ -1889,6 +1893,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
> struct sk_buff_head *queue = &tfile->sk.sk_write_queue;
> int queue_len;
>
> + tun_copy_ubuf_info(skb, zerocopy, msg_control);
> spin_lock_bh(&queue->lock);
> __skb_queue_tail(queue, skb);
> queue_len = skb_queue_len(queue);
> @@ -1899,8 +1904,10 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>
> local_bh_enable();
> } else if (!IS_ENABLED(CONFIG_4KSTACKS)) {
> + tun_copy_ubuf_info(skb, zerocopy, msg_control);
> tun_rx_batched(tun, tfile, skb, more);
> } else {
> + tun_copy_ubuf_info(skb, zerocopy, msg_control);
> netif_rx_ni(skb);
> }
> rcu_read_unlock();
So it looks to me you want to disable zerocopy in all of the possible
datapath?
Thanks
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