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Message-ID: <34EFBCA9F01B0748BEB6B629CE643AE60DB5CD27@DGGEMM533-MBX.china.huawei.com>
Date: Mon, 7 Dec 2020 13:38:14 +0000
From: wangyunjian <wangyunjian@...wei.com>
To: Jason Wang <jasowang@...hat.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
> -----Original Message-----
> From: Jason Wang [mailto:jasowang@...hat.com]
> Sent: Monday, December 7, 2020 11:54 AM
> 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 2020/12/4 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?
I think the newly added code is easy to miss this problem, so I want to
copy ubuf_info until we're sure there's no errors.
Thanks,
Yunjian
>
> Thanks
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