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Date: Sun, 21 Mar 2021 20:23:29 -0700
From: Cong Wang <xiyou.wangcong@...il.com>
To: John Fastabend <john.fastabend@...il.com>
Cc: Linux Kernel Network Developers <netdev@...r.kernel.org>,
bpf <bpf@...r.kernel.org>, duanxiongchun@...edance.com,
Dongdong Wang <wangdongdong.6@...edance.com>,
Jiang Wang <jiang.wang@...edance.com>,
Cong Wang <cong.wang@...edance.com>,
Daniel Borkmann <daniel@...earbox.net>,
Jakub Sitnicki <jakub@...udflare.com>,
Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [Patch bpf-next v5 04/11] skmsg: avoid lock_sock() in sk_psock_backlog()
On Fri, Mar 19, 2021 at 7:45 PM John Fastabend <john.fastabend@...il.com> wrote:
>
> Cong Wang wrote:
> > From: Cong Wang <cong.wang@...edance.com>
> >
> > We do not have to lock the sock to avoid losing sk_socket,
> > instead we can purge all the ingress queues when we close
> > the socket. Sending or receiving packets after orphaning
> > socket makes no sense.
> >
> > We do purge these queues when psock refcnt reaches zero but
> > here we want to purge them explicitly in sock_map_close().
> > There are also some nasty race conditions on testing bit
> > SK_PSOCK_TX_ENABLED and queuing/canceling the psock work,
> > we can expand psock->ingress_lock a bit to protect them too.
> >
> > Cc: John Fastabend <john.fastabend@...il.com>
> > Cc: Daniel Borkmann <daniel@...earbox.net>
> > Cc: Jakub Sitnicki <jakub@...udflare.com>
> > Cc: Lorenz Bauer <lmb@...udflare.com>
> > Signed-off-by: Cong Wang <cong.wang@...edance.com>
> > ---
> > include/linux/skmsg.h | 1 +
> > net/core/skmsg.c | 50 +++++++++++++++++++++++++++++++------------
> > net/core/sock_map.c | 1 +
> > 3 files changed, 38 insertions(+), 14 deletions(-)
> >
> > diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> > index f2d45a73b2b2..0f5e663f6c7f 100644
> > --- a/include/linux/skmsg.h
> > +++ b/include/linux/skmsg.h
> > @@ -347,6 +347,7 @@ static inline void sk_psock_report_error(struct sk_psock *psock, int err)
> > }
>
> Overall looks good, comment/question below.
>
> >
> > #if IS_ENABLED(CONFIG_BPF_STREAM_PARSER)
> > int sk_psock_init_strp(struct sock *sk, struct sk_psock *psock);
> > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> > index 305dddc51857..d0a227b0f672 100644
> > --- a/net/core/skmsg.c
> > +++ b/net/core/skmsg.c
> > @@ -497,7 +497,7 @@ static int sk_psock_handle_skb(struct sk_psock *psock, struct sk_buff *skb,
> > if (!ingress) {
> > if (!sock_writeable(psock->sk))
> > return -EAGAIN;
> > - return skb_send_sock_locked(psock->sk, skb, off, len);
> > + return skb_send_sock(psock->sk, skb, off, len);
> > }
> > return sk_psock_skb_ingress(psock, skb);
> > }
> > @@ -511,8 +511,6 @@ static void sk_psock_backlog(struct work_struct *work)
> > u32 len, off;
> > int ret;
> >
> > - /* Lock sock to avoid losing sk_socket during loop. */
> > - lock_sock(psock->sk);
> > if (state->skb) {
> > skb = state->skb;
> > len = state->len;
> > @@ -529,7 +527,7 @@ static void sk_psock_backlog(struct work_struct *work)
> > skb_bpf_redirect_clear(skb);
> > do {
> > ret = -EIO;
> > - if (likely(psock->sk->sk_socket))
> > + if (!sock_flag(psock->sk, SOCK_DEAD))
> > ret = sk_psock_handle_skb(psock, skb, off,
> > len, ingress);
> > if (ret <= 0) {
> > @@ -537,13 +535,13 @@ static void sk_psock_backlog(struct work_struct *work)
> > state->skb = skb;
> > state->len = len;
> > state->off = off;
> > - goto end;
> > + return;
>
> Unrelated to your series I'll add it to my queue of fixes, but I think we
> leak state->skb on teardown.
Ok. Please target all bug fixes to -net.
>
> > }
> > /* Hard errors break pipe and stop xmit. */
> > sk_psock_report_error(psock, ret ? -ret : EPIPE);
> > sk_psock_clear_state(psock, SK_PSOCK_TX_ENABLED);
> > kfree_skb(skb);
> > - goto end;
> > + return;
> > }
> > off += ret;
> > len -= ret;
> > @@ -552,8 +550,6 @@ static void sk_psock_backlog(struct work_struct *work)
> > if (!ingress)
> > kfree_skb(skb);
> > }
> > -end:
> > - release_sock(psock->sk);
> > }
> >
> > struct sk_psock *sk_psock_init(struct sock *sk, int node)
> > @@ -631,7 +627,7 @@ static void __sk_psock_purge_ingress_msg(struct sk_psock *psock)
> > }
> > }
> >
> > -static void sk_psock_zap_ingress(struct sk_psock *psock)
> > +static void __sk_psock_zap_ingress(struct sk_psock *psock)
> > {
> > struct sk_buff *skb;
> >
> > @@ -639,8 +635,13 @@ static void sk_psock_zap_ingress(struct sk_psock *psock)
> > skb_bpf_redirect_clear(skb);
> > kfree_skb(skb);
> > }
> > - spin_lock_bh(&psock->ingress_lock);
> > __sk_psock_purge_ingress_msg(psock);
> > +}
> > +
> > +static void sk_psock_zap_ingress(struct sk_psock *psock)
> > +{
> > + spin_lock_bh(&psock->ingress_lock);
> > + __sk_psock_zap_ingress(psock);
> > spin_unlock_bh(&psock->ingress_lock);
>
> I'm wondering about callers of sk_psock_zap_ingress() and why the lock is
> needed here. We have two callers
>
> sk_psock_destroy_deferred(), is deferred after an RCU grace period and after
> cancel_work_sync() so there should be no users to into the skb queue. If there
> are we have other problems I think.
Right, I think sk_psock_zap_ingress() can be completely removed here
as it is already called in sk_psock_drop() (as below).
>
> sk_psock_drop() is the other. It is called when the refcnt is zero and does
> a sk_psock_clear_state(psock, SK_PSOCK_TX_ENABLED). Should it just wrap
> up the clear_state and sk_psock_zap_ingress similar to other cases so it
> doesn't have to deal with the case where enqueue happens after
> sk_psock_zap_ingress.
>
> Something like this would be clearer?
Yes.
>
> void sk_psock_drop(struct sock *sk, struct sk_psock *psock)
> {
> sk_psock_stop()
> write_lock_bh(&sk->sk_callback_lock);
> sk_psock_restore_proto(sk, psock);
> rcu_assign_sk_user_data(sk, NULL);
> if (psock->progs.stream_parser)
> sk_psock_stop_strp(sk, psock);
> else if (psock->progs.stream_verdict)
> sk_psock_stop_verdict(sk, psock);
> write_unlock_bh(&sk->sk_callback_lock);
> call_rcu(&psock->rcu, sk_psock_destroy);
> }
> EXPORT_SYMBOL_GPL(sk_psock_drop)
>
> Then sk_psock_zap_ingress, as coded above, is not really needed anywhere and
> we just use the lockless variant, __sk_psock_zap_ingress(). WDYT, to I miss
> something.
This makes sense to me too.
Thanks!
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