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Message-ID: <87y2es37i3.fsf@cloudflare.com>
Date: Fri, 12 Mar 2021 13:02:12 +0100
From: Jakub Sitnicki <jakub@...udflare.com>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org,
duanxiongchun@...edance.com, wangdongdong.6@...edance.com,
jiang.wang@...edance.com, Cong Wang <cong.wang@...edance.com>,
John Fastabend <john.fastabend@...il.com>,
Daniel Borkmann <daniel@...earbox.net>,
Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [Patch bpf-next v4 04/11] skmsg: avoid lock_sock() in
sk_psock_backlog()
On Wed, Mar 10, 2021 at 06:32 AM CET, 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 0 but
> here we want to purge them explicitly in sock_map_close().
>
> 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 | 22 ++++++++++++++--------
> net/core/sock_map.c | 1 +
> 3 files changed, 16 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> index 7333bf881b81..91b357817bb8 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)
> }
>
> struct sk_psock *sk_psock_init(struct sock *sk, int node);
> +void sk_psock_purge(struct sk_psock *psock);
>
> #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 41a5f82c53e6..bf0f874780c1 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;
> }
> /* 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)
> @@ -654,6 +650,16 @@ static void sk_psock_link_destroy(struct sk_psock *psock)
> }
> }
>
> +void sk_psock_purge(struct sk_psock *psock)
> +{
> + sk_psock_clear_state(psock, SK_PSOCK_TX_ENABLED);
> +
> + cancel_work_sync(&psock->work);
> +
> + sk_psock_cork_free(psock);
> + sk_psock_zap_ingress(psock);
> +}
> +
> static void sk_psock_done_strp(struct sk_psock *psock);
>
> static void sk_psock_destroy_deferred(struct work_struct *gc)
> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> index dd53a7771d7e..26ba47b099f1 100644
> --- a/net/core/sock_map.c
> +++ b/net/core/sock_map.c
> @@ -1540,6 +1540,7 @@ void sock_map_close(struct sock *sk, long timeout)
> saved_close = psock->saved_close;
> sock_map_remove_links(sk, psock);
> rcu_read_unlock();
> + sk_psock_purge(psock);
> release_sock(sk);
> saved_close(sk, timeout);
> }
Nothing stops sk_psock_backlog from running after sk_psock_purge:
CPU 1 CPU 2
sk_psock_skb_redirect()
sk_psock(sk_other)
sock_flag(sk_other, SOCK_DEAD)
sk_psock_test_state(psock_other,
SK_PSOCK_TX_ENABLED)
sk_psock_purge()
skb_queue_tail(&psock_other->ingress_skb, skb)
schedule_work(&psock_other->work)
And sock_orphan can run while we're in sendmsg/sendpage_unlocked:
CPU 1 CPU 2
sk_psock_backlog
...
sendmsg_unlocked
sock = sk->sk_socket
tcp_close
__tcp_close
sock_orphan
kernel_sendmsg(sock, msg, vec, num, size)
So, after this change, without lock_sock in sk_psock_backlog, we will
not block tcp_close from running.
This makes me think that the process socket can get released from under
us, before kernel_sendmsg/sendpage runs.
What did I miss?
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