netdev - RE: [Patch bpf-next v3 3/9] udp: implement ->sendmsg

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Date:   Fri, 05 Mar 2021 17:20:58 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Cc:     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>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: RE: [Patch bpf-next v3 3/9] udp: implement ->sendmsg_locked()

Cong Wang wrote:
> From: Cong Wang <cong.wang@...edance.com>
> 
> UDP already has udp_sendmsg() which takes lock_sock() inside.
> We have to build ->sendmsg_locked() on top of it, by adding
> a new parameter for whether the sock has been locked.
> 
> 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/net/udp.h  |  1 +
>  net/ipv4/af_inet.c |  1 +
>  net/ipv4/udp.c     | 30 +++++++++++++++++++++++-------
>  3 files changed, 25 insertions(+), 7 deletions(-)

[...]

> -int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
> +static int __udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len, bool locked)
>  {

The lock_sock is also taken by BPF_CGROUP_RUN_PROG_UDP4_SENDMSG_LOCK() in
udp_sendmsg(),

 if (cgroup_bpf_enabled(BPF_CGROUP_UDP4_SENDMSG) && !connected) {
    err = BPF_CGROUP_RUN_PROG_UDP4_SENDMSG_LOCK(sk,
                                    (struct sockaddr *)usin, &ipc.addr);

so that will also need to be handled.

It also looks like sk_dst_set() wants the sock lock to be held, but I'm not
seeing how its covered in the current code,

 static inline void
 __sk_dst_set(struct sock *sk, struct dst_entry *dst)
 {
        struct dst_entry *old_dst;

        sk_tx_queue_clear(sk);
        sk->sk_dst_pending_confirm = 0;
        old_dst = rcu_dereference_protected(sk->sk_dst_cache,
                                            lockdep_sock_is_held(sk));
        rcu_assign_pointer(sk->sk_dst_cache, dst);
        dst_release(old_dst);
 }

I guess this could trip lockdep now, I'll dig a bit more Monday and see
if its actually the case.

In general I don't really like code that wraps locks in 'if' branches
like this. It seem fragile to me. I didn't walk every path in the code
to see if a lock is taken in any of the called functions but it looks
like ip_send_skb() can call into netfilter code and may try to take
the sock lock.

Do we need this locked send at all? We use it in sk_psock_backlog
but that routine needs an optimization rewrite for TCP anyways.
Its dropping a lot of performance on the floor for no good reason.

.John