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Date:   Sun, 24 Feb 2019 10:00:16 -0800
From:   Eric Dumazet <>
To:     Martin Lau <>, Eric Dumazet <>
Cc:     Lawrence Brakmo <>, netdev <>,
        Alexei Starovoitov <>,
        Daniel Borkmann <>,
        Kernel Team <>
Subject: Re: [PATCH v2 bpf-next 2/9] bpf: Add bpf helper bpf_tcp_enter_cwr

On 02/23/2019 07:08 PM, Martin Lau wrote:
> On Sat, Feb 23, 2019 at 05:32:14PM -0800, Eric Dumazet wrote:
>> On 02/22/2019 05:06 PM, brakmo wrote:
>>> From: Martin KaFai Lau <>
>>> This patch adds a new bpf helper BPF_FUNC_tcp_enter_cwr
>>> "int bpf_tcp_enter_cwr(struct bpf_tcp_sock *tp)".
>>> It is added to BPF_PROG_TYPE_CGROUP_SKB which can be attached
>>> to the egress path where the bpf prog is called by
>>> ip_finish_output() or ip6_finish_output().  The verifier
>>> ensures that the parameter must be a tcp_sock.
>>> This helper makes a tcp_sock enter CWR state.  It can be used
>>> by a bpf_prog to manage egress network bandwidth limit per
>>> cgroupv2.  A later patch will have a sample program to
>>> show how it can be used to limit bandwidth usage per cgroupv2.
>>> To ensure it is only called from BPF_CGROUP_INET_EGRESS, the
>>> attr->expected_attach_type must be specified as BPF_CGROUP_INET_EGRESS
>>> during load time if the prog uses this new helper.
>>> The newly added prog->enforce_expected_attach_type bit will also be set
>>> if this new helper is used.  This bit is for backward compatibility reason
>>> because currently prog->expected_attach_type has been ignored in
>>> BPF_PROG_TYPE_CGROUP_SKB.  During attach time,
>>> prog->expected_attach_type is only enforced if the
>>> prog->enforce_expected_attach_type bit is set.
>>> i.e. prog->expected_attach_type is only enforced if this new helper
>>> is used by the prog.
>> BTW, it seems to me that BPF_CGROUP_INET_EGRESS can be used while the socket lock is not held.
> Thanks for pointing it out.
> ic. I just noticed the comments at ip6_xmit():
> /*
>  * xmit an sk_buff (used by TCP, SCTP and DCCP)
>  * Note : socket lock is not held for SYNACK packets, but might be modified
>  * by calls to skb_set_owner_w() and ipv6_local_error(),
>  * which are using proper atomic operations or spinlocks.
>  */
> Is there other cases other than SYNACK?

Well, I was referring to various virtual devices, re-entering ip stack.

Since we can have a qdisc on any netdev, there is no way we can guarantee the socket is
locked by the current thread.

Random example :

     err = ip_local_out(net, skb->sk, skb);

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