netdev - Re: [PATCH bpf-next v6 05/11] bpf: Adds field bpf_sock_ops_cb_flags to tcp

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Date:   Sat, 20 Jan 2018 07:50:59 +0000
From:   Lawrence Brakmo <brakmo@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     netdev <netdev@...r.kernel.org>, Kernel Team <Kernel-team@...com>,
        "Blake Matheny" <bmatheny@...com>, Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH bpf-next v6 05/11] bpf: Adds field bpf_sock_ops_cb_flags
 to tcp_sock

On 1/19/18, 7:52 PM, "Alexei Starovoitov" <alexei.starovoitov@...il.com> wrote:

    On Fri, Jan 19, 2018 at 05:45:42PM -0800, Lawrence Brakmo wrote:
    > Adds field bpf_sock_ops_cb_flags to tcp_sock and bpf_sock_ops. Its primary
    > use is to determine if there should be calls to sock_ops bpf program at
    > various points in the TCP code. The field is initialized to zero,
    > disabling the calls. A sock_ops BPF program can set it, per connection and
    > as necessary, when the connection is established.
    > 
    > It also adds support for reading and writting the field within a
    > sock_ops BPF program. Reading is done by accessing the field directly.
    > However, writing is done through the helper function
    > bpf_sock_ops_cb_flags_set, in order to return an error if a BPF program
    > is trying to set a callback that is not supported in the current kernel
    > (i.e. running an older kernel). The helper function returns 0 if it was
    > able to set all of the bits set in the argument, a positive number
    > containing the bits that could not be set, or -EINVAL if the socket is
    > not a full TCP socket.
    ...
    > +/* Sock_ops bpf program related variables */
    > +#ifdef CONFIG_BPF
    > +	u8	bpf_sock_ops_cb_flags;  /* Control calling BPF programs
    > +					 * values defined in uapi/linux/tcp.h
    
    I guess we can extend u8 into u16 or more if necessary in the future.
    
Yes, that was my thought.

    > + * int bpf_sock_ops_cb_flags_set(bpf_sock_ops, flags)
    > + *     Set callback flags for sock_ops
    > + *     @bpf_sock_ops: pointer to bpf_sock_ops_kern struct
    > + *     @flags: flags value
    > + *     Return: 0 for no error
    > + *             -EINVAL if there is no full tcp socket
    > + *             bits in flags that are not supported by current kernel
    ...
    > +BPF_CALL_2(bpf_sock_ops_cb_flags_set, struct bpf_sock_ops_kern *, bpf_sock,
    > +	   int, argval)
    > +{
    > +	struct sock *sk = bpf_sock->sk;
    > +	int val = argval & BPF_SOCK_OPS_ALL_CB_FLAGS;
    > +
    > +	if (!sk_fullsock(sk))
    > +		return -EINVAL;
    > +
    > +#ifdef CONFIG_INET
    > +	if (val)
    > +		tcp_sk(sk)->bpf_sock_ops_cb_flags = val;
    > +
    > +	return argval & (~BPF_SOCK_OPS_ALL_CB_FLAGS);
    
    interesting idea! took me some time to realize the potential
    of such semantics, but now I like it a lot.
    It blends 'set good flag' with 'which flags are supported' logic
    into single helper. Nice.
    Thanks for adding a test for both ways.
    Acked-by: Alexei Starovoitov <ast@...nel.org>
    
    Eric, does this approach address your concerns?