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

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Message-ID: <20180120035220.rg55abc6gk2alere@ast-mbp>
Date:   Fri, 19 Jan 2018 19:52:22 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Lawrence Brakmo <brakmo@...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 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.

> + * 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?