netdev - Re: [PATCH bpf-next v5 05/11] bpf: Adds field bpf_sock_ops_flags to tcp

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Date:   Tue, 09 Jan 2018 15:30:28 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Lawrence Brakmo <brakmo@...com>, netdev <netdev@...r.kernel.org>
Cc:     Kernel Team <kernel-team@...com>, Blake Matheny <bmatheny@...com>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH bpf-next v5 05/11] bpf: Adds field bpf_sock_ops_flags to
 tcp_sock

On Tue, 2018-01-09 at 13:06 -0800, Lawrence Brakmo wrote:
> Adds field bpf_sock_ops_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, 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.
> 
> Examples of where to call the bpf program:
> 
> 1) When RTO fires
> 2) When a packet is retransmitted
> 3) When the connection terminates
> 4) When a packet is sent
> 5) When a packet is received
> 
> Signed-off-by: Lawrence Brakmo <brakmo@...com>
> ---
>  include/linux/tcp.h      | 8 ++++++++
>  include/uapi/linux/bpf.h | 1 +
>  net/core/filter.c        | 7 +++++++
>  3 files changed, 16 insertions(+)
> 
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index 4f93f095..62f4388 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -373,6 +373,14 @@ struct tcp_sock {
>  	 */
>  	struct request_sock *fastopen_rsk;
>  	u32	*saved_syn;
> +
> +/* Sock_ops bpf program related variables */
> +#ifdef CONFIG_BPF
> +	u32     bpf_sock_ops_flags;     /* values defined in uapi/linux/tcp.h */
> +#define BPF_SOCK_OPS_TEST_FLAG(TP, ARG) (TP->bpf_sock_ops_flags & ARG)
> +#else
> +#define BPF_SOCK_OPS_TEST_FLAG(TP, ARG) 0
> +#endif
>  };
> 

It looks like we add yet another TCP socket field for some feature that
 is only used by you or FB :/

At least please try to fill a hole (on 64bit kernels), instead of
adding one more.

Also, should not we reject attempts to use bits that are not yet
supported by the kernel ?

If a BPF filter expects to be called for every retransmit, but kernel
is too old, this wont work.