netdev - Re: [PATCH bpf-next v2 2/2] bpf: Require only one of cong_avoid() and cong

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Message-ID: <20220613171703.xoetc7dlr4qkss43@kafai-mbp>
Date:   Mon, 13 Jun 2022 10:17:03 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Jörn-Thorben Hinz <jthinz@...lbox.tu-berlin.de>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 2/2] bpf: Require only one of cong_avoid()
 and cong_control() from a TCP CC

On Thu, Jun 09, 2022 at 10:47:02PM +0200, Jörn-Thorben Hinz wrote:
> Remove the check for required and optional functions in a struct
> tcp_congestion_ops from bpf_tcp_ca.c. Rely on
> tcp_register_congestion_control() to reject a BPF CC that does not
> implement all required functions, as it will do for a non-BPF CC.
> 
> When a CC implements tcp_congestion_ops.cong_control(), the alternate
> cong_avoid() is not in use in the TCP stack. Previously, a BPF CC was
> still forced to implement cong_avoid() as a no-op since it was
> non-optional in bpf_tcp_ca.c.
> 
> Signed-off-by: Jörn-Thorben Hinz <jthinz@...lbox.tu-berlin.de>
> ---
>  net/ipv4/bpf_tcp_ca.c | 32 --------------------------------
>  1 file changed, 32 deletions(-)
> 
> diff --git a/net/ipv4/bpf_tcp_ca.c b/net/ipv4/bpf_tcp_ca.c
> index 1f5c53ede4e5..39b64f317499 100644
> --- a/net/ipv4/bpf_tcp_ca.c
> +++ b/net/ipv4/bpf_tcp_ca.c
> @@ -14,18 +14,6 @@
>  /* "extern" is to avoid sparse warning.  It is only used in bpf_struct_ops.c. */
>  extern struct bpf_struct_ops bpf_tcp_congestion_ops;
>  
> -static u32 optional_ops[] = {
> -	offsetof(struct tcp_congestion_ops, init),
> -	offsetof(struct tcp_congestion_ops, release),
> -	offsetof(struct tcp_congestion_ops, set_state),
> -	offsetof(struct tcp_congestion_ops, cwnd_event),
> -	offsetof(struct tcp_congestion_ops, in_ack_event),
> -	offsetof(struct tcp_congestion_ops, pkts_acked),
> -	offsetof(struct tcp_congestion_ops, min_tso_segs),
> -	offsetof(struct tcp_congestion_ops, sndbuf_expand),
> -	offsetof(struct tcp_congestion_ops, cong_control),
> -};
> -
>  static u32 unsupported_ops[] = {
>  	offsetof(struct tcp_congestion_ops, get_info),
>  };
> @@ -51,18 +39,6 @@ static int bpf_tcp_ca_init(struct btf *btf)
>  	return 0;
>  }
>  
> -static bool is_optional(u32 member_offset)
> -{
> -	unsigned int i;
> -
> -	for (i = 0; i < ARRAY_SIZE(optional_ops); i++) {
> -		if (member_offset == optional_ops[i])
> -			return true;
> -	}
> -
> -	return false;
> -}
> -
>  static bool is_unsupported(u32 member_offset)
>  {
>  	unsigned int i;
> @@ -268,14 +244,6 @@ static int bpf_tcp_ca_init_member(const struct btf_type *t,
>  		return 1;
>  	}
>  
> -	if (!btf_type_resolve_func_ptr(btf_vmlinux, member->type, NULL))
> -		return 0;
> -
> -	/* Ensure bpf_prog is provided for compulsory func ptr */
> -	prog_fd = (int)(*(unsigned long *)(udata + moff));
> -	if (!prog_fd && !is_optional(moff) && !is_unsupported(moff))
!is_unsupported() is still needed.

and remove 'int prog_fd' as reported by the test bot.

Test is still needed.  You can copy the simpler "bpf_dctcp"
to another tcp_congestion_ops.  Write+read the sk_packing
and also use .cong_control instead of .cong_avoid.  I think rs->acked_sacked
is the 'delivered' and the 'ack' is not used.