Subject: Re: [PATCH bpf-next v14 1/4] bpf: Add update_socket_protocol hook

From: Geliang Tang <[email-masked]>
Date: Fri, 18 Aug 2023 16:24:17 +0800

On Wed, Aug 16, 2023 at 09:11:56AM +0800, Geliang Tang wrote:
> Add a hook named update_socket_protocol in __sys_socket(), for bpf
> progs to attach to and update socket protocol. One user case is to
> force legacy TCP apps to create and use MPTCP sockets instead of
> TCP ones.
> 
> Define a fmod_ret set named bpf_mptcp_fmodret_ids, add the hook
> update_socket_protocol into this set, and register it in
> bpf_mptcp_kfunc_init().
> 
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/79
> Acked-by: Matthieu Baerts <[email-masked]>
> Acked-by: Yonghong Song <[email-masked]>
> Signed-off-by: Geliang Tang <[email-masked]>
> ---
>  net/mptcp/bpf.c | 15 +++++++++++++++
>  net/socket.c    | 26 +++++++++++++++++++++++++-
>  2 files changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/net/mptcp/bpf.c b/net/mptcp/bpf.c
> index 5a0a84ad94af..8a16672b94e2 100644
> --- a/net/mptcp/bpf.c
> +++ b/net/mptcp/bpf.c
> @@ -19,3 +19,18 @@ struct mptcp_sock *bpf_mptcp_sock_from_subflow(struct sock *sk)
>  
>  	return NULL;
>  }
> +
> +BTF_SET8_START(bpf_mptcp_fmodret_ids)
> +BTF_ID_FLAGS(func, update_socket_protocol)
> +BTF_SET8_END(bpf_mptcp_fmodret_ids)
> +
> +static const struct btf_kfunc_id_set bpf_mptcp_fmodret_set = {
> +	.owner = THIS_MODULE,
> +	.set   = &bpf_mptcp_fmodret_ids,
> +};
> +
> +static int __init bpf_mptcp_kfunc_init(void)
> +{
> +	return register_btf_fmodret_id_set(&bpf_mptcp_fmodret_set);
> +}
> +late_initcall(bpf_mptcp_kfunc_init);
> diff --git a/net/socket.c b/net/socket.c
> index 5d4e37595e9a..fdb5233bf560 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -1657,12 +1657,36 @@ struct file *__sys_socket_file(int family, int type, int protocol)
>  	return sock_alloc_file(sock, flags, NULL);
>  }
>  
> +/*	A hook for bpf progs to attach to and update socket protocol.
> + *
> + *	A static noinline declaration here could cause the compiler to
> + *	optimize away the function. A global noinline declaration will
> + *	keep the definition, but may optimize away the callsite.
> + *	Therefore, __weak is needed to ensure that the call is still
> + *	emitted, by telling the compiler that we don't know what the
> + *	function might eventually be.
> + *
> + *	__diag_* below are needed to dismiss the missing prototype warning.
> + */
> +
> +__diag_push();
> +__diag_ignore_all("-Wmissing-prototypes",
> +		  "A fmod_ret entry point for BPF programs");

Hi Martin & Yonghong,

I got a sparse warning for this new added 'update_socket_protocol':

 > touch net/socket.c && make C=1 net/socket.o

 net/socket.c:1676:21: warning: symbol 'update_socket_protocol' was not declared. Should it be static?

What should I do to fix it, or should I just leave it here? Please give
me some suggestions.

Thanks,
-Geliang

> +
> +__weak noinline int update_socket_protocol(int family, int type, int protocol)
> +{
> +	return protocol;
> +}
> +
> +__diag_pop();
> +
>  int __sys_socket(int family, int type, int protocol)
>  {
>  	struct socket *sock;
>  	int flags;
>  
> -	sock = __sys_socket_create(family, type, protocol);
> +	sock = __sys_socket_create(family, type,
> +				   update_socket_protocol(family, type, protocol));
>  	if (IS_ERR(sock))
>  		return PTR_ERR(sock);
>  
> -- 
> 2.35.3
>