netdev - Re: [PATCH bpf-next v9 1/4] bpf: Add update_socket

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Date: Thu, 3 Aug 2023 16:17:37 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Geliang Tang <geliang.tang@...e.com>, Alexei Starovoitov
 <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
 Yonghong Song <yhs@...com>, John Fastabend <john.fastabend@...il.com>,
 KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>,
 Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
 Florent Revest <revest@...omium.org>, Brendan Jackman
 <jackmanb@...omium.org>, Matthieu Baerts <matthieu.baerts@...sares.net>,
 Mat Martineau <martineau@...nel.org>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 John Johansen <john.johansen@...onical.com>, Paul Moore
 <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
 "Serge E. Hallyn" <serge@...lyn.com>,
 Stephen Smalley <stephen.smalley.work@...il.com>,
 Eric Paris <eparis@...isplace.org>, Mykola Lysenko <mykolal@...com>,
 Shuah Khan <shuah@...nel.org>, Simon Horman <horms@...nel.org>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, mptcp@...ts.linux.dev,
 apparmor@...ts.ubuntu.com, linux-security-module@...r.kernel.org,
 selinux@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next v9 1/4] bpf: Add update_socket_protocol hook

On 8/3/23 6:41 AM, 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 mod_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 <matthieu.baerts@...sares.net>
> Signed-off-by: Geliang Tang <geliang.tang@...e.com>
> ---
>   net/mptcp/bpf.c | 17 +++++++++++++++++
>   net/socket.c    | 24 ++++++++++++++++++++++++
>   2 files changed, 41 insertions(+)
> 
> diff --git a/net/mptcp/bpf.c b/net/mptcp/bpf.c
> index 5a0a84ad94af..c43aee31014d 100644
> --- a/net/mptcp/bpf.c
> +++ b/net/mptcp/bpf.c
> @@ -12,6 +12,23 @@
>   #include <linux/bpf.h>
>   #include "protocol.h"
>   
> +#ifdef CONFIG_BPF_JIT

Is this necessary? Most other register_btf_* functions do not have
a config like this.

> +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);
> +#endif /* CONFIG_BPF_JIT */
> +
>   struct mptcp_sock *bpf_mptcp_sock_from_subflow(struct sock *sk)
>   {
>   	if (sk && sk_fullsock(sk) && sk->sk_protocol == IPPROTO_TCP && sk_is_mptcp(sk))
> diff --git a/net/socket.c b/net/socket.c
> index 2b0e54b2405c..9f98ced88ac5 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -1644,11 +1644,35 @@ 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",
> +		  "kfuncs which will be used in BPF programs");
> +
> +__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;
>   
> +	protocol = update_socket_protocol(family, type, protocol);
>   	sock = __sys_socket_create(family, type, protocol);
>   	if (IS_ERR(sock))
>   		return PTR_ERR(sock);