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@tessares.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
---
 net/mptcp/bpf.c | 15 +++++++++++++++
 net/socket.c    | 24 ++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

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 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);
-- 
2.35.3

On 8/3/23 10:07 PM, 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@tessares.net>
> Acked-by: Yonghong Song <yonghong.song@linux.dev>
> Signed-off-by: Geliang Tang <geliang.tang@suse.com>
> ---
>   net/mptcp/bpf.c | 15 +++++++++++++++
>   net/socket.c    | 24 ++++++++++++++++++++++++
>   2 files changed, 39 insertions(+)
> 
> 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 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");
This "kfuns which will be used in BPF programs" piece is not accurate. It is a 
fmod_ret entry point for bpf prog.

> +
> +__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);

Paolo, could you help to take another look and ack this patch if it has 
addressed your earlier comment ?

>   	sock = __sys_socket_create(family, type, protocol);
>   	if (IS_ERR(sock))
>   		return PTR_ERR(sock);