Re: [PATCH bpf-next v9 1/4] bpf: Add update_socket_protocol hook

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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@xxxxxxxxxxxx>
Signed-off-by: Geliang Tang <geliang.tang@xxxxxxxx>
---
  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);



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