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

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On 8/18/23 1:24 AM, Geliang Tang wrote:
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 <matthieu.baerts@xxxxxxxxxxxx>
Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
Signed-off-by: Geliang Tang <geliang.tang@xxxxxxxx>
---
  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?

This is a sparse warning. Let us ignore it for now. We already have
__diag_ignore for missing prototypes in the above, but sparse won't recognize them. Also, 'static' is conflict with '__weak' attribute,
and we cannot remove '__weak' attribute.


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






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