bpf, xdp: Add tracepoint to xdp attaching failure

When error happens in dev_xdp_attach(), it should have a way to tell
users the error message like the netlink approach.

To avoid breaking uapi, adding a tracepoint in bpf_xdp_link_attach() is
an appropriate way to notify users the error message.

Hence, bpf libraries are able to retrieve the error message by this
tracepoint, and then report the error message to users.

Signed-off-by: Leon Hwang <hffilwlqm@gmail.com>
Link: https://lore.kernel.org/r/20230801142621.7925-2-hffilwlqm@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/trace/events/xdp.h

@@ -404,6 +404,23 @@ TRACE_EVENT(mem_return_failed,
 	)
 );
 
+TRACE_EVENT(bpf_xdp_link_attach_failed,
+
+	TP_PROTO(const char *msg),
+
+	TP_ARGS(msg),
+
+	TP_STRUCT__entry(
+		__string(msg, msg)
+	),
+
+	TP_fast_assign(
+		__assign_str(msg, msg);
+	),
+
+	TP_printk("errmsg=%s", __get_str(msg))
+);
+
 #endif /* _TRACE_XDP_H */
 
 #include <trace/define_trace.h>