netlink: add tracepoint at NL_SET_ERR_MSG

Often userspace won't request the extack information, or they don't log it
because of log level or so, and even when they do, sometimes it's not
enough to know exactly what caused the error.

Netlink extack is the standard way of reporting erros with descriptive
error messages. With a trace point on it, we then can know exactly where
the error happened, regardless of userspace app. Also, we can even see if
the err msg was overwritten.

The wrapper do_trace_netlink_extack() is because trace points shouldn't be
called from .h files, as trace points are not that small, and the function
call to do_trace_netlink_extack() on the macros is not protected by
tracepoint_enabled() because the macros are called from modules, and this
would require exporting some trace structs. As this is error path, it's
better to export just the wrapper instead.

v2: removed leftover tracepoint declaration
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/4546b63e67b2989789d146498b13cc09e1fdc543.1612403190.git.marcelo.leitner@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/netlink.h

@@ -11,6 +11,8 @@
 
 struct net;
 
+void do_trace_netlink_extack(const char *msg);
+
 static inline struct nlmsghdr *nlmsg_hdr(const struct sk_buff *skb)
 {
 	return (struct nlmsghdr *)skb->data;
@@ -90,6 +92,8 @@ struct netlink_ext_ack {
 	static const char __msg[] = msg;		\
 	struct netlink_ext_ack *__extack = (extack);	\
 							\
+	do_trace_netlink_extack(__msg);			\
+							\
 	if (__extack)					\
 		__extack->_msg = __msg;			\
 } while (0)
@@ -110,6 +114,8 @@ struct netlink_ext_ack {
 	static const char __msg[] = msg;			\
 	struct netlink_ext_ack *__extack = (extack);		\
 								\
+	do_trace_netlink_extack(__msg);				\
+								\
 	if (__extack) {						\
 		__extack->_msg = __msg;				\
 		__extack->bad_attr = (attr);			\

include/trace/events/netlink.h

+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM netlink
+
+#if !defined(_TRACE_NETLINK_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_NETLINK_H
+
+#include <linux/tracepoint.h>
+
+TRACE_EVENT(netlink_extack,
+
+	TP_PROTO(const char *msg),
+
+	TP_ARGS(msg),
+
+	TP_STRUCT__entry(
+		__string(	msg,	msg	)
+	),
+
+	TP_fast_assign(
+		__assign_str(msg, msg);
+	),
+
+	TP_printk("msg=%s", __get_str(msg))
+);
+
+#endif /* _TRACE_NETLINK_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>

net/netlink/af_netlink.c

@@ -67,6 +67,8 @@
 #include <net/sock.h>
 #include <net/scm.h>
 #include <net/netlink.h>
+#define CREATE_TRACE_POINTS
+#include <trace/events/netlink.h>
 
 #include "af_netlink.h"
 
@@ -147,6 +149,12 @@ static BLOCKING_NOTIFIER_HEAD(netlink_chain);
 
 static const struct rhashtable_params netlink_rhashtable_params;
 
+void do_trace_netlink_extack(const char *msg)
+{
+	trace_netlink_extack(msg);
+}
+EXPORT_SYMBOL(do_trace_netlink_extack);
+
 static inline u32 netlink_group_mask(u32 group)
 {
 	return group ? 1 << (group - 1) : 0;