netconsole: add support for sysdata and CPU population

Add infrastructure to automatically append kernel-generated data (sysdata)
to netconsole messages. As the first use case, implement CPU number
population, which adds the CPU that sent the message.

This change introduces three distinct data types:
- extradata: The complete set of appended data (sysdata + userdata)
- userdata: User-provided key-value pairs from userspace
- sysdata: Kernel-populated data (e.g. cpu=XX)

The implementation adds a new configfs attribute 'cpu_nr' to control CPU
number population per target. When enabled, each message is tagged with
its originating CPU. The sysdata is dynamically updated at message time
and appended after any existing userdata.

The CPU number is formatted as "cpu=XX" and is added to the extradata
buffer, respecting the existing size limits.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: leitao

drivers/net/netconsole.c

@@ -1117,6 +1117,40 @@ static void populate_configfs_item(struct netconsole_target *nt,
 	init_target_config_group(nt, target_name);
 }
 
+/*
+ * prepare_extradata - append sysdata at extradata_complete in runtime
+ * @nt: target to send message to
+ */
+static int prepare_extradata(struct netconsole_target *nt)
+{
+	int sysdata_len, extradata_len;
+
+	/* userdata was appended when configfs write helper was called
+	 * by update_userdata().
+	 */
+	extradata_len = nt->userdata_length;
+
+	if (!(nt->sysdata_fields & CPU_NR))
+		goto out;
+
+	/* Append cpu=%d at extradata_complete after userdata str */
+	sysdata_len = scnprintf(&nt->extradata_complete[nt->userdata_length],
+				MAX_EXTRADATA_ENTRY_LEN, " cpu=%u\n",
+				raw_smp_processor_id());
+
+	extradata_len += sysdata_len;
+
+	WARN_ON_ONCE(extradata_len >
+		     MAX_EXTRADATA_ENTRY_LEN * MAX_EXTRADATA_ITEMS);
+
+out:
+	return extradata_len;
+}
+#else /* CONFIG_NETCONSOLE_DYNAMIC not set */
+static int prepare_extradata(struct netconsole_target *nt)
+{
+	return 0;
+}
 #endif	/* CONFIG_NETCONSOLE_DYNAMIC */
 
 /* Handle network interface device notifications */
@@ -1251,16 +1285,14 @@ static void append_release(char *buf)
 
 static void send_fragmented_body(struct netconsole_target *nt,
 				 const char *msgbody, int header_len,
-				 int msgbody_len)
+				 int msgbody_len, int extradata_len)
 {
 	int sent_extradata, preceding_bytes;
 	const char *extradata = NULL;
 	int body_len, offset = 0;
-	int extradata_len = 0;
 
 #ifdef CONFIG_NETCONSOLE_DYNAMIC
 	extradata = nt->extradata_complete;
-	extradata_len = nt->userdata_length;
 #endif
 
 	/* body_len represents the number of bytes that will be sent. This is
@@ -1341,7 +1373,8 @@ static void send_fragmented_body(struct netconsole_target *nt,
 static void send_msg_fragmented(struct netconsole_target *nt,
 				const char *msg,
 				int msg_len,
-				int release_len)
+				int release_len,
+				int extradata_len)
 {
 	int header_len, msgbody_len;
 	const char *msgbody;
@@ -1369,7 +1402,8 @@ static void send_msg_fragmented(struct netconsole_target *nt,
 	/* for now on, the header will be persisted, and the msgbody
 	 * will be replaced
 	 */
-	send_fragmented_body(nt, msgbody, header_len, msgbody_len);
+	send_fragmented_body(nt, msgbody, header_len, msgbody_len,
+			     extradata_len);
 }
 
 /**
@@ -1385,20 +1419,19 @@ static void send_msg_fragmented(struct netconsole_target *nt,
 static void send_ext_msg_udp(struct netconsole_target *nt, const char *msg,
 			     int msg_len)
 {
-	int extradata_len = 0;
 	int release_len = 0;
+	int extradata_len;
 
-#ifdef CONFIG_NETCONSOLE_DYNAMIC
-	extradata_len = nt->userdata_length;
-#endif
+	extradata_len = prepare_extradata(nt);
 
 	if (nt->release)
 		release_len = strlen(init_utsname()->release) + 1;
 
 	if (msg_len + release_len + extradata_len <= MAX_PRINT_CHUNK)
 		return send_msg_no_fragmentation(nt, msg, msg_len, release_len);
 
-	return send_msg_fragmented(nt, msg, msg_len, release_len);
+	return send_msg_fragmented(nt, msg, msg_len, release_len,
+				   extradata_len);
 }
 
 static void write_ext_msg(struct console *con, const char *msg,