Merge branch 'netconsole-cpu-population'

Breno Leitao says:

====================
netconsole: Add support for CPU population

The current implementation of netconsole sends all log messages in
parallel, which can lead to an intermixed and interleaved output on the
receiving side. This makes it challenging to demultiplex the messages
and attribute them to their originating CPUs.

As a result, users and developers often struggle to effectively analyze
and debug the parallel log output received through netconsole.

Example of a message got from produciton hosts:

	------------[ cut here ]------------
	------------[ cut here ]------------
	refcount_t: saturated; leaking memory.
	WARNING: CPU: 2 PID: 1613668 at lib/refcount.c:22 refcount_warn_saturate+0x5e/0xe0
	refcount_t: addition on 0; use-after-free.
	WARNING: CPU: 26 PID: 4139916 at lib/refcount.c:25 refcount_warn_saturate+0x7d/0xe0
	Modules linked in: bpf_preload(E) vhost_net(E) tun(E) vhost(E)

This series of patches introduces a new feature to the netconsole
subsystem that allows the automatic population of the CPU number in the
userdata field for each log message. This enhancement provides several
benefits:

* Improved demultiplexing of parallel log output: When multiple CPUs are
  sending messages concurrently, the added CPU number in the userdata
  makes it easier to differentiate and attribute the messages to their
  originating CPUs.

* Better visibility into message sources: The CPU number information
  gives users and developers more insight into which specific CPU a
  particular log message came from, which can be valuable for debugging
  and analysis.

The changes in this series are as follows Patches::

Patch "consolidate send buffers into netconsole_target struct"
=================================================

Move the static buffers to netconsole target, from static declaration
in send_msg_no_fragmentation() and send_msg_fragmented().

Patch "netconsole: Rename userdata to extradata"
=================================================
Create the a concept of extradata, which encompasses the concept of
userdata and the upcoming sysdatao

Sysdata is a new concept being added, which is basically fields that are
populated by the kernel. At this time only the CPU#, but, there is a
desire to add current task name, kernel release version, etc.

Patch "netconsole: Helper to count number of used entries"
===========================================================
Create a simple helper to count number of entries in extradata. I am
separating this in a function since it will need to count userdata and
sysdata. For instance, when the user adds an extra userdata, we need to
check if there is space, counting the previous data entries (from
userdata and cpu data)

Patch "Introduce configfs helpers for sysdata features"
======================================================
Create the concept of sysdata feature in the netconsole target, and
create the configfs helpers to enable the bit in nt->sysdata

Patch "Include sysdata in extradata entry count"
================================================
Add the concept of sysdata when counting for available space in the
buffer. This will protect users from creating new userdata/sysdata if
there is no more space

Patch "netconsole: add support for sysdata and CPU population"
===============================================================
This is the core patch. Basically add a new option to enable automatic
CPU number population in the netconsole userdata Provides a new "cpu_nr"
sysfs attribute to control this feature

Patch "netconsole: selftest: test CPU number auto-population"
=============================================================
Expands the existing netconsole selftest to verify the CPU number
auto-population functionality Ensures the received netconsole messages
contain the expected "cpu=<CPU>" entry in the message. Test different
permutation with userdata

Patch "netconsole: docs: Add documentation for CPU number auto-population"
=============================================================================
Updates the netconsole documentation to explain the new CPU number
auto-population feature Provides instructions on how to enable and use
the feature

I believe these changes will be a valuable addition to the netconsole
subsystem, enhancing its usefulness for kernel developers and users.

PS: This patchset is on top of the patch that created
netcons_fragmented_msg selftest:

https://lore.kernel.org/all/20250203-netcons_frag_msgs-v1-1-5bc6bedf2ac0@debian.org/

---
Changes in v5:
- Fixed a kernel doc syntax syntax (Simon)
- Link to v4: https://lore.kernel.org/r/20250204-netcon_cpu-v4-0-9480266ef556@debian.org

Changes in v4:
- Fixed Kernel doc for netconsole_target (Simon)
- Fixed a typo in disable_sysdata_feature (Simon)
- Improved sysdata_cpu_nr_show() to return !! in a bit-wise operation
- Link to v3: https://lore.kernel.org/r/20250124-netcon_cpu-v3-0-12a0d286ba1d@debian.org

Changes in v3:
- Moved the buffer into netconsole_target, avoiding static functions in
  the send path (Jakub).
- Fix a documentation error (Randy Dunlap)
- Created a function that handle all the extradata, consolidating it in
  a single place (Jakub)
- Split the patch even more, trying to simplify the review.
- Link to v2: https://lore.kernel.org/r/20250115-netcon_cpu-v2-0-95971b44dc56@debian.org

Changes in v2:
- Create the concept of extradata and sysdata. This will make the design
  easier to understand, and the code easier to read.
  * Basically extradata encompasses userdata and the new sysdata.
    Userdata originates from user, and sysdata originates in kernel.
- Improved the test to send from a very specific CPU, which can be
  checked to be correct on the other side, as suggested by Jakub.
- Fixed a bug where CPU # was populated at the wrong place
- Link to v1: https://lore.kernel.org/r/20241113-netcon_cpu-v1-0-d187bf7c0321@debian.org
====================

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

5 files changed, 427 insertions, 64 deletions

diff --git a/Documentation/networking/netconsole.rst b/Documentation/networking/netconsole.rst
index 94c4680fdf3e..84803c59968a 100644
--- a/Documentation/networking/netconsole.rst
+++ b/Documentation/networking/netconsole.rst
@@ -17,6 +17,8 @@ Release prepend support by Breno Leitao <leitao@debian.org>, Jul 7 2023
 
 Userdata append support by Matthew Wood <thepacketgeek@gmail.com>, Jan 22 2024
 
+Sysdata append support by Breno Leitao <leitao@debian.org>, Jan 15 2025
+
 Please send bug reports to Matt Mackall <mpm@selenic.com>
 Satyam Sharma <satyam.sharma@gmail.com>, and Cong Wang <xiyou.wangcong@gmail.com>
 
@@ -238,6 +240,49 @@ Delete `userdata` entries with `rmdir`::
 
    It is recommended to not write user data values with newlines.
 
+CPU number auto population in userdata
+--------------------------------------
+
+Inside the netconsole configfs hierarchy, there is a file called
+`cpu_nr` under the `userdata` directory. This file is used to enable or disable
+the automatic CPU number population feature. This feature automatically
+populates the CPU number that is sending the message.
+
+To enable the CPU number auto-population::
+
+  echo 1 > /sys/kernel/config/netconsole/target1/userdata/cpu_nr
+
+When this option is enabled, the netconsole messages will include an additional
+line in the userdata field with the format `cpu=<cpu_number>`. This allows the
+receiver of the netconsole messages to easily differentiate and demultiplex
+messages originating from different CPUs, which is particularly useful when
+dealing with parallel log output.
+
+Example::
+
+  echo "This is a message" > /dev/kmsg
+  12,607,22085407756,-;This is a message
+   cpu=42
+
+In this example, the message was sent by CPU 42.
+
+.. note::
+
+   If the user has set a conflicting `cpu` key in the userdata dictionary,
+   both keys will be reported, with the kernel-populated entry appearing after
+   the user one. For example::
+
+     # User-defined CPU entry
+     mkdir -p /sys/kernel/config/netconsole/target1/userdata/cpu
+     echo "1" > /sys/kernel/config/netconsole/target1/userdata/cpu/value
+
+   Output might look like::
+
+     12,607,22085407756,-;This is a message
+      cpu=1
+      cpu=42    # kernel-populated value
+
+
 Extended console:
 =================
 
diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 86ab4a42769a..f77eddf22185 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -45,12 +45,12 @@ MODULE_DESCRIPTION("Console driver for network interfaces");
 MODULE_LICENSE("GPL");
 
 #define MAX_PARAM_LENGTH		256
-#define MAX_USERDATA_ENTRY_LENGTH	256
-#define MAX_USERDATA_VALUE_LENGTH	200
+#define MAX_EXTRADATA_ENTRY_LEN		256
+#define MAX_EXTRADATA_VALUE_LEN	200
 /* The number 3 comes from userdata entry format characters (' ', '=', '\n') */
-#define MAX_USERDATA_NAME_LENGTH	(MAX_USERDATA_ENTRY_LENGTH - \
-					MAX_USERDATA_VALUE_LENGTH - 3)
-#define MAX_USERDATA_ITEMS		16
+#define MAX_EXTRADATA_NAME_LEN		(MAX_EXTRADATA_ENTRY_LEN - \
+					MAX_EXTRADATA_VALUE_LEN - 3)
+#define MAX_EXTRADATA_ITEMS		16
 #define MAX_PRINT_CHUNK			1000
 
 static char config[MAX_PARAM_LENGTH];
@@ -97,13 +97,23 @@ struct netconsole_target_stats  {
 	struct u64_stats_sync syncp;
 };
 
+/* Features enabled in sysdata. Contrary to userdata, this data is populated by
+ * the kernel. The fields are designed as bitwise flags, allowing multiple
+ * features to be set in sysdata_fields.
+ */
+enum sysdata_feature {
+	/* Populate the CPU that sends the message */
+	CPU_NR = BIT(0),
+};
+
 /**
  * struct netconsole_target - Represents a configured netconsole target.
  * @list:	Links this target into the target_list.
  * @group:	Links us into the configfs subsystem hierarchy.
  * @userdata_group:	Links to the userdata configfs hierarchy
- * @userdata_complete:	Cached, formatted string of append
- * @userdata_length:	String length of userdata_complete
+ * @extradata_complete:	Cached, formatted string of append
+ * @userdata_length:	String length of usedata in extradata_complete.
+ * @sysdata_fields:	Sysdata features enabled.
  * @stats:	Packet send stats for the target. Used for debugging.
  * @enabled:	On / off knob to enable / disable target.
  *		Visible from userspace (read-write).
@@ -123,20 +133,25 @@ struct netconsole_target_stats  {
  *		remote_ip	(read-write)
  *		local_mac	(read-only)
  *		remote_mac	(read-write)
+ * @buf:	The buffer used to send the full msg to the network stack
  */
 struct netconsole_target {
 	struct list_head	list;
 #ifdef	CONFIG_NETCONSOLE_DYNAMIC
 	struct config_group	group;
 	struct config_group	userdata_group;
-	char userdata_complete[MAX_USERDATA_ENTRY_LENGTH * MAX_USERDATA_ITEMS];
+	char extradata_complete[MAX_EXTRADATA_ENTRY_LEN * MAX_EXTRADATA_ITEMS];
 	size_t			userdata_length;
+	/* bit-wise with sysdata_feature bits */
+	u32			sysdata_fields;
 #endif
 	struct netconsole_target_stats stats;
 	bool			enabled;
 	bool			extended;
 	bool			release;
 	struct netpoll		np;
+	/* protected by target_list_lock */
+	char			buf[MAX_PRINT_CHUNK];
 };
 
 #ifdef	CONFIG_NETCONSOLE_DYNAMIC
@@ -396,6 +411,19 @@ static ssize_t transmit_errors_show(struct config_item *item, char *buf)
 	return sysfs_emit(buf, "%llu\n", xmit_drop_count + enomem_count);
 }
 
+/* configfs helper to display if cpu_nr sysdata feature is enabled */
+static ssize_t sysdata_cpu_nr_enabled_show(struct config_item *item, char *buf)
+{
+	struct netconsole_target *nt = to_target(item->ci_parent);
+	bool cpu_nr_enabled;
+
+	mutex_lock(&dynamic_netconsole_mutex);
+	cpu_nr_enabled = !!(nt->sysdata_fields & CPU_NR);
+	mutex_unlock(&dynamic_netconsole_mutex);
+
+	return sysfs_emit(buf, "%d\n", cpu_nr_enabled);
+}
+
 /*
  * This one is special -- targets created through the configfs interface
  * are not enabled (and the corresponding netpoll activated) by default.
@@ -659,6 +687,24 @@ out_unlock:
 	return ret;
 }
 
+/* Count number of entries we have in extradata.
+ * This is important because the extradata_complete only supports
+ * MAX_EXTRADATA_ITEMS entries. Before enabling any new {user,sys}data
+ * feature, number of entries needs to checked for available space.
+ */
+static size_t count_extradata_entries(struct netconsole_target *nt)
+{
+	size_t entries;
+
+	/* Userdata entries */
+	entries = list_count_nodes(&nt->userdata_group.cg_children);
+	/* Plus sysdata entries */
+	if (nt->sysdata_fields & CPU_NR)
+		entries += 1;
+
+	return entries;
+}
+
 static ssize_t remote_mac_store(struct config_item *item, const char *buf,
 		size_t count)
 {
@@ -687,7 +733,7 @@ out_unlock:
 
 struct userdatum {
 	struct config_item item;
-	char value[MAX_USERDATA_VALUE_LENGTH];
+	char value[MAX_EXTRADATA_VALUE_LEN];
 };
 
 static struct userdatum *to_userdatum(struct config_item *item)
@@ -724,13 +770,13 @@ static void update_userdata(struct netconsole_target *nt)
 
 	/* Clear the current string in case the last userdatum was deleted */
 	nt->userdata_length = 0;
-	nt->userdata_complete[0] = 0;
+	nt->extradata_complete[0] = 0;
 
 	list_for_each(entry, &nt->userdata_group.cg_children) {
 		struct userdatum *udm_item;
 		struct config_item *item;
 
-		if (WARN_ON_ONCE(child_count >= MAX_USERDATA_ITEMS))
+		if (WARN_ON_ONCE(child_count >= MAX_EXTRADATA_ITEMS))
 			break;
 		child_count++;
 
@@ -738,19 +784,19 @@ static void update_userdata(struct netconsole_target *nt)
 		udm_item = to_userdatum(item);
 
 		/* Skip userdata with no value set */
-		if (strnlen(udm_item->value, MAX_USERDATA_VALUE_LENGTH) == 0)
+		if (strnlen(udm_item->value, MAX_EXTRADATA_VALUE_LEN) == 0)
 			continue;
 
-		/* This doesn't overflow userdata_complete since it will write
-		 * one entry length (1/MAX_USERDATA_ITEMS long), entry count is
+		/* This doesn't overflow extradata_complete since it will write
+		 * one entry length (1/MAX_EXTRADATA_ITEMS long), entry count is
 		 * checked to not exceed MAX items with child_count above
 		 */
-		complete_idx += scnprintf(&nt->userdata_complete[complete_idx],
-					  MAX_USERDATA_ENTRY_LENGTH, " %s=%s\n",
+		complete_idx += scnprintf(&nt->extradata_complete[complete_idx],
+					  MAX_EXTRADATA_ENTRY_LEN, " %s=%s\n",
 					  item->ci_name, udm_item->value);
 	}
-	nt->userdata_length = strnlen(nt->userdata_complete,
-				      sizeof(nt->userdata_complete));
+	nt->userdata_length = strnlen(nt->extradata_complete,
+				      sizeof(nt->extradata_complete));
 }
 
 static ssize_t userdatum_value_store(struct config_item *item, const char *buf,
@@ -761,7 +807,7 @@ static ssize_t userdatum_value_store(struct config_item *item, const char *buf,
 	struct userdata *ud;
 	ssize_t ret;
 
-	if (count > MAX_USERDATA_VALUE_LENGTH)
+	if (count > MAX_EXTRADATA_VALUE_LEN)
 		return -EMSGSIZE;
 
 	mutex_lock(&dynamic_netconsole_mutex);
@@ -780,7 +826,62 @@ out_unlock:
 	return ret;
 }
 
+/* disable_sysdata_feature - Disable sysdata feature and clean sysdata
+ * @nt: target that is disabling the feature
+ * @feature: feature being disabled
+ */
+static void disable_sysdata_feature(struct netconsole_target *nt,
+				    enum sysdata_feature feature)
+{
+	nt->sysdata_fields &= ~feature;
+	nt->extradata_complete[nt->userdata_length] = 0;
+}
+
+/* configfs helper to sysdata cpu_nr feature */
+static ssize_t sysdata_cpu_nr_enabled_store(struct config_item *item,
+					    const char *buf, size_t count)
+{
+	struct netconsole_target *nt = to_target(item->ci_parent);
+	bool cpu_nr_enabled, curr;
+	ssize_t ret;
+
+	ret = kstrtobool(buf, &cpu_nr_enabled);
+	if (ret)
+		return ret;
+
+	mutex_lock(&dynamic_netconsole_mutex);
+	curr = nt->sysdata_fields & CPU_NR;
+	if (cpu_nr_enabled == curr)
+		/* no change requested */
+		goto unlock_ok;
+
+	if (cpu_nr_enabled &&
+	    count_extradata_entries(nt) >= MAX_EXTRADATA_ITEMS) {
+		/* user wants the new feature, but there is no space in the
+		 * buffer.
+		 */
+		ret = -ENOSPC;
+		goto unlock;
+	}
+
+	if (cpu_nr_enabled)
+		nt->sysdata_fields |= CPU_NR;
+	else
+		/* This is special because extradata_complete might have
+		 * remaining data from previous sysdata, and it needs to be
+		 * cleaned.
+		 */
+		disable_sysdata_feature(nt, CPU_NR);
+
+unlock_ok:
+	ret = strnlen(buf, count);
+unlock:
+	mutex_unlock(&dynamic_netconsole_mutex);
+	return ret;
+}
+
 CONFIGFS_ATTR(userdatum_, value);
+CONFIGFS_ATTR(sysdata_, cpu_nr_enabled);
 
 static struct configfs_attribute *userdatum_attrs[] = {
 	&userdatum_attr_value,
@@ -808,15 +909,13 @@ static struct config_item *userdatum_make_item(struct config_group *group,
 	struct netconsole_target *nt;
 	struct userdatum *udm;
 	struct userdata *ud;
-	size_t child_count;
 
-	if (strlen(name) > MAX_USERDATA_NAME_LENGTH)
+	if (strlen(name) > MAX_EXTRADATA_NAME_LEN)
 		return ERR_PTR(-ENAMETOOLONG);
 
 	ud = to_userdata(&group->cg_item);
 	nt = userdata_to_target(ud);
-	child_count = list_count_nodes(&nt->userdata_group.cg_children);
-	if (child_count >= MAX_USERDATA_ITEMS)
+	if (count_extradata_entries(nt) >= MAX_EXTRADATA_ITEMS)
 		return ERR_PTR(-ENOSPC);
 
 	udm = kzalloc(sizeof(*udm), GFP_KERNEL);
@@ -842,6 +941,7 @@ static void userdatum_drop(struct config_group *group, struct config_item *item)
 }
 
 static struct configfs_attribute *userdata_attrs[] = {
+	&sysdata_attr_cpu_nr_enabled,
 	NULL,
 };
 
@@ -1017,6 +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 */
@@ -1117,29 +1251,28 @@ static void send_msg_no_fragmentation(struct netconsole_target *nt,
 				      int msg_len,
 				      int release_len)
 {
-	static char buf[MAX_PRINT_CHUNK]; /* protected by target_list_lock */
-	const char *userdata = NULL;
+	const char *extradata = NULL;
 	const char *release;
 
 #ifdef CONFIG_NETCONSOLE_DYNAMIC
-	userdata = nt->userdata_complete;
+	extradata = nt->extradata_complete;
 #endif
 
 	if (release_len) {
 		release = init_utsname()->release;
 
-		scnprintf(buf, MAX_PRINT_CHUNK, "%s,%s", release, msg);
+		scnprintf(nt->buf, MAX_PRINT_CHUNK, "%s,%s", release, msg);
 		msg_len += release_len;
 	} else {
-		memcpy(buf, msg, msg_len);
+		memcpy(nt->buf, msg, msg_len);
 	}
 
-	if (userdata)
-		msg_len += scnprintf(&buf[msg_len],
+	if (extradata)
+		msg_len += scnprintf(&nt->buf[msg_len],
 				     MAX_PRINT_CHUNK - msg_len,
-				     "%s", userdata);
+				     "%s", extradata);
 
-	send_udp(nt, buf, msg_len);
+	send_udp(nt, nt->buf, msg_len);
 }
 
 static void append_release(char *buf)
@@ -1150,28 +1283,27 @@ static void append_release(char *buf)
 	scnprintf(buf, MAX_PRINT_CHUNK, "%s,", release);
 }
 
-static void send_fragmented_body(struct netconsole_target *nt, 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)
 {
-	const char *userdata = NULL;
+	int sent_extradata, preceding_bytes;
+	const char *extradata = NULL;
 	int body_len, offset = 0;
-	int userdata_len = 0;
 
 #ifdef CONFIG_NETCONSOLE_DYNAMIC
-	userdata = nt->userdata_complete;
-	userdata_len = nt->userdata_length;
+	extradata = nt->extradata_complete;
 #endif
 
 	/* body_len represents the number of bytes that will be sent. This is
 	 * bigger than MAX_PRINT_CHUNK, thus, it will be split in multiple
 	 * packets
 	 */
-	body_len = msgbody_len + userdata_len;
+	body_len = msgbody_len + extradata_len;
 
 	/* In each iteration of the while loop below, we send a packet
 	 * containing the header and a portion of the body. The body is
-	 * composed of two parts: msgbody and userdata. We keep track of how
+	 * composed of two parts: msgbody and extradata. We keep track of how
 	 * many bytes have been sent so far using the offset variable, which
 	 * ranges from 0 to the total length of the body.
 	 */
@@ -1181,7 +1313,7 @@ static void send_fragmented_body(struct netconsole_target *nt, char *buf,
 		int this_offset = 0;
 		int this_chunk = 0;
 
-		this_header += scnprintf(buf + this_header,
+		this_header += scnprintf(nt->buf + this_header,
 					 MAX_PRINT_CHUNK - this_header,
 					 ",ncfrag=%d/%d;", offset,
 					 body_len);
@@ -1192,47 +1324,48 @@ static void send_fragmented_body(struct netconsole_target *nt, char *buf,
 					 MAX_PRINT_CHUNK - this_header);
 			if (WARN_ON_ONCE(this_chunk <= 0))
 				return;
-			memcpy(buf + this_header, msgbody + offset, this_chunk);
+			memcpy(nt->buf + this_header, msgbody + offset,
+			       this_chunk);
 			this_offset += this_chunk;
 		}
 
 		/* msgbody was finally written, either in the previous
 		 * messages and/or in the current buf. Time to write
-		 * the userdata.
+		 * the extradata.
 		 */
 		msgbody_written |= offset + this_offset >= msgbody_len;
 
-		/* Msg body is fully written and there is pending userdata to
-		 * write, append userdata in this chunk
+		/* Msg body is fully written and there is pending extradata to
+		 * write, append extradata in this chunk
 		 */
 		if (msgbody_written && offset + this_offset < body_len) {
 			/* Track how much user data was already sent. First
 			 * time here, sent_userdata is zero
 			 */
-			int sent_userdata = (offset + this_offset) - msgbody_len;
+			sent_extradata = (offset + this_offset) - msgbody_len;
 			/* offset of bytes used in current buf */
-			int preceding_bytes = this_chunk + this_header;
+			preceding_bytes = this_chunk + this_header;
 
-			if (WARN_ON_ONCE(sent_userdata < 0))
+			if (WARN_ON_ONCE(sent_extradata < 0))
 				return;
 
-			this_chunk = min(userdata_len - sent_userdata,
+			this_chunk = min(extradata_len - sent_extradata,
 					 MAX_PRINT_CHUNK - preceding_bytes);
 			if (WARN_ON_ONCE(this_chunk < 0))
 				/* this_chunk could be zero if all the previous
 				 * message used all the buffer. This is not a
-				 * problem, userdata will be sent in the next
+				 * problem, extradata will be sent in the next
 				 * iteration
 				 */
 				return;
 
-			memcpy(buf + this_header + this_offset,
-			       userdata + sent_userdata,
+			memcpy(nt->buf + this_header + this_offset,
+			       extradata + sent_extradata,
 			       this_chunk);
 			this_offset += this_chunk;
 		}
 
-		send_udp(nt, buf, this_header + this_offset);
+		send_udp(nt, nt->buf, this_header + this_offset);
 		offset += this_offset;
 	}
 }
@@ -1240,9 +1373,9 @@ static void send_fragmented_body(struct netconsole_target *nt, char *buf,
 static void send_msg_fragmented(struct netconsole_target *nt,
 				const char *msg,
 				int msg_len,
-				int release_len)
+				int release_len,
+				int extradata_len)
 {
-	static char buf[MAX_PRINT_CHUNK]; /* protected by target_list_lock */
 	int header_len, msgbody_len;
 	const char *msgbody;
 
@@ -1260,16 +1393,17 @@ static void send_msg_fragmented(struct netconsole_target *nt,
 	 * "ncfrag=<byte-offset>/<total-bytes>"
 	 */
 	if (release_len)
-		append_release(buf);
+		append_release(nt->buf);
 
 	/* Copy the header into the buffer */
-	memcpy(buf + release_len, msg, header_len);
+	memcpy(nt->buf + release_len, msg, header_len);
 	header_len += release_len;
 
 	/* for now on, the header will be persisted, and the msgbody
 	 * will be replaced
 	 */
-	send_fragmented_body(nt, buf, msgbody, header_len, msgbody_len);
+	send_fragmented_body(nt, msgbody, header_len, msgbody_len,
+			     extradata_len);
 }
 
 /**
@@ -1285,20 +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 userdata_len = 0;
 	int release_len = 0;
+	int extradata_len;
 
-#ifdef CONFIG_NETCONSOLE_DYNAMIC
-	userdata_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 + userdata_len <= MAX_PRINT_CHUNK)
+	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,
diff --git a/tools/testing/selftests/drivers/net/Makefile b/tools/testing/selftests/drivers/net/Makefile
index c7f1c443f2af..28b6d47f812d 100644
--- a/tools/testing/selftests/drivers/net/Makefile
+++ b/tools/testing/selftests/drivers/net/Makefile
@@ -9,6 +9,7 @@ TEST_PROGS := \
 	netcons_basic.sh \
 	netcons_fragmented_msg.sh \
 	netcons_overflow.sh \
+	netcons_sysdata.sh \
 	ping.py \
 	queues.py \
 	stats.py \
diff --git a/tools/testing/selftests/drivers/net/lib/sh/lib_netcons.sh b/tools/testing/selftests/drivers/net/lib/sh/lib_netcons.sh
index 0c262b123fdd..3c96b022954d 100644
--- a/tools/testing/selftests/drivers/net/lib/sh/lib_netcons.sh
+++ b/tools/testing/selftests/drivers/net/lib/sh/lib_netcons.sh
@@ -230,3 +230,20 @@ function check_for_dependencies() {
 		exit "${ksft_skip}"
 	fi
 }
+
+function check_for_taskset() {
+	if ! which taskset > /dev/null ; then
+		echo "SKIP: taskset(1) is not available" >&2
+		exit "${ksft_skip}"
+	fi
+}
+
+# This is necessary if running multiple tests in a row
+function pkill_socat() {
+	PROCESS_NAME="socat UDP-LISTEN:6666,fork ${OUTPUT_FILE}"
+	# socat runs under timeout(1), kill it if it is still alive
+	# do not fail if socat doesn't exist anymore
+	set +e
+	pkill -f "${PROCESS_NAME}"
+	set -e
+}
diff --git a/tools/testing/selftests/drivers/net/netcons_sysdata.sh b/tools/testing/selftests/drivers/net/netcons_sysdata.sh
new file mode 100755
index 000000000000..2b78fd1f5982
--- /dev/null
+++ b/tools/testing/selftests/drivers/net/netcons_sysdata.sh
@@ -0,0 +1,167 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: GPL-2.0
+
+# A test that makes sure that sysdata runtime CPU data is properly set
+# when a message is sent.
+#
+# There are 3 different tests, every time sent using a random CPU.
+#  - Test #1
+#    * Only enable cpu_nr sysdata feature.
+#  - Test #2
+#    * Keep cpu_nr sysdata feature enable and enable userdata.
+#  - Test #3
+#    * keep userdata enabled, and disable sysdata cpu_nr feature.
+#
+# Author: Breno Leitao <leitao@debian.org>
+
+set -euo pipefail
+
+SCRIPTDIR=$(dirname "$(readlink -e "${BASH_SOURCE[0]}")")
+
+source "${SCRIPTDIR}"/lib/sh/lib_netcons.sh
+
+# Enable the sysdata cpu_nr feature
+function set_cpu_nr() {
+	if [[ ! -f "${NETCONS_PATH}/userdata/cpu_nr_enabled" ]]
+	then
+		echo "Populate CPU configfs path not available in ${NETCONS_PATH}/userdata/cpu_nr_enabled" >&2
+		exit "${ksft_skip}"
+	fi
+
+	echo 1 > "${NETCONS_PATH}/userdata/cpu_nr_enabled"
+}
+
+# Disable the sysdata cpu_nr feature
+function unset_cpu_nr() {
+	echo 0 > "${NETCONS_PATH}/userdata/cpu_nr_enabled"
+}
+
+# Test if MSG content and `cpu=${CPU}` exists in OUTPUT_FILE
+function validate_sysdata_cpu_exists() {
+	# OUTPUT_FILE will contain something like:
+	# 6.11.1-0_fbk0_rc13_509_g30d75cea12f7,13,1822,115075213798,-;netconsole selftest: netcons_gtJHM
+	#  userdatakey=userdatavalue
+	#  cpu=X
+
+	if [ ! -f "$OUTPUT_FILE" ]; then
+		echo "FAIL: File was not generated." >&2
+		exit "${ksft_fail}"
+	fi
+
+	if ! grep -q "${MSG}" "${OUTPUT_FILE}"; then
+		echo "FAIL: ${MSG} not found in ${OUTPUT_FILE}" >&2
+		cat "${OUTPUT_FILE}" >&2
+		exit "${ksft_fail}"
+	fi
+
+	# Check if cpu=XX exists in the file and matches the one used
+	# in taskset(1)
+	if ! grep -q "cpu=${CPU}\+" "${OUTPUT_FILE}"; then
+		echo "FAIL: 'cpu=${CPU}' not found in ${OUTPUT_FILE}" >&2
+		cat "${OUTPUT_FILE}" >&2
+		exit "${ksft_fail}"
+	fi
+
+	rm "${OUTPUT_FILE}"
+	pkill_socat
+}
+
+# Test if MSG content exists in OUTPUT_FILE but no `cpu=` string
+function validate_sysdata_no_cpu() {
+	if [ ! -f "$OUTPUT_FILE" ]; then
+		echo "FAIL: File was not generated." >&2
+		exit "${ksft_fail}"
+	fi
+
+	if ! grep -q "${MSG}" "${OUTPUT_FILE}"; then
+		echo "FAIL: ${MSG} not found in ${OUTPUT_FILE}" >&2
+		cat "${OUTPUT_FILE}" >&2
+		exit "${ksft_fail}"
+	fi
+
+	if grep -q "cpu=" "${OUTPUT_FILE}"; then
+		echo "FAIL: 'cpu=  found in ${OUTPUT_FILE}" >&2
+		cat "${OUTPUT_FILE}" >&2
+		exit "${ksft_fail}"
+	fi
+
+	rm "${OUTPUT_FILE}"
+}
+
+# Start socat, send the message and wait for the file to show up in the file
+# system
+function runtest {
+	# Listen for netconsole port inside the namespace and destination
+	# interface
+	listen_port_and_save_to "${OUTPUT_FILE}" &
+	# Wait for socat to start and listen to the port.
+	wait_local_port_listen "${NAMESPACE}" "${PORT}" udp
+	# Send the message
+	taskset -c "${CPU}" echo "${MSG}: ${TARGET}" > /dev/kmsg
+	# Wait until socat saves the file to disk
+	busywait "${BUSYWAIT_TIMEOUT}" test -s "${OUTPUT_FILE}"
+}
+
+# ========== #
+# Start here #
+# ========== #
+
+modprobe netdevsim 2> /dev/null || true
+modprobe netconsole 2> /dev/null || true
+
+# Check for basic system dependency and exit if not found
+check_for_dependencies
+# This test also depends on taskset(1). Check for it before starting the test
+check_for_taskset
+
+# Set current loglevel to KERN_INFO(6), and default to KERN_NOTICE(5)
+echo "6 5" > /proc/sys/kernel/printk
+# Remove the namespace, interfaces and netconsole target on exit
+trap cleanup EXIT
+# Create one namespace and two interfaces
+set_network
+# Create a dynamic target for netconsole
+create_dynamic_target
+
+#====================================================
+# TEST #1
+# Send message from a random CPU
+#====================================================
+# Random CPU in the system
+CPU=$((RANDOM % $(nproc)))
+OUTPUT_FILE="/tmp/${TARGET}_1"
+MSG="Test #1 from CPU${CPU}"
+# Enable the auto population of cpu_nr
+set_cpu_nr
+runtest
+# Make sure the message was received in the dst part
+# and exit
+validate_sysdata_cpu_exists
+
+#====================================================
+# TEST #2
+# This test now adds userdata together with sysdata
+# ===================================================
+# Get a new random CPU
+CPU=$((RANDOM % $(nproc)))
+OUTPUT_FILE="/tmp/${TARGET}_2"
+MSG="Test #2 from CPU${CPU}"
+set_user_data
+runtest
+validate_sysdata_cpu_exists
+
+# ===================================================
+# TEST #3
+# Unset cpu_nr, so, no CPU should be appended.
+# userdata is still set
+# ===================================================
+CPU=$((RANDOM % $(nproc)))
+OUTPUT_FILE="/tmp/${TARGET}_3"
+MSG="Test #3 from CPU${CPU}"
+# Enable the auto population of cpu_nr
+unset_cpu_nr
+runtest
+# At this time, cpu= shouldn't be present in the msg
+validate_sysdata_no_cpu
+
+exit "${ksft_pass}"