Message-Id: <20250115-netcon_cpu-v2-3-95971b44dc56@debian.org>
Date: Wed, 15 Jan 2025 05:35:20 -0800
From: Breno Leitao <leitao@...ian.org>
To: Andrew Lunn <andrew+netdev@...n.ch>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
 Simon Horman <horms@...nel.org>, Jonathan Corbet <corbet@....net>, 
 Shuah Khan <shuah@...nel.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 Breno Leitao <leitao@...ian.org>, kernel-team@...a.com, max@...sevol.com, 
 thepacketgeek@...il.com
Subject: [PATCH net-next v2 3/5] 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@...ian.org>
---
 drivers/net/netconsole.c | 131 +++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 128 insertions(+), 3 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 108ec4f836b62860832c601768546c0ecbdb1153..d89b4ef23965ac0a25090d0a1e461de4e56c5fa7 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -97,6 +97,15 @@ 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.
@@ -104,6 +113,8 @@ struct netconsole_target_stats  {
  * @userdata_group:	Links to the userdata configfs hierarchy
  * @extradata_complete:	Cached, formatted string of append
  * @userdata_length:	String length of usedata in extradata_complete.
+ * @sysdata_length:	String length of sysdata 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).
@@ -130,7 +141,9 @@ struct netconsole_target {
 	struct config_group	group;
 	struct config_group	userdata_group;
 	char extradata_complete[MAX_EXTRADATA_ENTRY_LEN * MAX_EXTRADATA_ITEMS];
-	size_t			userdata_length;
+	size_t			userdata_length; /* updated at update_userdata() */
+	size_t			sysdata_length;  /* updated at every message */
+	u32			sysdata_fields;  /* bit-wise with sysdata_feature bits */
 #endif
 	struct netconsole_target_stats stats;
 	bool			enabled;
@@ -396,6 +409,19 @@ 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_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.
@@ -666,7 +692,15 @@ static ssize_t remote_ip_store(struct config_item *item, const char *buf,
  */
 static size_t count_extradata_entries(struct netconsole_target *nt)
 {
-	return list_count_nodes(&nt->userdata_group.cg_children);
+	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,
@@ -790,7 +824,67 @@ static ssize_t userdatum_value_store(struct config_item *item, const char *buf,
 	return ret;
 }
 
+/* disable_sysdata_feature - Disable sysdata feature and clean sysdata
+ * @nt: target that is diabling the feature
+ * @feature: feature being disabled
+ */
+static void disable_sysdata_feature(struct netconsole_target *nt,
+				    enum sysdata_feature feature)
+{
+	nt->sysdata_fields &= ~feature;
+	nt->sysdata_length = 0;
+	/* extradata_complete might have sysdata appended after
+	 * userdata. Clean everything, and it will be re-appened
+	 * when a new message is sent.
+	 */
+	nt->extradata_complete[nt->userdata_length] = 0;
+}
+
+/* configfs helper to sysdata cpu_nr feature */
+static ssize_t sysdata_cpu_nr_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);
 
 static struct configfs_attribute *userdatum_attrs[] = {
 	&userdatum_attr_value,
@@ -850,6 +944,7 @@ static void userdatum_drop(struct config_group *group, struct config_item *item)
 }
 
 static struct configfs_attribute *userdata_attrs[] = {
+	&sysdata_attr_cpu_nr,
 	NULL,
 };
 
@@ -1025,6 +1120,35 @@ static void populate_configfs_item(struct netconsole_target *nt,
 	init_target_config_group(nt, target_name);
 }
 
+/*
+ * append_runtime_sysdata - append sysdata at extradata_complete in runtime
+ * @nt: target to send message to
+ */
+static void append_runtime_sysdata(struct netconsole_target *nt)
+{
+	size_t userdata_len = nt->userdata_length;
+	size_t sysdata_len;
+
+	if (!(nt->sysdata_fields & CPU_NR))
+		return;
+
+	/* Append cpu=%d at extradata_complete after userdata str */
+	sysdata_len = scnprintf(&nt->extradata_complete[userdata_len],
+				MAX_EXTRADATA_ENTRY_LEN, " cpu=%u\n",
+				raw_smp_processor_id());
+
+	WARN_ON_ONCE(userdata_len + sysdata_len >
+		     MAX_EXTRADATA_ENTRY_LEN * MAX_EXTRADATA_ITEMS);
+
+	/* nt->sysdata_length will be used later to decide if the message
+	 * needs to be fragmented.
+	 * userdata_len cannot be used for it, once next sysdata append should
+	 * start from the same userdata_len location, and only overwrite old
+	 * sysdata.
+	 */
+	nt->sysdata_length = sysdata_len;
+}
+
 #endif	/* CONFIG_NETCONSOLE_DYNAMIC */
 
 /* Handle network interface device notifications */
@@ -1297,7 +1421,8 @@ static void send_ext_msg_udp(struct netconsole_target *nt, const char *msg,
 	int release_len = 0;
 
 #ifdef CONFIG_NETCONSOLE_DYNAMIC
-	extradata_len = nt->userdata_length;
+	append_runtime_sysdata(nt);
+	extradata_len = nt->userdata_length + nt->sysdata_length;
 #endif
 
 	if (nt->release)

-- 
2.43.5

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