[PATCH RFC net-next v3 4/8] netconsole: Introduce configfs helpers for sysdata features

[Breno Leitao <leitao@debian.org>]

This patch introduces a bitfield to store sysdata features in the
netconsole_target struct. It also adds configfs helpers to enable
or disable the CPU_NR feature, which populates the CPU number in
sysdata.

The patch provides the necessary infrastructure to set or unset the
CPU_NR feature, but does not modify the message itself.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 drivers/net/netconsole.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 1c197da806962a751a3da8794def6a97cff5f701..4cefa43555aada25769b705dd8c8c89964f51a52 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 +113,7 @@ 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_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).
@@ -131,6 +141,8 @@ struct netconsole_target {
 	struct config_group	userdata_group;
 	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;
@@ -398,6 +410,19 @@ 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.
@@ -792,7 +817,62 @@ 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->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,
@@ -852,6 +932,7 @@ static void userdatum_drop(struct config_group *group, struct config_item *item)
 }
 
 static struct configfs_attribute *userdata_attrs[] = {
+	&sysdata_attr_cpu_nr,
 	NULL,
 };
 

-- 
2.43.5