net: netconsole: cache userdata formatted string in netconsole_target

Store a formatted string for userdata that will be appended to netconsole
messages. The string has a capacity of 4KB, as calculated by the userdatum
entry length of 256 bytes and a max of 16 userdata entries.

Update the stored netconsole_target->userdata_complete string with the new
formatted userdata values when a userdatum is created, edited, or
removed. Each userdata entry contains a trailing newline, which will be
formatted as such in netconsole messages::

    6.7.0-rc8-virtme,12,500,1646292204,-;test
    release=foo
    something=bar
    6.7.0-rc8-virtme,12,500,1646292204,-;another test
    release=foo
    something=bar

Enforcement of MAX_USERDATA_ITEMS is done in userdatum_make_item;
update_userdata will not check for this case but will skip any userdata
children over the limit of MAX_USERDATA_ITEMs.

If a userdata entry/dir is created but no value is provided, that entry
will be skipped. This is in part because update_userdata() can't be
called in userdatum_make_item() since the item will not have been added
to the userdata config_group children yet. To preserve the experience of
adding an empty userdata that doesn't show up in the netconsole
messages, purposefully skip empty userdata items even when
update_userdata() can be called.
Co-developed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Matthew Wood <thepacketgeek@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/netconsole.c

@@ -85,6 +85,8 @@ static struct console netconsole_ext;
  * @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
  * @enabled:	On / off knob to enable / disable target.
  *		Visible from userspace (read-write).
  *		We maintain a strict 1:1 correspondence between this and
@@ -109,6 +111,8 @@ struct netconsole_target {
 #ifdef	CONFIG_NETCONSOLE_DYNAMIC
 	struct config_group	group;
 	struct config_group	userdata_group;
+	char userdata_complete[MAX_USERDATA_ENTRY_LENGTH * MAX_USERDATA_ITEMS];
+	size_t			userdata_length;
 #endif
 	bool			enabled;
 	bool			extended;
@@ -638,10 +642,48 @@ static ssize_t userdatum_value_show(struct config_item *item, char *buf)
 	return sysfs_emit(buf, "%s\n", &(to_userdatum(item)->value[0]));
 }
 
+static void update_userdata(struct netconsole_target *nt)
+{
+	int complete_idx = 0, child_count = 0;
+	struct list_head *entry;
+
+	/* Clear the current string in case the last userdatum was deleted */
+	nt->userdata_length = 0;
+	nt->userdata_complete[0] = 0;
+
+	list_for_each(entry, &nt->userdata_group.cg_children) {
+		struct userdatum *udm_item;
+		struct config_item *item;
+
+		if (child_count >= MAX_USERDATA_ITEMS)
+			break;
+		child_count++;
+
+		item = container_of(entry, struct config_item, ci_entry);
+		udm_item = to_userdatum(item);
+
+		/* Skip userdata with no value set */
+		if (strnlen(udm_item->value, MAX_USERDATA_VALUE_LENGTH) == 0)
+			continue;
+
+		/* This doesn't overflow userdata_complete since it will write
+		 * one entry length (1/MAX_USERDATA_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",
+					  item->ci_name, udm_item->value);
+	}
+	nt->userdata_length = strnlen(nt->userdata_complete,
+				      sizeof(nt->userdata_complete));
+}
+
 static ssize_t userdatum_value_store(struct config_item *item, const char *buf,
 				     size_t count)
 {
 	struct userdatum *udm = to_userdatum(item);
+	struct netconsole_target *nt;
+	struct userdata *ud;
 	int ret;
 
 	if (count > MAX_USERDATA_VALUE_LENGTH)
@@ -654,6 +696,10 @@ static ssize_t userdatum_value_store(struct config_item *item, const char *buf,
 		goto out_unlock;
 	trim_newline(udm->value, sizeof(udm->value));
 
+	ud = to_userdata(item->ci_parent);
+	nt = userdata_to_target(ud);
+	update_userdata(nt);
+
 	mutex_unlock(&dynamic_netconsole_mutex);
 	return count;
 out_unlock:
@@ -708,12 +754,27 @@ static struct config_item *userdatum_make_item(struct config_group *group,
 	return &udm->item;
 }
 
+static void userdatum_drop(struct config_group *group, struct config_item *item)
+{
+	struct netconsole_target *nt;
+	struct userdata *ud;
+
+	ud = to_userdata(&group->cg_item);
+	nt = userdata_to_target(ud);
+
+	mutex_lock(&dynamic_netconsole_mutex);
+	update_userdata(nt);
+	config_item_put(item);
+	mutex_unlock(&dynamic_netconsole_mutex);
+}
+
 static struct configfs_attribute *userdata_attrs[] = {
 	NULL,
 };
 
 static struct configfs_group_operations userdata_ops = {
 	.make_item		= userdatum_make_item,
+	.drop_item		= userdatum_drop,
 };
 
 static struct config_item_type userdata_type = {