netconsole: Dynamic allocation of userdata buffer

The userdata buffer in struct netconsole_target is currently statically
allocated with a size of MAX_USERDATA_ITEMS * MAX_EXTRADATA_ENTRY_LEN
(16 * 256 = 4096 bytes). This wastes memory when userdata entries are
not used or when only a few entries are configured, which is common in
typical usage scenarios. It also forces us to keep MAX_USERDATA_ITEMS
small to limit the memory wasted.

Change the userdata buffer from a static array to a dynamically
allocated pointer. The buffer is now allocated on-demand in
update_userdata() whenever userdata entries are added, modified, or
removed via configfs. The implementation calculates the exact size
needed for all current userdata entries, allocates a new buffer of that
size, formats the entries into it, and atomically swaps it with the old
buffer.

This approach provides several benefits:
- Memory efficiency: Targets with no userdata use zero bytes instead of
  4KB, and targets with userdata only allocate what they need;
- Scalability: Makes it practical to increase MAX_USERDATA_ITEMS to a
  much larger value without imposing a fixed memory cost on every
  target;
- No hot-path overhead: Allocation occurs during configuration (write to
  configfs), not during message transmission

If memory allocation fails during userdata update, -ENOMEM is returned
to userspace through the configfs attribute write operation.

The sysdata buffer remains statically allocated since it has a smaller
fixed size (MAX_SYSDATA_ITEMS * MAX_EXTRADATA_ENTRY_LEN = 4 * 256 = 1024
bytes) and its content length is less predictable.

Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20251119-netconsole_dynamic_extradata-v3-3-497ac3191707@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/netconsole.c

@@ -155,7 +155,7 @@ struct netconsole_target {
 #ifdef	CONFIG_NETCONSOLE_DYNAMIC
 	struct config_group	group;
 	struct config_group	userdata_group;
-	char			userdata[MAX_EXTRADATA_ENTRY_LEN * MAX_USERDATA_ITEMS];
+	char			*userdata;
 	size_t			userdata_length;
 	char			sysdata[MAX_EXTRADATA_ENTRY_LEN * MAX_SYSDATA_ITEMS];
 
@@ -875,45 +875,77 @@ 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)
+/* Navigate configfs and calculate the lentgh of the formatted string
+ * representing userdata.
+ * Must be called holding netconsole_subsys.su_mutex
+ */
+static int calc_userdata_len(struct netconsole_target *nt)
 {
+	struct userdatum *udm_item;
+	struct config_item *item;
 	struct list_head *entry;
-	int child_count = 0;
+	int len = 0;
-	unsigned long flags;
-
-	spin_lock_irqsave(&target_list_lock, flags);
-
-	/* Clear the current string in case the last userdatum was deleted */
-	nt->userdata_length = 0;
-	nt->userdata[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) {
-			spin_unlock_irqrestore(&target_list_lock, flags);
-			WARN_ON_ONCE(1);
-			return;
-		}
-		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_EXTRADATA_VALUE_LEN) == 0)
+		if (udm_item->value[0]) {
-			continue;
+			len += snprintf(NULL, 0, " %s=%s\n", item->ci_name,
-
+					udm_item->value);
-		/* This doesn't overflow userdata 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
-		 */
-		nt->userdata_length += scnprintf(&nt->userdata[nt->userdata_length],
-						 MAX_EXTRADATA_ENTRY_LEN, " %s=%s\n",
-						 item->ci_name, udm_item->value);
 	}
+	return len;
+}
+
+static int update_userdata(struct netconsole_target *nt)
+{
+	struct userdatum *udm_item;
+	struct config_item *item;
+	struct list_head *entry;
+	char *old_buf = NULL;
+	char *new_buf = NULL;
+	unsigned long flags;
+	int offset = 0;
+	int len;
+
+	/* Calculate required buffer size */
+	len = calc_userdata_len(nt);
+
+	if (WARN_ON_ONCE(len > MAX_EXTRADATA_ENTRY_LEN * MAX_USERDATA_ITEMS))
+		return -ENOSPC;
+
+	/* Allocate new buffer */
+	if (len) {
+		new_buf = kmalloc(len + 1, GFP_KERNEL);
+		if (!new_buf)
+			return -ENOMEM;
+	}
+
+	/* Write userdata to new buffer */
+	list_for_each(entry, &nt->userdata_group.cg_children) {
+		item = container_of(entry, struct config_item, ci_entry);
+		udm_item = to_userdatum(item);
+		/* Skip userdata with no value set */
+		if (udm_item->value[0]) {
+			offset += scnprintf(&new_buf[offset], len + 1 - offset,
+					    " %s=%s\n", item->ci_name,
+					    udm_item->value);
+		}
+	}
+
+	WARN_ON_ONCE(offset != len);
+
+	/* Switch to new buffer and free old buffer */
+	spin_lock_irqsave(&target_list_lock, flags);
+	old_buf = nt->userdata;
+	nt->userdata = new_buf;
+	nt->userdata_length = offset;
 	spin_unlock_irqrestore(&target_list_lock, flags);
+
+	kfree(old_buf);
+
+	return 0;
 }
 
 static ssize_t userdatum_value_store(struct config_item *item, const char *buf,
@@ -937,7 +969,9 @@ static ssize_t userdatum_value_store(struct config_item *item, const char *buf,
 
 	ud = to_userdata(item->ci_parent);
 	nt = userdata_to_target(ud);
-	update_userdata(nt);
+	ret = update_userdata(nt);
+	if (ret < 0)
+		goto out_unlock;
 	ret = count;
 out_unlock:
 	mutex_unlock(&dynamic_netconsole_mutex);
@@ -1193,7 +1227,10 @@ static struct configfs_attribute *netconsole_target_attrs[] = {
 
 static void netconsole_target_release(struct config_item *item)
 {
-	kfree(to_target(item));
+	struct netconsole_target *nt = to_target(item);
+
+	kfree(nt->userdata);
+	kfree(nt);
 }
 
 static struct configfs_item_operations netconsole_target_item_ops = {
@@ -1874,6 +1911,9 @@ static struct netconsole_target *alloc_param_target(char *target_config,
 static void free_param_target(struct netconsole_target *nt)
 {
 	netpoll_cleanup(&nt->np);
+#ifdef	CONFIG_NETCONSOLE_DYNAMIC
+	kfree(nt->userdata);
+#endif
 	kfree(nt);
 }