Commit eb83801a authored by Gustavo Luiz Duarte's avatar Gustavo Luiz Duarte Committed by Jakub Kicinski
Browse files

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: default avatarGustavo Luiz Duarte <gustavold@gmail.com>
Reviewed-by: default avatarBreno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20251119-netconsole_dynamic_extradata-v3-3-497ac3191707@meta.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 9dc10f50

drivers/net/netconsole.c

+70 −30
Original line number Diff line number Diff line
@@ -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;

	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;

	int len = 0;



	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]) {

			len += snprintf(NULL, 0, " %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;



		if (child_count >= MAX_USERDATA_ITEMS) {

			spin_unlock_irqrestore(&target_list_lock, flags);

			WARN_ON_ONCE(1);

			return;

	/* 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;

	}

		child_count++;



	/* 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 (strnlen(udm_item->value, MAX_EXTRADATA_VALUE_LEN) == 0)

			continue;



		/* 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);

		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);

}