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netconsole: Dynamic allocation of userdata buffer

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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>
This commit is contained in:
Gustavo Luiz Duarte 2025-11-19 16:14:51 -08:00 committed by Jakub Kicinski
parent 9dc10f50c4
commit eb83801af2
1 changed files with 73 additions and 33 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

106
drivers/net/netconsole.c
View File

@ -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) {
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)
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]) {
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;
/* 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);
}