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netconsole: publish the userdata payload with RCU

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update_userdata() takes target_list_lock to swap nt->userdata and
nt->userdata_length, then frees the old buffer. Since commit
7eab73b186 ("netconsole: convert to NBCON console infrastructure")
that lock is also the console's device_lock, so writing a userdata value
from configfs serialises against the printk core emitting messages.

The buffer is immutable once published, which is what RCU is for. Move
the string and its length into a single netcons_userdata object and
publish it with rcu_replace_pointer(), freeing the old one with
kfree_rcu().

New userdata design:

0) Unify the userdata fields into a struct netcons_userdata
1) update_userdata() no longer needs target_list_lock.
2) writers stay serialised by dynamic_netconsole_mutex.
3) reading userdata needs an RCU read lock.

No functional change intended.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Gustavo Luiz Duarte <gustavold@gmail.com>
Link: https://patch.msgid.link/20260810-netcons-userdata-rcu-v3-1-f65557f769ce@debian.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Breno Leitao
2026-08-10 02:36:11 -07:00
committed by Paolo Abeni
parent 885a48b521
commit a9560343d4
1 changed files with 56 additions and 36 deletions
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92
drivers/net/netconsole.c
View File

@@ -151,13 +151,27 @@ enum target_state {
STATE_DEACTIVATED,
};
/**
* struct netcons_userdata - Formatted userdata payload of a target.
* @rcu: Used to free the payload after a grace period.
* @length: Length of @data, excluding the NUL terminator.
* @data: Formatted " key=value\n" entries, NUL terminated.
*
* Immutable once published, so the transmit path never observes @data and
* @length disagreeing.
*/
struct netcons_userdata {
struct rcu_head rcu;
size_t length;
char data[];
};
/**
* struct netconsole_target - Represents a configured netconsole target.
* @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: Cached, formatted string of append
* @userdata_length: String length of userdata.
* @userdata: Cached, formatted userdata payload. RCU protected.
* @sysdata: Cached, formatted string of append
* @sysdata_fields: Sysdata features enabled.
* @msgcounter: Message sent counter.
@@ -198,8 +212,7 @@ struct netconsole_target {
#ifdef CONFIG_NETCONSOLE_DYNAMIC
struct config_group group;
struct config_group userdata_group;
char *userdata;
size_t userdata_length;
struct netcons_userdata __rcu *userdata;
char sysdata[MAX_EXTRADATA_ENTRY_LEN * MAX_SYSDATA_ITEMS];
/* bit-wise with sysdata_feature bits */
@@ -1351,12 +1364,11 @@ static int calc_userdata_len(struct netconsole_target *nt)
static int update_userdata(struct netconsole_target *nt)
{
struct netcons_userdata *new = NULL;
struct netcons_userdata *old;
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;
@@ -1368,8 +1380,8 @@ static int update_userdata(struct netconsole_target *nt)
/* Allocate new buffer */
if (len) {
new_buf = kmalloc(len + 1, GFP_KERNEL);
if (!new_buf)
new = kmalloc_flex(*new, data, len + 1);
if (!new)
return -ENOMEM;
}
@@ -1379,22 +1391,21 @@ static int update_userdata(struct netconsole_target *nt)
udm_item = to_userdatum(item);
/* Skip userdata with no value set */
if (udm_item->value[0]) {
offset += scnprintf(&new_buf[offset], len + 1 - offset,
offset += scnprintf(&new->data[offset],
len + 1 - offset,
" %s=%s\n", item->ci_name,
udm_item->value);
}
}
WARN_ON_ONCE(offset != len);
if (new)
new->length = offset;
/* 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);
/* Writers are serialized by dynamic_netconsole_mutex. */
old = rcu_replace_pointer(nt->userdata, new,
lockdep_is_held(&dynamic_netconsole_mutex));
kfree_rcu(old, rcu);
return 0;
}
@@ -1684,7 +1695,7 @@ static void netconsole_target_release(struct config_item *item)
{
struct netconsole_target *nt = to_target(item);
kfree(nt->userdata);
kfree(rcu_access_pointer(nt->userdata));
kfree(nt);
}
@@ -2227,14 +2238,13 @@ static void send_udp(struct netconsole_target *nt, const char *msg, int len)
static void send_msg_no_fragmentation(struct netconsole_target *nt,
const char *msg,
int msg_len,
int release_len)
int release_len,
const struct netcons_userdata *userdata)
{
const char *userdata = NULL;
const char *sysdata = NULL;
const char *release;
#ifdef CONFIG_NETCONSOLE_DYNAMIC
userdata = nt->userdata;
sysdata = nt->sysdata;
#endif
@@ -2251,7 +2261,7 @@ static void send_msg_no_fragmentation(struct netconsole_target *nt,
if (userdata)
msg_len += scnprintf(&nt->buf[msg_len],
sizeof(nt->buf) - msg_len, "%s",
userdata);
userdata->data);
if (sysdata)
msg_len += scnprintf(&nt->buf[msg_len],
@@ -2271,7 +2281,8 @@ static void append_release(char *buf)
static void send_fragmented_body(struct netconsole_target *nt,
const char *msgbody_ptr, int header_len,
int msgbody_len, int sysdata_len)
int msgbody_len, int sysdata_len,
const struct netcons_userdata *userdata)
{
const char *userdata_ptr = NULL;
const char *sysdata_ptr = NULL;
@@ -2282,12 +2293,12 @@ static void send_fragmented_body(struct netconsole_target *nt,
int userdata_len = 0;
#ifdef CONFIG_NETCONSOLE_DYNAMIC
userdata_ptr = nt->userdata;
sysdata_ptr = nt->sysdata;
userdata_len = nt->userdata_length;
#endif
if (WARN_ON_ONCE(!userdata_ptr && userdata_len != 0))
return;
if (userdata) {
userdata_ptr = userdata->data;
userdata_len = userdata->length;
}
if (WARN_ON_ONCE(!sysdata_ptr && sysdata_len != 0))
return;
@@ -2364,7 +2375,8 @@ static void send_msg_fragmented(struct netconsole_target *nt,
const char *msg,
int msg_len,
int release_len,
int sysdata_len)
int sysdata_len,
const struct netcons_userdata *userdata)
{
int header_len, msgbody_len;
const char *msgbody;
@@ -2393,7 +2405,7 @@ static void send_msg_fragmented(struct netconsole_target *nt,
* will be replaced
*/
send_fragmented_body(nt, msgbody, header_len, msgbody_len,
sysdata_len);
sysdata_len, userdata);
}
/**
@@ -2408,25 +2420,33 @@ static void send_msg_fragmented(struct netconsole_target *nt,
static void send_ext_msg_udp(struct netconsole_target *nt,
struct nbcon_write_context *wctxt)
{
const struct netcons_userdata *userdata = NULL;
int userdata_len = 0;
int release_len = 0;
int sysdata_len = 0;
int len;
/* Keeps the payload picked below alive until the last send_udp(). */
rcu_read_lock();
#ifdef CONFIG_NETCONSOLE_DYNAMIC
sysdata_len = prepare_sysdata(nt, wctxt);
userdata_len = nt->userdata_length;
userdata = rcu_dereference(nt->userdata);
if (userdata)
userdata_len = userdata->length;
#endif
if (nt->release)
release_len = strlen(init_utsname()->release) + 1;
len = wctxt->len + release_len + sysdata_len + userdata_len;
if (len <= MAX_PRINT_CHUNK)
return send_msg_no_fragmentation(nt, wctxt->outbuf,
wctxt->len, release_len);
send_msg_no_fragmentation(nt, wctxt->outbuf, wctxt->len,
release_len, userdata);
else
send_msg_fragmented(nt, wctxt->outbuf, wctxt->len, release_len,
sysdata_len, userdata);
return send_msg_fragmented(nt, wctxt->outbuf, wctxt->len, release_len,
sysdata_len);
rcu_read_unlock();
}
static void send_msg_udp(struct netconsole_target *nt, const char *msg,
@@ -2669,7 +2689,7 @@ static void free_param_target(struct netconsole_target *nt)
netconsole_skb_pool_flush(nt);
netpoll_cleanup(&nt->np);
#ifdef CONFIG_NETCONSOLE_DYNAMIC
kfree(nt->userdata);
kfree(rcu_access_pointer(nt->userdata));
#endif
kfree(nt);
}