[PATCH net-next 1/2] netconsole: avoid printing partially updated target attributes

[Gustavo Luiz Duarte <gustavold@gmail.com>]

The configfs store callbacks all serialize on dynamic_netconsole_mutex
but not on the read side, so reading an attribute while it is being
written returns a partially updated value.

Hold dynamic_netconsole_mutex on *_show() callbacks to avoid racing with
writers.

The dev_name_show() callback can also race with
netconsole_netdev_event() writing to np.dev_name due to
NETDEV_CHANGENAME. So it needs to hold RTNL in addition to
dynamic_netconsole_mutex.

Reported-by: Sashiko <netdev-bot+sashiko@kernel.org>
Closes: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260814-netcons_ipv6-v3-7-bc0915e8c75f@gmail.com
Closes: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260928-netcons-fixes-v1-0-bb5ffe5e698a%40gmail.com
Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
---
 drivers/net/netconsole.c | 62 +++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 51 insertions(+), 11 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 267254f046de..188beacb308d 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -859,7 +859,19 @@ static ssize_t release_show(struct config_item *item, char *buf)
 
 static ssize_t dev_name_show(struct config_item *item, char *buf)
 {
-	return sysfs_emit(buf, "%s\n", to_target(item)->np.dev_name);
+	struct netconsole_target *nt = to_target(item);
+	int ret;
+
+	dynamic_netconsole_mutex_lock();
+	/* Hold RTNL to prevent racing against netconsole_netdev_event()
+	 * changing np.dev_name.
+	 */
+	rtnl_lock();
+	ret = sysfs_emit(buf, "%s\n", nt->np.dev_name);
+	rtnl_unlock();
+	dynamic_netconsole_mutex_unlock();
+
+	return ret;
 }
 
 static ssize_t local_port_show(struct config_item *item, char *buf)
@@ -875,25 +887,39 @@ static ssize_t remote_port_show(struct config_item *item, char *buf)
 static ssize_t local_ip_show(struct config_item *item, char *buf)
 {
 	struct netconsole_target *nt = to_target(item);
+	int ret;
+
+	dynamic_netconsole_mutex_lock();
 
 	if (nt->local_ip.family == AF_UNSPEC)
-		return sysfs_emit(buf, "\n");
-	if (nt->local_ip.family == AF_INET6)
-		return sysfs_emit(buf, "%pI6c\n", &nt->local_ip.in6);
+		ret = sysfs_emit(buf, "\n");
+	else if (nt->local_ip.family == AF_INET6)
+		ret = sysfs_emit(buf, "%pI6c\n", &nt->local_ip.in6);
 	else
-		return sysfs_emit(buf, "%pI4\n", &nt->local_ip.ip);
+		ret = sysfs_emit(buf, "%pI4\n", &nt->local_ip.ip);
+
+	dynamic_netconsole_mutex_unlock();
+
+	return ret;
 }
 
 static ssize_t remote_ip_show(struct config_item *item, char *buf)
 {
 	struct netconsole_target *nt = to_target(item);
+	int ret;
+
+	dynamic_netconsole_mutex_lock();
 
 	if (nt->remote_ip.family == AF_UNSPEC)
-		return sysfs_emit(buf, "\n");
-	if (nt->remote_ip.family == AF_INET6)
-		return sysfs_emit(buf, "%pI6c\n", &nt->remote_ip.in6);
+		ret = sysfs_emit(buf, "\n");
+	else if (nt->remote_ip.family == AF_INET6)
+		ret = sysfs_emit(buf, "%pI6c\n", &nt->remote_ip.in6);
 	else
-		return sysfs_emit(buf, "%pI4\n", &nt->remote_ip.ip);
+		ret = sysfs_emit(buf, "%pI4\n", &nt->remote_ip.ip);
+
+	dynamic_netconsole_mutex_unlock();
+
+	return ret;
 }
 
 static ssize_t local_mac_show(struct config_item *item, char *buf)
@@ -906,7 +932,14 @@ static ssize_t local_mac_show(struct config_item *item, char *buf)
 
 static ssize_t remote_mac_show(struct config_item *item, char *buf)
 {
-	return sysfs_emit(buf, "%pM\n", to_target(item)->remote_mac);
+	struct netconsole_target *nt = to_target(item);
+	int ret;
+
+	dynamic_netconsole_mutex_lock();
+	ret = sysfs_emit(buf, "%pM\n", nt->remote_mac);
+	dynamic_netconsole_mutex_unlock();
+
+	return ret;
 }
 
 static ssize_t transmit_errors_show(struct config_item *item, char *buf)
@@ -1342,7 +1375,14 @@ static struct netconsole_target *userdata_to_target(struct userdata *ud)
 
 static ssize_t userdatum_value_show(struct config_item *item, char *buf)
 {
-	return sysfs_emit(buf, "%s\n", &(to_userdatum(item)->value[0]));
+	struct userdatum *udm = to_userdatum(item);
+	int ret;
+
+	dynamic_netconsole_mutex_lock();
+	ret = sysfs_emit(buf, "%s\n", udm->value);
+	dynamic_netconsole_mutex_unlock();
+
+	return ret;
 }
 
 /* Navigate configfs and calculate the lentgh of the formatted string

-- 
2.55.0