kobject: make uevent_seqnum atomic

We will soon no longer acquire uevent_sock_mutex
for most kobject_uevent_net_broadcast() calls,
and also while calling uevent_net_broadcast().

Make uevent_seqnum an atomic64_t to get its own protection.

This fixes a race while reading /sys/kernel/uevent_seqnum.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Christian Brauner <brauner@kernel.org>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20240214084829.684541-2-edumazet@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/kobject.h

@@ -38,7 +38,7 @@ extern char uevent_helper[];
 #endif
 
 /* counter to tag the uevent, read only except for the kobject core */
-extern u64 uevent_seqnum;
+extern atomic64_t uevent_seqnum;
 
 /*
  * The actions here must match the index to the string array

kernel/ksysfs.c

@@ -39,7 +39,7 @@ static struct kobj_attribute _name##_attr = __ATTR_RW(_name)
 static ssize_t uevent_seqnum_show(struct kobject *kobj,
 				  struct kobj_attribute *attr, char *buf)
 {
-	return sysfs_emit(buf, "%llu\n", (unsigned long long)uevent_seqnum);
+	return sysfs_emit(buf, "%llu\n", (u64)atomic64_read(&uevent_seqnum));
 }
 KERNEL_ATTR_RO(uevent_seqnum);
 

lib/kobject_uevent.c

@@ -30,7 +30,7 @@
 #include <net/net_namespace.h>
 
 
-u64 uevent_seqnum;
+atomic64_t uevent_seqnum;
 #ifdef CONFIG_UEVENT_HELPER
 char uevent_helper[UEVENT_HELPER_PATH_LEN] = CONFIG_UEVENT_HELPER_PATH;
 #endif
@@ -44,7 +44,7 @@ struct uevent_sock {
 static LIST_HEAD(uevent_sock_list);
 #endif
 
-/* This lock protects uevent_seqnum and uevent_sock_list */
+/* This lock protects uevent_sock_list */
 static DEFINE_MUTEX(uevent_sock_mutex);
 
 /* the strings here must match the enum in include/linux/kobject.h */
@@ -583,13 +583,13 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
 		break;
 	}
 
-	mutex_lock(&uevent_sock_mutex);
 	/* we will send an event, so request a new sequence number */
-	retval = add_uevent_var(env, "SEQNUM=%llu", ++uevent_seqnum);
-	if (retval) {
-		mutex_unlock(&uevent_sock_mutex);
+	retval = add_uevent_var(env, "SEQNUM=%llu",
+				atomic64_inc_return(&uevent_seqnum));
+	if (retval)
 		goto exit;
-	}
+
+	mutex_lock(&uevent_sock_mutex);
 	retval = kobject_uevent_net_broadcast(kobj, env, action_string,
 					      devpath);
 	mutex_unlock(&uevent_sock_mutex);
@@ -688,7 +688,8 @@ static int uevent_net_broadcast(struct sock *usk, struct sk_buff *skb,
 	int ret;
 
 	/* bump and prepare sequence number */
-	ret = snprintf(buf, sizeof(buf), "SEQNUM=%llu", ++uevent_seqnum);
+	ret = snprintf(buf, sizeof(buf), "SEQNUM=%llu",
+		       atomic64_inc_return(&uevent_seqnum));
 	if (ret < 0 || (size_t)ret >= sizeof(buf))
 		return -ENOMEM;
 	ret++;