Re: [PATCH] kobject_uevent: add uevent_helper exist check

[Andrew Morton <akpm@linux-foundation.org>]

On Sat,  5 Apr 2025 01:31:02 +0800 zhoumin <teczm@foxmail.com> wrote:

> The kernel creates uevent_helper process for every uevent sent,
> even if the uevent_helper does not exist. Before the rootfs is
> mounted, a large number of events are generated. This change
> introduces uevent_helper existence check to prevent unnecessary
> operations.
> 
> Logs a debug messase before call_usermodehelper_setup.
> e.g.: pr_emerg("action:%s devpath:%s\n", action_string, devpath);
> You will see a large number of uevent_helper processes
> are attempted to be created before the rootfs is mounted.

Is there any measurable reduction in boot time?

> ...
>
> --- a/lib/kobject_uevent.c
> +++ b/lib/kobject_uevent.c
> @@ -28,6 +28,7 @@
>  #include <net/sock.h>
>  #include <net/netlink.h>
>  #include <net/net_namespace.h>
> +#include <linux/namei.h>
>  
>  
>  atomic64_t uevent_seqnum;
> @@ -58,6 +59,23 @@ static const char *kobject_actions[] = {
>  	[KOBJ_UNBIND] =		"unbind",
>  };
>  
> +#ifdef CONFIG_UEVENT_HELPER
> +static int uevent_helper_lookup(void)
> +{
> +	static int ret = -ENOENT;
> +	struct path path;
> +
> +	if (!ret)
> +		return ret;

Cached in a static variable.  So if a uevent helper later becomes
available, we won't know that and a reboot will be needed?

> +
> +	ret = kern_path(uevent_helper, LOOKUP_FOLLOW, &path);

I wonder if this is the official/preferred way of detecting the
presence of a file.

> +	if (!ret)
> +		path_put(&path);
> +
> +	return ret;
> +}
> +#endif
> +
>  static int kobject_action_type(const char *buf, size_t count,
>  			       enum kobject_action *type,
>  			       const char **args)
> @@ -610,7 +628,7 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
>  
>  #ifdef CONFIG_UEVENT_HELPER
>  	/* call uevent_helper, usually only enabled during early boot */
> -	if (uevent_helper[0] && !kobj_usermode_filter(kobj)) {
> +	if (uevent_helper[0] && !uevent_helper_lookup() && !kobj_usermode_filter(kobj)) {
>  		struct subprocess_info *info;
>  
>  		retval = add_uevent_var(env, "HOME=/");
> -- 
> 2.43.0