Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put_uevent().

[Dave Hansen <dave.hansen@intel.com>]

On 10/22/25 17:04, Kuniyuki Iwashima wrote:
> --- a/include/linux/eventpoll.h
> +++ b/include/linux/eventpoll.h
> @@ -82,11 +82,14 @@ static inline struct epoll_event __user *
>  epoll_put_uevent(__poll_t revents, __u64 data,
>  		 struct epoll_event __user *uevent)
>  {
> -	if (__put_user(revents, &uevent->events) ||
> -	    __put_user(data, &uevent->data))
> -		return NULL;
> -
> -	return uevent+1;
> +	__user_write_access_begin(uevent, sizeof(*uevent));
> +	unsafe_put_user(revents, &uevent->events, efault);
> +	unsafe_put_user(data, &uevent->data, efault);
> +	user_access_end();
> +	return uevent + 1;
> +efault:
> +	user_access_end();
> +	return NULL;
>  }
>  #endif

This makes me nervous. The access_ok() check is quite a distance away.
I'd kinda want to see some performance numbers before doing this. Is
removing a single access_ok() even measurable?

Also, even if we go do this, shouldn't __user_write_access_begin() be
called something more like unsafe_user_write_access_begin()?