Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

[Al Viro]

On Thu, Jan 04, 2018 at 03:10:51AM +0000, Williams, Dan J wrote:

> diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
> index 1c65817673db..dbc12007da51 100644
> --- a/include/linux/fdtable.h
> +++ b/include/linux/fdtable.h
> @@ -82,8 +82,10 @@ static inline struct file *__fcheck_files(struct files_struct *files, unsigned i
>  {
>  	struct fdtable *fdt = rcu_dereference_raw(files->fdt);
>  
> -	if (fd < fdt->max_fds)
> +	if (fd < fdt->max_fds) {
> +		osb();
>  		return rcu_dereference_raw(fdt->fd[fd]);
> +	}
>  	return NULL;
>  }

... and the point of that would be?  Possibly revealing the value of files->fdt?
Why would that be a threat, assuming you manage to extract the information in
question in the first place?