Re: [PATCH v4 4/9] cxl/mem: Add basic IOCTL interface

[Al Viro]

On Mon, Feb 15, 2021 at 05:45:33PM -0800, Ben Widawsky wrote:
> +	if (cmd->info.size_in) {
> +		mbox_cmd.payload_in = kvzalloc(cmd->info.size_in, GFP_KERNEL);
> +		if (!mbox_cmd.payload_in) {
> +			rc = -ENOMEM;
> +			goto out;
> +		}
> +
> +		if (copy_from_user(mbox_cmd.payload_in,
> +				   u64_to_user_ptr(in_payload),
> +				   cmd->info.size_in)) {
> +			rc = -EFAULT;
> +			goto out;
> +		}

Umm...  Do you need to open-code vmemdup_user()?  The only difference is
GFP_KERNEL allocation instead of GFP_USER one, and the latter is arguably
saner here...  Zeroing is definitely pointless - you either overwrite
the entire buffer with copy_from_user(), or you fail and free the damn
thing.