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Date:   Fri, 1 May 2020 18:43:06 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] eventfd: convert to f_op->read_iter()

On Fri, May 01, 2020 at 11:18:05AM -0600, Jens Axboe wrote:

> -	if (res > 0 && put_user(ucnt, (__u64 __user *)buf))
> +	if (res > 0 && copy_to_iter(&ucnt, res, iov) < res)

*whoa*

It is correct, but only because here res > 0 <=> res == 8.
And that's not trivial at the first glance.

Please, turn that into something like

	if (iov_iter_count(to) < sizeof(ucnt))
		return -EINVAL;
	spin_lock_irq(&ctx->wqh.lock);
	if (!ctx->count) {
		if (unlikely(file->f_flags & O_NONBLOCK) {
			spin_unlock_irq(&ctx->wqh.lock)
			return -EAGAIN;
		}
		__add_wait_queue(&ctx->wqh, &wait);
		for (;;) {
			set_current_state(TASK_INTERRUPTIBLE);
			if (ctx->count)
				break;
			if (signal_pending(current)) {
				spin_unlock_irq(&ctx->wqh.lock)
				return -ERESTARTSYS;
			}
			spin_unlock_irq(&ctx->wqh.lock);
			schedule();
			spin_lock_irq(&ctx->wqh.lock);
		}
		__remove_wait_queue(&ctx->wqh, &wait);
		__set_current_state(TASK_RUNNING);
	}
	eventfd_ctx_do_read(ctx, &ucnt);
	if (waitqueue_active(&ctx->wqh))
		wake_up_locked_poll(&ctx->wqh, EPOLLOUT);
	spin_unlock_irq(&ctx->wqh.lock);
	if (unlikely(copy_to_iter(&ucnt, sizeof(ucnt), to) != sizeof(ucnt)))
		return -EFAULT;
	return sizeof(ucnt);

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