linux-kernel - Re: [RFC PATCH for-next 4/4] epoll: epoll() syscall definition

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Date:	Tue, 25 Feb 2014 10:21:44 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Nathaniel Yazdani <n1ght.4nd.d4y@...il.com>
Cc:	viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH for-next 4/4] epoll: epoll() syscall definition

Nathaniel Yazdani <n1ght.4nd.d4y@...il.com> wrote:
> + * stores triggered eventpoll entries in the 'out' array. The input array is
> + * _not_ read-only, because the resulting event mask gets written back to each
> + * entry's ->ep_events field. When successful, this will be the same as before
> + * (plus EPOLLERR & EPOLLHUP). If ->ep_events gets cleared, then it is reasonable
> + * to infer that the entry's ->ep_fildes was a bad file descriptor.
> + */

> +	if (!access_ok(VERIFY_WRITE, in, inc * sizeof(struct epoll)))
> +		goto out;
> +	for (i = 0; i < inc; ++i) {
> +		int fd, io;
> +		long long id;
> +
> +		ret = -EFAULT;
> +		if (__get_user(fd, &in[i].ep_fildes) ||
> +		    __get_user(io, &in[i].ep_events) ||
> +		    __get_user(id, &in[i].ep_ident))
> +			goto out;
> +
> +		ep_control(file->private_data, fd, &io, id, 0);
> +		ret = -EFAULT;
> +		if (__put_user(io, &in[i].ep_events))
> +			goto out;

I don't think we should waste cycles writing to 'in' on success.
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