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Date:   Thu, 18 Aug 2022 08:26:38 +0200
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Axel Rasmussen <axelrasmussen@google.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Dmitry V . Levin" <ldv@altlinux.org>,
        Gleb Fotengauer-Malinovskiy <glebfm@altlinux.org>,
        Hugh Dickins <hughd@google.com>, Jan Kara <jack@suse.cz>,
        Jonathan Corbet <corbet@lwn.net>,
        Mel Gorman <mgorman@techsingularity.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>, Nadav Amit <namit@vmware.com>,
        Peter Xu <peterx@redhat.com>, Shuah Khan <shuah@kernel.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        zhangyi <yi.zhang@huawei.com>, linux-doc@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org, linux-mm@kvack.org,
        linux-security-module@vger.kernel.org,
        Mike Rapoport <rppt@linux.ibm.com>
Subject: Re: [PATCH v6 2/5] userfaultfd: add /dev/userfaultfd for fine
 grained access control
Message-ID: <Yv3bnouKb7242Ama@kroah.com>
References: <20220817214728.489904-1-axelrasmussen@google.com>
 <20220817214728.489904-3-axelrasmussen@google.com>
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On Wed, Aug 17, 2022 at 02:47:25PM -0700, Axel Rasmussen wrote:
> +static int userfaultfd_dev_open(struct inode *inode, struct file *file)
> +{
> +	return 0;

If your open does nothing, no need to list it here at all, right?

> +}
> +
> +static long userfaultfd_dev_ioctl(struct file *file, unsigned int cmd, unsigned long flags)
> +{
> +	if (cmd != USERFAULTFD_IOC_NEW)
> +		return -EINVAL;
> +
> +	return new_userfaultfd(flags);
> +}
> +
> +static const struct file_operations userfaultfd_dev_fops = {
> +	.open = userfaultfd_dev_open,
> +	.unlocked_ioctl = userfaultfd_dev_ioctl,
> +	.compat_ioctl = userfaultfd_dev_ioctl,

Why do you need to set compat_ioctl?  Shouldn't it just default to the
existing one?

And why is this a device node at all?  Shouldn't the syscall handle all
of this (to be honest, I didn't read anything but the misc code, sorry.)

thanks,

greg k-h