Re: [PATCH v6 2/5] userfaultfd: add /dev/userfaultfd for fine grained access control

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Aug 18, 2022 at 08:26:38AM +0200, Greg KH wrote:
> 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.)

Ah, read the documentation now.  Seems you want to make it easier for
people to get permissions on a system.  Doesn't seem wise, but hey, it's
not my feature...

thanks,

greg k-h