Re: [PATCH net-next v3 3/3] eventpoll: Add epoll ioctl for epoll_params

[Greg Kroah-Hartman]

On Thu, Jan 25, 2024 at 04:11:28PM -0800, Joe Damato wrote:
> On Thu, Jan 25, 2024 at 03:21:46PM -0800, Greg Kroah-Hartman wrote:
> > On Thu, Jan 25, 2024 at 10:56:59PM +0000, Joe Damato wrote:
> > > +struct epoll_params {
> > > +	u64 busy_poll_usecs;
> > > +	u16 busy_poll_budget;
> > > +
> > > +	/* for future fields */
> > > +	u8 data[118];
> > > +} EPOLL_PACKED;
> > 
> > variables that cross the user/kernel boundry need to be __u64, __u16,
> > and __u8 here.
> 
> I'll make that change for the next version, thank you.
> 
> > And why 118?
> 
> I chose this arbitrarily. I figured that a 128 byte struct would support 16
> u64s in the event that other fields needed to be added in the future. 118
> is what was left after the existing fields. There's almost certainly a
> better way to do this - or perhaps it is unnecessary as per your other
> message.
> 
> I am not sure if leaving extra space in the struct is a recommended
> practice for ioctls or not - I thought I noticed some code that did and
> some that didn't in the kernel so I err'd on the side of leaving the space
> and probably did it in the worst way possible.

It's not really a good idea unless you know exactly what you are going
to do with it.  Why not just have a new ioctl if you need new
information in the future?  That's simpler, right?

thanks,

greg k-h