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

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Date: Thu, 25 Jan 2024 16:23:58 -0800
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Joe Damato <jdamato@...tly.com>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	chuck.lever@...cle.com, jlayton@...nel.org,
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	Jonathan Corbet <corbet@....net>,
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nathan Lynch <nathanl@...ux.ibm.com>,
	Steve French <stfrench@...rosoft.com>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Jiri Slaby <jirislaby@...nel.org>,
	Julien Panis <jpanis@...libre.com>, Arnd Bergmann <arnd@...db.de>,
	Andrew Waterman <waterman@...s.berkeley.edu>,
	Thomas Huth <thuth@...hat.com>, Palmer Dabbelt <palmer@...belt.com>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	"open list:FILESYSTEMS (VFS and infrastructure)" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH net-next v3 3/3] eventpoll: Add epoll ioctl for
 epoll_params

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