Received-SPF: pass (google.com: domain of linux-kernel+bounces-39457-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Date: Thu, 25 Jan 2024 16:23:58 -0800
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Joe Damato <jdamato@fastly.com>
Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	chuck.lever@oracle.com, jlayton@kernel.org,
	linux-api@vger.kernel.org, brauner@kernel.org, edumazet@google.com,
	davem@davemloft.net, alexander.duyck@gmail.com,
	sridhar.samudrala@intel.com, kuba@kernel.org,
	willemdebruijn.kernel@gmail.com, weiwan@google.com,
	Jonathan Corbet <corbet@lwn.net>,
	Alexander Viro <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.cz>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nathan Lynch <nathanl@linux.ibm.com>,
	Steve French <stfrench@microsoft.com>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	Jiri Slaby <jirislaby@kernel.org>,
	Julien Panis <jpanis@baylibre.com>, Arnd Bergmann <arnd@arndb.de>,
	Andrew Waterman <waterman@eecs.berkeley.edu>,
	Thomas Huth <thuth@redhat.com>, Palmer Dabbelt <palmer@dabbelt.com>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	"open list:FILESYSTEMS (VFS and infrastructure)" <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH net-next v3 3/3] eventpoll: Add epoll ioctl for
 epoll_params
Message-ID: <2024012525-outdoors-district-2660@gregkh>
References: <20240125225704.12781-1-jdamato@fastly.com>
 <20240125225704.12781-4-jdamato@fastly.com>
 <2024012551-anyone-demeaning-867b@gregkh>
 <20240126001128.GC1987@fastly.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20240126001128.GC1987@fastly.com>

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