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Date: Fri, 26 Jan 2024 07:16:29 +0100
From: "Arnd Bergmann" <arnd@arndb.de>
To: "Joe Damato" <jdamato@fastly.com>,
 "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>
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Subject: Re: [PATCH net-next v3 3/3] eventpoll: Add epoll ioctl for epoll_params
Content-Type: text/plain

On Fri, Jan 26, 2024, at 03:36, Joe Damato wrote:
> On Thu, Jan 25, 2024 at 04:23:58PM -0800, Greg Kroah-Hartman wrote:
>> 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;
>> > > 
>
> Sure, that makes sense to me. I'll remove it in the v4 alongside the other
> changes you've requested.
>
> Thanks for your time and patience reviewing my code. I greatly appreciate
> your helpful comments and feedback.

Note that you should still pad the structure to its normal
alignment. On non-x86 targets this would currently mean a
multiple of 64 bits.

I would suggest dropping the EPOLL_PACKED here entirely and
just using a fully aligned structure on all architectures, like

struct epoll_params {
      __aligned_u64 busy_poll_usecs;
      __u16 busy_poll_budget;
      __u8 __pad[6];
};

The explicit padding can help avoid leaking stack data when
a structure is copied back from kernel to userspace, so I would
just always use it in ioctl data structures.

      Arnd