linux-kernel - Re: systemd-rfkill regression on 5.11 and later kernels

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Date:   Thu, 18 Mar 2021 12:16:54 +0100
From:   Johannes Berg <johannes@...solutions.net>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
        linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: systemd-rfkill regression on 5.11 and later kernels

On Thu, 2021-03-18 at 12:11 +0100, Takashi Iwai wrote:
> > That said, we can "fix" this like this, and hope we'll not get this
> > again? And if we do get it again ... well, we keep renaming the structs
> > and add "struct rfkill_event_v3" next time?
> 
> Yeah, that's a dilemma.  An oft-seen trick is to add more bytes for
> the future use, e.g. extend to 16 bytes and fill 0 for the remaining.

Yeah, I guess I could stick a reserved[15] there, it's small enough.

> In the sound driver, we introduced an ioctl to inform from user-space
> which API protocol it can speak, and the kernel falls back to the old
> API/ABI if it's a lower version or it's not told at all.  But I'm not
> sure whether such an implementation is optimal for rfkill.

I thought about it, but it ... doesn't really help.

Somebody's going to do

	ioctl(..., sizeof(ev)) == sizeof(ev)

and break on older kernels, or == my_fixed_size, or ... something. It's
not really going to address the issue entirely.

And it's more complexity.

johannes