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From:   Emmanuel Grumbach <egrumbach@gmail.com>
Date:   Thu, 18 Mar 2021 11:26:22 +0200
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Subject: Re: systemd-rfkill regression on 5.11 and later kernels
To:     Takashi Iwai <tiwai@suse.de>
Cc:     Emmanuel Grumbach <emmanuel.grumbach@intel.com>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        Linux Kernel <linux-kernel@vger.kernel.org>
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Hi,

On Thu, Mar 18, 2021 at 10:31 AM Takashi Iwai <tiwai@suse.de> wrote:
>
> Hi,
>
> we've received a bug report about rfkill change that was introduced in
> 5.11.  While the systemd-rfkill expects the same size of both read and
> write, the kernel rfkill write cuts off to the old 8 bytes while read
> gives 9 bytes, hence it leads the error:
>   https://github.com/systemd/systemd/issues/18677
>   https://bugzilla.opensuse.org/show_bug.cgi?id=1183147

If you use an old kernel that expects only 8 bytes and you write 9,
then yes, the kernel will read only 8.
If you use a new kernel (5.11) and you send 9 bytes, the kernel will
read all the 9 bytes, so I am not sure I understand the problem here.
If you have a new header file that makes you send 9 bytes, then, in
order to run against an old kernel (which seems to have been the case
with the report in github), then you must be ready to have the kernel
read less than 9 bytes.
What am I missing?

>
> As far as I understand from the log in the commit 14486c82612a, this
> sounds like the intended behavior.  But if this was implemented in
> that way just for the compatibility reason, it actually is worse,
> introducing a regression.
>
> Although this can be addressed easily in the systemd side, the current
> kernel behavior needs reconsideration, IMO.
>
>
> thanks,
>
> Takashi