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Date:   Thu, 4 Feb 2021 17:48:10 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     David Laight <David.Laight@aculab.com>
Cc:     'Jiri Slaby' <jirislaby@kernel.org>,
        Jari Ruusu <jariruusu@protonmail.com>,
        Sasha Levin <sashal@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        "masahiroy@kernel.org" <masahiroy@kernel.org>
Subject: Re: Kernel version numbers after 4.9.255 and 4.4.255
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On Thu, Feb 04, 2021 at 04:28:19PM +0000, David Laight wrote:
> From: Jiri Slaby
> > Sent: 04 February 2021 11:01
> > 
> > On 04. 02. 21, 9:51, Greg Kroah-Hartman wrote:
> > >> It might work somewhere, but there are a lot of (X * 65536 + Y * 256 + Z)
> > >> assumptions all around the world. So this doesn't look like a good idea.
> > >
> > > Ok, so what happens if we "wrap"?  What will break with that?  At first
> > > glance, I can't see anything as we keep the padding the same, and our
> > > build scripts seem to pick the number up from the Makefile and treat it
> > > like a string.
> > >
> > > It's only the crazy out-of-tree kernel stuff that wants to do minor
> > > version checks that might go boom.  And frankly, I'm not all that
> > > concerned if they have problems :)
> > 
> > Agreed. But currently, sublevel won't "wrap", it will "overflow" to
> > patchlevel. And that might be a problem. So we might need to update the
> > header generation using e.g. "sublevel & 0xff" (wrap around) or
> > "sublevel > 255 : 255 : sublevel" (be monotonic and get stuck at 255).
> > 
> > In both LINUX_VERSION_CODE generation and KERNEL_VERSION proper.
> 
> A full wrap might catch checks for less than (say) 4.4.2 which
> might be present to avoid very early versions.

Who does that?

> So sticking at 255 or wrapping onto (say) 128 to 255 might be better.

Better how?

> I'm actually intrigued about how often you expect people to update
> systems running these LTS kernels.

Whenever they can, and should.

> At a release every week it takes 5 years to run out of sublevels.
> No one is going to reboot a server anywhere near that often.

Why not?

Usually kernels this old are stuck in legacy embedded systems, like last
year's new phone models :)

thanks,

greg k-h