Re: Kernel version numbers after 4.9.255 and 4.4.255 - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Jiri Slaby <jirislaby@kernel.org>
Cc: 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
Subject: Re: Kernel version numbers after 4.9.255 and 4.4.255
Date: Thu, 4 Feb 2021 09:51:03 +0100	[thread overview]
Message-ID: <YBu1d0+nfbWGfMtj@kroah.com> (raw)
In-Reply-To: <78ada91b-21ee-563f-9f75-3cbaeffafad4@kernel.org>

On Thu, Feb 04, 2021 at 08:26:04AM +0100, Jiri Slaby wrote:
> On 04. 02. 21, 7:20, Greg Kroah-Hartman wrote:
> > On Thu, Feb 04, 2021 at 05:59:42AM +0000, Jari Ruusu wrote:
> > > Greg,
> > > I hope that your linux kernel release scripts are
> > > implemented in a way that understands that PATCHLEVEL= and
> > > SUBLEVEL= numbers in top-level linux Makefile are encoded
> > > as 8-bit numbers for LINUX_VERSION_CODE and
> > > KERNEL_VERSION() macros, and must stay in range 0...255.
> > > These 8-bit limits are hardcoded in both kernel source and
> > > userspace ABI.
> > > 
> > > After 4.9.255 and 4.4.255, your scripts should be
> > > incrementing a number in EXTRAVERSION= in top-level
> > > linux Makefile.
> > 
> > Should already be fixed in linux-next, right?
> 
> I assume you mean:
> commit 537896fabed11f8d9788886d1aacdb977213c7b3
> Author: Sasha Levin <sashal@kernel.org>
> Date:   Mon Jan 18 14:54:53 2021 -0500
> 
>     kbuild: give the SUBLEVEL more room in KERNEL_VERSION
> 
> That would IMO break userspace as definition of kernel version has changed.
> And that one is UAPI/ABI (see include/generated/uapi/linux/version.h) as
> Jari writes. For example will glibc still work:
> http://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/configure.ac;h=13abda0a51484c5951ffc6d718aa36b72f3a9429;hb=HEAD#l14
> 
> ? Or gcc 10 (11 will have this differently):
> https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/config/bpf/bpf.c;hb=ee5c3db6c5b2c3332912fb4c9cfa2864569ebd9a#l165
> 
> and
> 
> https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/config/bpf/bpf-helpers.h;hb=ee5c3db6c5b2c3332912fb4c9cfa2864569ebd9a#l53

Ugh, I thought this was an internal representation, not an external one
:(

> 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 :)

So, let's leave it alone and just see what happens!

greg k-h