linux-kernel - Re: [PATCH] kbuild: give SUBLEVEL more room in KERNEL

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Date:   Mon, 18 Jan 2021 14:52:02 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Sasha Levin <sashal@...nel.org>
Cc:     masahiroy@...nel.org, michal.lkml@...kovi.net,
        torvalds@...ux-foundation.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...nel.org
Subject: Re: [PATCH] kbuild: give SUBLEVEL more room in KERNEL_VERSION

On Mon, Jan 18, 2021 at 08:39:59AM -0500, Sasha Levin wrote:
> On Mon, Jan 18, 2021 at 10:24:33AM +0100, Greg KH wrote:
> > On Mon, Jan 18, 2021 at 10:21:16AM +0100, Greg KH wrote:
> > > On Sun, Jan 17, 2021 at 08:49:51PM -0500, Sasha Levin wrote:
> > > > SUBLEVEL only has 8 bits of space, which means that we'll overflow it
> > > > once it reaches 256.
> > > >
> > > > Few of the stable branches will imminently overflow SUBLEVEL while
> > > > there's no risk of overflowing VERSION.
> > > >
> > > > Thus, give SUBLEVEL 8 more bits which will be stolen from VERSION, this
> > > > should create a better balance between the different version numbers we
> > > > use.
> > > >
> > > > The downside here is that Linus will have 8 bits less to play with, but
> > > > given our current release cadence (~10 weeks), the number of Linus's
> > > > fingers & toes (20), and the current VERSION (5) we can calculate that
> > > > VERSION will overflow in just over 1,000 years, so I'm kicking this can
> > > > down the road.
> > > >
> > > > Cc: stable@...nel.org
> > > > Signed-off-by: Sasha Levin <sashal@...nel.org>
> > > > ---
> > > >  Makefile | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/Makefile b/Makefile
> > > > index 9e73f82e0d863..dc2bad7a440d8 100644
> > > > --- a/Makefile
> > > > +++ b/Makefile
> > > > @@ -1252,8 +1252,8 @@ endef
> > > >
> > > >  define filechk_version.h
> > > >  	echo \#define LINUX_VERSION_CODE $(shell                         \
> > > > -	expr $(VERSION) \* 65536 + 0$(PATCHLEVEL) \* 256 + 0$(SUBLEVEL)); \
> > > > -	echo '#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))'
> > > > +	expr $(VERSION) \* 16777216 + 0$(PATCHLEVEL) \* 65536 + 0$(SUBLEVEL)); \
> > > > +	echo '#define KERNEL_VERSION(a,b,c) (((a) << 24) + ((b) << 16) + (c))'
> > > 
> > > As much as I agree, this will break in-tree users of LINUX_VERSION_CODE
> > > that try to suck out the version/patchlevel number of the kernel release
> > > into their own fields.  Things like USB host controller strings, v4l
> > > ioctl reports, scsi driver ioctls, and other places do fun bit-movements
> > > to try to unreverse this bit packing.
> > > 
> > > So how about we just provide a "real" version/subversion/revision
> > > #define as well, and clean up all in-kernel users, so we can get this to
> > > work, and we can change it in the future more easily.
> > 
> > Or, I can just stop doing stable releases at .255 and then abuse the
> > EXTRAVERSION field to put in sub-revision values.
> > 
> > Or, we can just not worry about it as anyone using these really old
> > kernels, userspace will work just fine (the number going backwards for
> > these fields isn't going to break anything), it's only any crazy
> > out-of-tree code that will get confused if they are trying to do
> > different build options based on SUBLEVEL :)
> 
> I think it would also affect code that doesn't do things based on
> SBULEVEL. Consider something like:
> 
> 	if (LINUX_VERSION_CODE < KERNEL_VERSION(4,5,0))
> 
> Which will cause 4.4.256 to now change the result of that comparison.

Sure, but there are no in-kernel users like this, so my sympathy is
quite low :)