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

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Date:   Mon, 18 Jan 2021 08:39:59 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.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 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.

-- 
Thanks,
Sasha