linux-kernel - Re: [PATCH 2/3] kbuild: clamp SUBLEVEL to 255

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Message-ID: <CAK7LNATDts980vwwUOHV4V-HcrYgXMZueFCYzL6ayP3LZegqNA@mail.gmail.com>
Date:   Tue, 9 Feb 2021 10:58:25 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Sasha Levin <sashal@...nel.org>,
        "michal.lkml@...kovi.net" <michal.lkml@...kovi.net>,
        "linux-kbuild@...r.kernel.org" <linux-kbuild@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 2/3] kbuild: clamp SUBLEVEL to 255

On Mon, Feb 8, 2021 at 10:48 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Sasha Levin
> > Sent: 06 February 2021 03:51
> >
> > Right now if SUBLEVEL becomes larger than 255 it will overflow into the
> > territory of PATCHLEVEL, causing havoc in userspace that tests for
> > specific kernel version.
> >
> > While userspace code tests for MAJOR and PATCHLEVEL, it doesn't test
> > SUBLEVEL at any point as ABI changes don't happen in the context of
> > stable tree.
> >
> > Thus, to avoid overflows, simply clamp SUBLEVEL to it's maximum value in
> > the context of LINUX_VERSION_CODE. This does not affect "make
> > kernelversion" and such.
> >
> > Signed-off-by: Sasha Levin <sashal@...nel.org>
> > ---
> >  Makefile | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/Makefile b/Makefile
> > index 49ac1b7fe8e99..157be50c691e5 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -1258,9 +1258,15 @@ define filechk_utsrelease.h
> >  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))'
> > +     if [ $(SUBLEVEL) -gt 255 ]; then                                 \
> > +             echo \#define LINUX_VERSION_CODE $(shell                 \
> > +             expr $(VERSION) \* 65536 + 0$(PATCHLEVEL) \* 256 + 255); \
> > +     else                                                             \
> > +             echo \#define LINUX_VERSION_CODE $(shell                 \
> > +             expr $(VERSION) \* 65536 + 0$(PATCHLEVEL) \* 256 + $(SUBLEVEL)); \
> > +     fi;                                                              \
> > +     echo '#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) +  \
> > +     ((c) > 255 ? 255 : (c)))'
> >  endef
>
> Why not use KERNEL_VERSION to define LINUX_VERSION_CODE ?
> Basically just:
>         echo '#define LINUX_VERSION_CODE KERNEL_VERSION($(VERSION), $(PATCHLEVEL)+0, $(SUBLEVEL)+0)'




It was not possible to macrofy LINUX_VERSION_CODE.
(build error when CONFIG_KALLSYMS is disabled)


Presumably, this restriction will go away
with the following commit in linux-next.



commit e06af0b2ba02fc0cc2219a14c4c04ff0296a6f9f
Author: Masahiro Yamada <masahiroy@...nel.org>
Date:   Thu Jan 28 18:42:30 2021 +1100

    init/version.c: remove Version_<LINUX_VERSION_CODE> symbol





My plan is to refactor LINUX_VERSION_CODE in the next
development cycle.






> If PATCHLEVEL and SUBLEVEL are guaranteed to be non-empty the +0
> can be removed.
> The patch assumes they are non-empty, the original pre-prended 0
> to stop syntax error for empty version strings.
>
> Note that the expr version will process 08 and 09.
> gcc will treat them as octal, and may error them.
>
>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>
--
Best Regards
Masahiro Yamada