linux-kernel - Re: 4.17.x won't boot due to "x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G"

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Date:   Mon, 9 Jul 2018 13:10:05 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Gabriel C <nix.or.die@...il.com>,
        Benjamin Gilbert <bgilbert@...hat.com>,
        linux-x86_64@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        bero@...dev.ch, Andi Kleen <ak@...ux.intel.com>,
        Michal Marek <michal.lkml@...kovi.net>
Subject: Re: 4.17.x won't boot due to "x86/boot/compressed/64: Handle 5-level
 paging boot if kernel is above 4G"

On Sat, Jul 07, 2018 at 10:21:47AM +0900, Masahiro Yamada wrote:
> 2018-07-07 1:29 GMT+09:00 Kirill A. Shutemov <kirill@...temov.name>:
> > On Fri, Jul 06, 2018 at 11:13:02PM +0900, Masahiro Yamada wrote:
> >> >> LDFLAGS is for internal-use.
> >> >> Please do not override it from the command line.
> >> >
> >> > Can we generate a build error if a user try to override LDFLAGS, CFLAGS or
> >> > other critical internal-use-only variables?
> >>
> >> Yes, Make can check where variables came from.
> >
> > I think we should do this.
> >
> >> >> make LDFLAGS_KERNEL=...  LDFLAGS_MODULE=...
> >> >> will allow you to append linker flags.
> >> >
> >> > Okay. It makes me wounder if we should taint kernel in such cases?
> >> > Custom compiler/linker flags are risky and can lead to weird bugs.
> >>
> >> OK.
> >> So, what problem are we discussing?
> >
> > Users set custom LDFLAGS/CFLAGS and break kernel. Then report bug that
> > hard to debug. See
> >
> > https://bugzilla.kernel.org/show_bug.cgi?id=200385
> 
> 
> CFLAGS is only used under tools/.
> Passing CFLAGS is probably no effect to the kernel.
> 
> In Linux makefiles,
> KBUILD_ prefixed variables are used internally.
> 
> KBUILD_CFLAGS, KBUILD_CPPFLAGS, KBUILD_AFLAGS, etc.
> 
> 
> LDFLAGS is an exception.  I do not know why.
> Renaming LDFLAGS to KBUILD_LDFLAGS
> will make the code consistent.
> 
> At least, it will avoid overriding flags by accident.
> 
> Of course, users still can change KBUILD_LDFLAGS
> if they really want.
> 
> The build system could add belt and braces checks for that,
> but it is arguable since
> there are lots of lots of internal variables.

I think renaming LDFLAGS to KBUILD_LDFLAGS is good idea.
Would you prepare patch?

-- 
 Kirill A. Shutemov