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:   Fri, 6 Jul 2018 13:41:44 +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 Fri, Jul 06, 2018 at 03:37:58PM +0900, Masahiro Yamada wrote:
> >> > > Also see https://bugzilla.kernel.org/show_bug.cgi?id=200385 ,
> >> > >
> >> > > 0a1756bd2897951c03c1cb671bdfd40729ac2177 is acting up
> >> > > too with the same symptoms
> >> >
> >> > I tracked it down to -flto in LDFLAGS. I'll look more into this.
> >>
> >> -flto in LDFLAGS screws up this part of paging_prepare():
> >
> > +Masahiro, Michal.
> >
> > I've got it wrong. *Any* LDFLAGS option passed to make this way:
> >
> >   make LDFLAGS="..."
> >
> > would cause a issue. Even empty.
> >
> > It overrides all assignments to the variable in the makefile.
> > As result the image is built without -pie and linker doesn't generate
> > position independed code.
> >
> > Looks like the patch below helps, but my make-fu is poor.
> > I don't see many override directives in kernel makefiles.
> > It makes me think that there's a better way to fix this.
> >
> > Hm?
> 
> 
> 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?

This breakage was rather hard to debug. We need to have some kind of
fail-safe for the future.

> You want to pass your own linker flags
> for building vmlinux and modules,
> but do not want to pass them to
> the decompressor (arch/x86/boot/compressed).
> 
> Correct?

I personally don't think that changing compiler/linker options for kernel
build is good idea in general.

> Kbuild provides a way for users
> to pass additional linker flags to modules.
> (LDFLAGS_MODULE)
> 
> 
> But, there is no way to do that for vmlinux.
> 
> It is easy to support it, though.
> 
> https://patchwork.kernel.org/patch/10510833/
> 
> If this is the one you want, I can merge 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.

-- 
 Kirill A. Shutemov