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 19:29:28 +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 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

and start of this thread:

https://lore.kernel.org/lkml/20180701213243.GA20180@trogon.sfo.coreos.systems/

It took me a while to track down the issue. I blamed linker for a while.

> > I've got it wrong. *Any* LDFLAGS option passed to make this way:
> >
> >  make LDFLAGS="..."
> 
> In your previous mail, I thought you were asking me how to pass
> custom linker flags.
> 
> If not, we do not need to think about that case.
> Just say "Do not do that".

At least we need to make user aware about risk of setting custom flags.

-- 
 Kirill A. Shutemov