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:   Sat, 7 Jul 2018 10:21:47 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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"

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.




> 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



-- 
Best Regards
Masahiro Yamada