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

From: Masahiro Yamada <yamada.masahiro@socionext.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Gabriel C <nix.or.die@gmail.com>,
	Benjamin Gilbert <bgilbert@redhat.com>,
	linux-x86_64@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
	bero@lindev.ch, Andi Kleen <ak@linux.intel.com>,
	Michal Marek <michal.lkml@markovi.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"
Date: Mon, 9 Jul 2018 19:37:44 +0900	[thread overview]
Message-ID: <CAK7LNAT1G+rziRXewk=kVWg5NNK_NQyvC-y51Xib6-fxqvHtkw@mail.gmail.com> (raw)
In-Reply-To: <20180709101005.43yp6b2g3b4dgg37@kshutemo-mobl1>

2018-07-09 19:10 GMT+09:00 Kirill A. Shutemov <kirill@shutemov.name>:
> 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@shutemov.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?

Yes, targeting for 4.19-rc1.

-- 
Best Regards
Masahiro Yamada