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: Gabriel C <nix.or.die@gmail.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
	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: Sat, 7 Jul 2018 09:55:58 +0900	[thread overview]
Message-ID: <CAK7LNASt3E178H8Ho0+bh3mTvE=3UF6Fj9AX=vWCZ=mWX-72jg@mail.gmail.com> (raw)
In-Reply-To: <CAEJqkgiSDdu7a6VyhsrA=rnjevyz2VZZdRVRHBgYHkmHjvAULw@mail.gmail.com>

2018-07-06 23:39 GMT+09:00 Gabriel C <nix.or.die@gmail.com>:
> 2018-07-06 16:13 GMT+02:00 Masahiro Yamada <yamada.masahiro@socionext.com>:
>> Hi.
>>
>> 2018-07-06 19:41 GMT+09:00 Kirill A. Shutemov <kirill@shutemov.name>:
>>> 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?
>>
>> Yes, Make can check where variables came from.
>>
>>
>>> 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.
>>
>> OK.
>> So, what problem are we discussing?
>>
>>
>>> 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".
>
> I am sorry but I have a hard time to get your logic here.
>
> You are saying : the *env* variable LDFLAGS as well passing
> LDFLAGS to make , which your build allows should not be use
> because is for 'internal usage' .. ?
>
> Well that logic you have here is wrong and wrong for any project
> not just for the kernel,

Why 'my logic'?

LDFLAGS has been long used internally since the old days,
before I ever worked on the kernel.

I shared my knowledge about the kernel build system.

The current situation is not nice,
but why are you blaming me for the code I did not add ?

Note:
I have never said 'the *env* variable LDFLAGS'

> If you know 'parts' need have particular flags then 'you' have to
> ensure nothing
> overrides these or nothing at all can chage these.
>
> So swap your logic and apped LDFLAGS to your private
> 'call_it_whatever_you_wish_KERNEL_NEED_BE_THERE_ANY_KIND_FLAGS'
> and don't allow these to be changed at all , when you
> *know* they have be there.
>
>
> Teling users to not use LD/C/CXX flags is not really going to work right ?
>
>
> BR

-- 
Best Regards
Masahiro Yamada