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

[Masahiro Yamada]

Hi.

2018-07-06 19:41 GMT+09:00 Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>:
> 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".



-- 
Best Regards
Masahiro Yamada