linux-kernel - Re: Build/qemu test results for v4.18-rc4

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Date:   Mon, 9 Jul 2018 20:16:04 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Kees Cook <keescook@...omium.org>,
        Guenter Roeck <linux@...ck-us.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: Build/qemu test results for v4.18-rc4



On 07/09/2018 08:06 PM, Kees Cook wrote:
> On Mon, Jul 9, 2018 at 10:55 AM, Guenter Roeck <linux@...ck-us.net> wrote:
>> s390:allmodconfig:
>>
>> arch/s390/kernel/als.o: In function `verify_facilities':
>> als.c:(.init.text+0x24): undefined reference to `latent_entropy'
>> als.c:(.init.text+0xae): undefined reference to `latent_entropy'
>> make[3]: *** [arch/s390/boot/compressed/vmlinux] Error 1
>> make[2]: *** [arch/s390/boot/compressed/vmlinux] Error 2
>> make[1]: *** [bzImage] Error 2
>>
>> This problem is only seen when using a compiler which has the relevant
>> plugins enabled. Bisect points to commit 1658dcee3d43ed ("gcc-plugins:
>> allow to enable GCC_PLUGINS for COMPILE_TEST") as the culprit. I don't
>> know if a fix for v4.18 has been submitted. The s390 boot code in -next
>> has been rearranged and the problem is no longer seen there.
> 
> Hm, that would imply that mm/page_alloc.o wasn't visible during the
> als.o linking? But ... if the problem is gone, I guess... good?

als.o is used to detect if the kernel was compiled for a newer generation
than the hardware that we are running on (very early). So it is compiled
without gcov, kcov,ubsan and with a different march -in other words special.
In linux-next we moved the als part to the decompressor which avoids all
these kind of special handling. As this is part of a bigger patch set it 
would be  non-trivial to backport that for 4.18.
So unless we want to have it fixed for 4.18 I think we are fine.