Re: [PATCH] x86/boot: allow a relocatable kernel to be linked with lld

From: Fangrui Song <maskray@google.com>
To: Dmitry Golovin <dima@golovin.in>
Cc: Borislav Petkov <bp@alien8.de>,
	"clang-built-linux@googlegroups.com" 
	<clang-built-linux@googlegroups.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "x86@kernel.org" <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Daniel Kiper <daniel.kiper@oracle.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86/boot: allow a relocatable kernel to be linked with lld
Date: Sun, 17 May 2020 12:44:29 -0700	[thread overview]
Message-ID: <20200517194429.scwhfr4l4bv4h3ux@google.com> (raw)
In-Reply-To: <602331589572661@mail.yandex.ru>

On 2020-05-16, Dmitry Golovin wrote:
>15.05.2020, 21:50, "Borislav Petkov" <bp@alien8.de>:
>>
>> I need more info here about which segment is read-only?
>>
>> Is this something LLD does by default or what's happening?
>>
>
>Probably should have quoted the original error message:
>
>    ld.lld: error: can't create dynamic relocation R_386_32 against
>    symbol: _bss in readonly segment; recompile object files with -fPIC
>    or pass '-Wl,-z,notext' to allow text relocations in the output

Do we know where do these R_386_32 come from?

When linking in -shared mode, the linker assumes the image is a shared
object and has undetermined image base at runtime. An absolute
relocation needs a text relocation (a relocation against a readonly
segment).

When neither -z notext nor -z text is specified, GNU ld is in an
indefinite state where it will enable text relocations (DT_TEXTREL
DF_TEXTREL) on demand. It is not considered a good practice for
userspace applications to do this.

Of course the kernel is different....... I know little about the kernel,
but if there is a way to make the sections containing R_386_32
relocations writable (SHF_WRITE), that will be a better solution to me.
In LLD, -z notext is like making every section SHF_WRITE.

>>
>> IOW, don't be afraid to be more verbose in the commit message. :)
>>
>
>Tried both BFD and LLD for linking to understand the difference more and
>rewrite the commit message, and came to the conclusion that the patch is
>wrong. I will submit v2 when I figure out the correct solution.
>
>Regards,
>Dmitry
>
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