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

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Message-ID: <20200517194429.scwhfr4l4bv4h3ux@google.com>
Date:   Sun, 17 May 2020 12:44:29 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Dmitry Golovin <dima@...ovin.in>
Cc:     Borislav Petkov <bp@...en8.de>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/boot: allow a relocatable kernel to be linked with
 lld

On 2020-05-16, Dmitry Golovin wrote:
>15.05.2020, 21:50, "Borislav Petkov" <bp@...en8.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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