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

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Message-ID: <20200515185051.GC19017@zn.tnic>
Date:   Fri, 15 May 2020 20:50:51 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Dmitry Golovin <dima@...ovin.in>
Cc:     clang-built-linux@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, 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
Subject: Re: [PATCH] x86/boot: allow a relocatable kernel to be linked with
 lld

On Fri, May 01, 2020 at 08:42:13AM +0000, Dmitry Golovin wrote:
> LLD by default disallows relocations in read-only segments. For a

I need more info here about which segment is read-only?

Is this something LLD does by default or what's happening?

Because my BFD-linked vmlinux has:

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000001000 0x0000000000000000 0x0000000000000000
                 0x000000000070fa28 0x0000000000726b00  RWE    0x1000
  LOAD           0x0000000000000000 0x0000000000727000 0x0000000000727000
                 0x0000000000000000 0x0000000000007000  RW     0x1000
  DYNAMIC        0x00000000007108f8 0x000000000070f8f8 0x000000000070f8f8
                 0x0000000000000130 0x0000000000000130  RW     0x8
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RWE    0x10

so what's up?

> relocatable kernel, we pass -z notext to the linker to explicitly
> allow relocations. This behavior is the default for BFD.

Or are you saying that ld.bfd makes the text segment by default RW while
ld.lld makes it read-only like the elf manpage says:

"p_flags
              This member holds a bit mask of flags relevant to the segment:

              PF_X   An executable segment.
              PF_W   A writable segment.
              PF_R   A readable segment.

              A text segment commonly has the flags PF_X and PF_R."

IOW, don't be afraid to be more verbose in the commit message. :)

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette