[llvm-dev] Linking Linux kernel with LLD

[Sean Silva via llvm-dev]

On Mon, Jan 23, 2017 at 8:16 AM, Dmitry Golovin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> >>  - D28094 (Implemented support for R_386_PC8/R_386_8 relocations)
> > Do you remember where it was used ?
>
> setup.elf:
>       ld.lld -m elf_i386 -T arch/x86/boot/setup.ld arch/x86/boot/a20.o
> arch/x86/boot/bioscall.o arch/x86/boot/cmdline.o arch/x86/boot/copy.o
> arch/x86/boot/cpu.o arch/x86/boot/cpuflags.o arch/x86/boot/cpucheck.o
> arch/x86/boot/early_serial_console.o arch/x86/boot/edd.o
> arch/x86/boot/header.o arch/x86/boot/main.o arch/x86/boot/mca.o
> arch/x86/boot/memory.o arch/x86/boot/pm.o arch/x86/boot/pmjump.o
> arch/x86/boot/printf.o arch/x86/boot/regs.o arch/x86/boot/string.o
> arch/x86/boot/tty.o arch/x86/boot/video.o arch/x86/boot/video-mode.o
> arch/x86/boot/version.o arch/x86/boot/video-vga.o
> arch/x86/boot/video-vesa.o arch/x86/boot/video-bios.o -o
> arch/x86/boot/setup.elf
>     ld.lld: error: do not know how to handle relocation 'R_386_PC8' (23)
>
> > I updated patch for --emit-relocs, now they do not fail.
>
> Thanks, applied it, doesn't fail.
>
> I still didn't do anything with "Setup too big!" problem, just commented
> out the assert. Tried booting the resulting bzImage and vmlinux with qemu.
> The bzImage only did reboot over and over, but the vmlinux did show an
> adorable picture (attached).
>


That's beautiful! Looks like some kernel or loader text got copied into the
VGA text buffer. Those "e with two dots above" looks like Code Page 437 (
https://en.wikipedia.org/wiki/Code_page_437) for 0x89 which is a common MOV
opcode in x86 machine code. "Capital H" and "capital Phi" also look like
common x86 opcodes.

I remember when we were getting the FreeBSD kernel to link correctly, one
of the bugs was that the kernel load address was too low (because of our
MAXPAGESIZE value) which meant that the location where the kernel was
asking to be copied into actually overlapped the bootloader's text, which
ended about as well as you would expect it to. We didn't get a pretty
picture though :)

-- Sean Silva


>
> Regards,
> Dmitry
>
> 20.01.2017, 22:49, "Dmitry Golovin via llvm-dev" <llvm-dev at lists.llvm.org
> >:
> >>>  - D28094 (Implemented support for R_386_PC8/R_386_8 relocations)
> >>  Do you remember where it was used ?
> >
> > I can undo the patch (but I can access the build machine only on Monday)
> and see what breaks.
> >
> >>  CONSTRUCTORS can be just removed, they do nothing for ELF.
> >
> > Okay, this is what I did (I thought it will break things, but it is
> okay). I will apply the patch.
> >
> >>  LLD does not support "symbol = 5*6", but accepts "symbol = 5 * 6"
> currently. Not sure what is easy fix here.
> >
> > Just ignore it for now, it's not really a big deal.
> >
> >>  I updated patch for --emit-relocs, now they do not fail:
> https://reviews.llvm.org/D28612
> >
> > Thank you, I will apply the updated patch and hope that it will boot.
> >
> > Regards,
> > Dmitry
> >
> > 20.01.2017, 18:36, "George Rimar" <grimar at accesssoftek.com>:
> >>  Hi Dmitry,
> >>
> >>  thanls for sharing. Few comments/questions below:
> >>
> >>>  Here is the list of modifications I had to do in order to link the
> kernel (I have used llvmlinux with clang and mainline with gcc, the
> >results are similar):
> >>>
> >>>  1. LLD patches:
> >>
> >>   > - D28094 (Implemented support for R_386_PC8/R_386_8 relocations)
> >>
> >>  Do you remember where it was used ?
> >>
> >>>  5. In arch/x86/kernel/vmlinux.lds.S commented out the "CONSTRUCTORS",
> because LLD doesn't support it.
> >>
> >>  It is https://reviews.llvm.org/D28951. CONSTRUCTORS can be just
> removed, they do nothing for ELF.
> >>
> >>>  6. In arch/x86/boot/setup.ld replaced 5*512 with precalculated value
> of 2560 because it doesn't seem that LLD supports math inside >ASSERT in
> linker scripts.
> >>
> >>  It is actually not relative with ASSERT. LLD does not support "symbol
> = 5*6", but accepts "symbol = 5 * 6" currently.
> >>  Not sure what is easy fix here.
> >>
> >>>  Finally the kernel was built and it obviously didn't run (only
> printed out "No setup signature found..." but this is some result as well).
> >Probably, the result could be better if the --emit-relocs option didn't
> fail and CONSTRUCTORS were supported. I really don't know what >to do about
> the assertion that I have commented out.
> >>
> >>  I updated patch for --emit-relocs, now they do not fail:
> https://reviews.llvm.org/D28612
> >>  It looks to be important feature for self relocations, so it is not
> surprising it did not run without :)
> >>
> >>  George.
> >
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