OSDev.org

Posted: **Fri Dec 11, 2020 7:01 am**

Hello,
I am currently working on a kernel that can be built with both a GNU build system and an LLVM system. It currently works with GNU, but not LLVM. Here is my linker script

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ENTRY(_start)

SECTIONS 
{
    . = 0x100000;
    .stivale2hdr : {
        *(.stivale2hdr)
    }
    .text : ALIGN(4096) {
        KEEP(*(.text*))
    }
    .data : ALIGN(4096) {
        KEEP(*(.data*))
    }
    .rodata : ALIGN(4096) {
        KEEP(*(.rodata*))
    }
    .bss : ALIGN(4096) {
        KEEP(*(COMMON))
        KEEP(*(.bss*))
    }
    /DISCARD/ : { 
        *(.comment) 
        *(.gnu*) 
        *(.note*)
        *(.interp)
        *(.dynsym)
        *(.dynstr)
        *(.dynamic)
        *(.eh_frame*) 
    }
    _end = .;
}

And here is my bootstrap code

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// start-i386-mb2.s  - contains start code for Multiboot 2 on i386
// Distributed with NexNix, licensed under the MIT license
// See LICENSE

.section .stivale2hdr
.align 8
header_start:
    .quad 0                 // Use ELF header entry
    .long stack_top         // Stack address
    .long 0
    .quad 0                 // Flags
    .long framebuffer_tag   // Pointer to tags
    .long 0

// Start of nexldr

.section .text
.global _start
_start:
    cli
    hlt

.section .data
framebuffer_tag:
    .quad 0x3ecc1bc43d0f7971    // Framebuffer tag
    .quad 0                     // Only tag in list
    .word 0                     // Let bootloader pick values
    .word 0
    .word 0

.section .bss
.align 16
stack:
    .skip 8192
stack_top:

And yet it appears that LLD is ignoring the . = 0x100000; line and offseting everything to 0.
But if I move the .stivale2hdr section after the .text section, .text get placed at the correct address, but everything else is offset from 0!
And here is my readelf output
https://gist.github.com/nexos-dev/a0797 ... 89e749f3c5

EDIT: I just looked at it again, and it is making a shared library! Why is that? My linker line is

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clang --target=i386-elf -fuse-ld=lld -nostdlib -Tarch/x86/i386-stivale2-link.ld arch/x86/start-i386-stivale2.o   -o nldr,

Thanks,
nexos

Posted: **Fri Dec 11, 2020 8:10 am**

nexos wrote:Hello,
I am currently working on a kernel that can be built with both a GNU build system and an LLVM system. It currently works with GNU, but not LLVM.

I also had problems with the LLVM lld and address calculation in the linker scripts. I've managed to create a workaround that works for both GNU ld and LLVM lld by using the SEGMENT_START macro and placing the section at "." (watch the second line below carefully, there's an extra "." before the colon). I don't know if that matters, but I also always use a PHDRS block in the linker scripts.

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    . = SEGMENT_START("text-segment", LOAD_ADDRESS) + SIZEOF_HEADERS;
    .text . : {

Hopefully this helps.

You could also try

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    . = 0x100000;
    .text : ALIGN(4096) {
        KEEP(*(.stivale2hdr))
        *(.text*)
    }

(There's not much point in using KEEP if you only have one block in the section definition. It tells the linker not to reorder those parts within the section when there are more. So in this example above it makes sense as the section has two definitons it's used to keep ".stivale2hdr" at the beginning of the text section)

Cheers,
bzt

Posted: **Fri Dec 11, 2020 11:47 am**

Unfortunately, the workaround didn't work. The second option won't work, as my kernel boots with Multiboot2 and Stivale2 (the boot protocol of Limine), and stivale2 requires the header in its own section. I guess I won't allow compiling with LLVM when using Stivale2

.

Posted: **Fri Dec 11, 2020 2:46 pm**

nexos wrote:Unfortunately, the workaround didn't work. The second option won't work, as my kernel boots with Multiboot2 and Stivale2 (the boot protocol of Limine), and stivale2 requires the header in its own section. I guess I won't allow compiling with LLVM when using Stivale2 .

Ah, I see. Does the workaround work if you specify a PHDRS block in which you put both the stivale2 and text section into the same segment? It should, but this is just a well educated guess.

Cheers,
bzt

Posted: **Fri Dec 11, 2020 7:37 pm**

Does marking stivale2hdr as allocatable make any difference in the situation?

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.section .stivale2hdr, "a"

Posted: **Sat Dec 12, 2020 7:35 am**

linuxyne wrote:Does marking stivale2hdr as allocatable make any difference in the situation?
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.section .stivale2hdr, "a"

That fixed it! Thanks!

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