OSDev.org

Posted: **Fri Dec 07, 2018 7:27 am**

Hello

I'm writing my own kernel, and after month of hitting the wall with my head I've found a way to
a) boot 64bit kernels from GRUB without assembly files
b) eliminate global offset table

I hope it will be useful for you.

My goal was to write 32bit code which deals with GRUB, creates page tables, GDT and other stuff in C++. Linker refuses to link 32bit and 64bit object files, but feels ok if you translate 32bit sources to assembly(with gcc -S), and them compile them as 64bit object files. The tool detects "#define MULTIBOOT_INTERFACE" line in source files and builds them with -m32 -S flags, then postprocesses assembly code and compiles the result into 64bit object files. Another problem is GOT with I don't want to see in my binary. This problem is solved during postprocessing stage.

During postprocessing stage I insert dummy section in file header:

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.text
.Ltext_offset:

Also I remove .file directive because I dislike it, and #APP / #NOAPP lines - gcc wraps my inline assembly with them.

Then each GOT address calculation is modified:

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	call	__x86.get_pc_thunk.bx
//	addl	$_GLOBAL_OFFSET_TABLE_, %ebx
	addl	$(.Ltext_offset - .), %ebx

Then each reference to GOT is replaced with address of label relative to .Ltext_offset:

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//	leal	.LC0@GOTOFF(%ebx), %eax
	movl	$(.LC0-.Ltext_offset), %eax
	addl	%ebx, %eax

Upd: there is an unfixed problem with static variables:

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int foo;
void bar() {
    m = foo;
    ...
    foo = n;
}

Please move all such variables into a static structure and get a pointer to this structure:

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struct fb_parameters {
    uint64 address = 0xB8000;
    uint32 width   = 80;
    uint32 height  = 24;
    uint32 pitch   = 0;
    uint32 bpp     = 0;
    uint32 color   = 0;
    uint32 x = 0, w = 0;
    static void (*putchar)(int);
};
static fb_parameters fb;

void print32 (const char *format, ...) {
    auto f = &::fb;
    ...
}

There are still problems with instructions like movzbl, I'll fix them soon.

Regards,
Dmitry

Posted: **Fri Dec 07, 2018 7:36 pm**

Why not use objcopy to convert 32-bit object to 64-bit objects?

Posted: **Sat Dec 08, 2018 2:21 am**

ckotinko wrote:b) eliminate global offset table

There's an easier fix: -fno-pic.

There's an even easier fix: use a proper cross-compiler.

OSDev.org

I've created an extremely useful tool for kernel developers

I've created an extremely useful tool for kernel developers

Re: I've created an extremely useful tool for kernel develop

Re: I've created an extremely useful tool for kernel develop