Linker error setting loading GDT register with LGDT instruction using Inline assembly

I was compiling my prototype of prototype of a kernel (sounds weird, but it really doesn't matter) and in the installation I need to link the ASM file to a C file compiled with gcc to get a executable that could be used as kernel. The problem is that, after implementing a swap to protected mode from real mode, I get this error at linking the kernel.c and loader.asm scripts:

Code:

kernel.c:(.text+0x1e1): undefined reference to `gdtr'

I will explain how all process of installation is and I will put the codes below. Installation steps: 1: Compile asm: Code:

nasm -f elf32 loader.asm -o kasm.o

2: Compile .c : Code:

gcc -m32 -ffreestanding -c kernel.c -o kc.o

3: Link both: Code:

ld -m elf_i386 -T linker.ld -o kernel kasm.o kc.o

The complete error output is: Code:

kc.o: In function `k_enter_protected_mode':
kernel.c:(.text+0x1e1): undefined reference to `gdtr'

The code looks like: Code:

/*
 *
 * kernel.c - version 0.0.1
 * This script is under the license of the distributed package, this license
 * can be found in the package itself
 * Script coded by Cristian Simón for the CKA Proyect
 * ----
 * License: GNU GPL v3
 * Coder: Cristian Simón
 * Proyect: CKA 
 * 
 */
/* Output defines */
#define BLACK_BGROUND 0X07 /* black background */
#define WHITE_TXT 0x07 /* light gray on black text */
#define GREEN_TXT 0x02 /* light green on black text */
#define RED_TXT 0x04 /* light red on black text*/
#define CYAN_TXT 0x03 /*light cyan on black text */
#include <stddef.h>
#include <stdint.h>
#include <cpuid.h>
void k_clear_screen();
void k_sleep_3sec();
unsigned int k_printf(char *message, unsigned int line, float color);
void k_malloc(size_t sz);
void k_free(void *mem);
/* k_clear_screen : to clear the entire text screen */
void k_clear_screen()
{
    char *vidmem = (char *) 0xC00B8000;
    unsigned int i=0;
    while(i < (80*25*2))
    {
        vidmem[i]=' ';
        i++;
        vidmem[i]=BLACK_BGROUND;
        i++;
    };
}

/* k_printf : the message and the line # */
unsigned int k_printf(char *message, unsigned int line, float color)
{
    char *vidmem = (char *) 0xC00B8000;
    unsigned int i=0;

    i=(line*80*2);

    while(*message!=0)
    {
        if(*message=='\n') /* check for a new line */
        {
            line++;
            i=(line*80*2);
            *message++;
        } else {
            vidmem[i]=*message;
            *message++;
            i++;
            vidmem[i]=color;
            i++;
        };
    };

    return(1);
}

/* 
* k_sleep_3sec : to make a simple delay of aprox 3 sec, since is a nasty sleep, 
* duration will vary
* from system to system
*/
void k_sleep_3sec()
{
    int c = 1, d = 1;
    for ( c = 1 ; c <= 20000 ; c++ )
    for ( d = 1 ; d <= 20000 ; d++ )
    {}
}
/* 
* Malloc and free functions for this kernel
* Maybe change in the future, sure
*/
static unsigned char our_memory[1024 * 1024]; /* reserve 1 MB for malloc */
static size_t next_index = 0;
int k_malloc_err;
void k_malloc(size_t sz)
{
    void *mem;
    if(sizeof our_memory - next_index < sz){
        return NULL;
        k_malloc_err = 1;
    }

    mem = &our_memory[next_index];
    next_index += sz;
    return mem;
}
void k_free(void *mem)
{
   /* we cheat, and don't free anything. */
}
/* Schreduler */
/*---*/
/*
* Our schreduler is a RTC (Run to Completion)
* In the future we will add more schredulers or change the type
* but for now this is what we got
*/
int proc_number_count = 0;
void k_schreduler(char *proc_name, unsigned int proc_prior)
{
    proc_number_count = proc_number_count + 1;
    int proc_number = proc_number_count;
}
void k_enter_protected_mode()
{
    __asm__ volatile ("cli;"          
        "lgdt (gdtr);"  
        "mov %eax, cr0;" 
        "or %al, 1;"    
        "mov cr0, %eax;" 
        "jmp 0x8,PModeMain;"
        "PModeMain:");
}
/*main function*/
void k_main() 
{
    k_clear_screen();
    k_printf(" Wellcome to", 0, WHITE_TXT);
    k_printf(" CKA!", 1, GREEN_TXT);
    k_printf("==============>", 2, WHITE_TXT);
    k_printf(" CKA stands for C Kernel with Assembly", 3, WHITE_TXT);
    k_printf(" Version 0.0.1, => based in the job of Debashis Barman", 4, WHITE_TXT);
    k_printf(" Contact => assemblyislaw@gmail.com / blueshell@mail2tor.com", 5, WHITE_TXT);
    k_printf("           or in the github repository page", 6, WHITE_TXT);
    k_sleep_3sec();
    k_clear_screen();
    /* here start the magic */
    k_printf(" !===> Starting Checkup <===!", 0, WHITE_TXT);
    k_printf(" =-=-=-=-=-=-=-=-=-=-=-=-=-=-", 1, WHITE_TXT);
    k_printf("[KernelInfo] Woah! No Kernel Panic for now! Well, lets fix that...", 2, CYAN_TXT);
    k_printf("[Proc1] Checking for k_malloc() and k_free() kernel functions", 3, WHITE_TXT);
    k_malloc(15);
    if (k_malloc_err == 1){
        k_printf("[F-ERROR] Unable to use k_malloc, do you have enough memory?", 4, RED_TXT);
        while(1){
            int error_stayer = 1;
        }

    } else{ 
        k_printf("[Proc1] k_malloc and k_free found, resuming boot...", 4, GREEN_TXT);
    }
    k_enter_protected_mode();
    k_printf("[KernelInfo] Switched to protected mode successfully", 5, CYAN_TXT);

}

This was kernel.c

Code:

ENTRY(loader)
OUTPUT_FORMAT(elf32-i386)

SECTIONS {
   /* The kernel will live at 3GB + 1MB in the virtual
      address space, which will be mapped to 1MB in the
      physical address space. */
   . = 0xC0100000;

   .text : AT(ADDR(.text) - 0xC0000000) {
       *(.text)
       *(.rodata*)
   }

   .data ALIGN (0x1000) : AT(ADDR(.data) - 0xC0000000) {
       *(.data)
   }

   .bss : AT(ADDR(.bss) - 0xC0000000) {
       _sbss = .;
       *(COMMON)
       *(.bss)
       _ebss = .;
   }
}

This was the linker.ld

Code:

global _loader                          ; Make entry point visible to linker.
extern k_main                           ; _main is defined elsewhere

; setting up the Multiboot header - see GRUB docs for details
MODULEALIGN equ  1<<0             ; align loaded modules on page boundaries
MEMINFO     equ  1<<1             ; provide memory map
FLAGS       equ  MODULEALIGN | MEMINFO  ; this is the Multiboot 'flag' field
MAGIC       equ    0x1BADB002     ; 'magic number' lets bootloader find the header
CHECKSUM    equ -(MAGIC + FLAGS)  ; checksum required

; This is the virtual base address of kernel space. It must be used to convert virtual
; addresses into physical addresses until paging is enabled. Note that this is not
; the virtual address where the kernel image itself is loaded -- just the amount that must
; be subtracted from a virtual address to get a physical address.
KERNEL_VIRTUAL_BASE equ 0xC0000000                  ; 3GB
KERNEL_PAGE_NUMBER equ (KERNEL_VIRTUAL_BASE >> 22)  ; Page directory index of kernel's 4MB PTE.


section .data
align 0x1000
BootPageDirectory:
    ; This page directory entry identity-maps the first 4MB of the 32-bit physical address space.
    ; All bits are clear except the following:
    ; bit 7: PS The kernel page is 4MB.
    ; bit 1: RW The kernel page is read/write.
    ; bit 0: P  The kernel page is present.
    ; This entry must be here -- otherwise the kernel will crash immediately after paging is
    ; enabled because it can't fetch the next instruction! It's ok to unmap this page later.
    dd 0x00000083
    times (KERNEL_PAGE_NUMBER - 1) dd 0                 ; Pages before kernel space.
    ; This page directory entry defines a 4MB page containing the kernel.
    dd 0x00000083
    times (1024 - KERNEL_PAGE_NUMBER - 1) dd 0  ; Pages after the kernel image.


section .text
align 4
MultiBootHeader:
    dd MAGIC
    dd FLAGS
    dd CHECKSUM

; reserve initial kernel stack space -- that's 16k.
STACKSIZE equ 0x4000

; setting up entry point for linker
loader equ (_loader - 0xC0000000)
global loader

_loader:
    ; NOTE: Until paging is set up, the code must be position-independent and use physical
    ; addresses, not virtual ones!
    mov ecx, (BootPageDirectory - KERNEL_VIRTUAL_BASE)
    mov cr3, ecx                                        ; Load Page Directory Base Register.

    mov ecx, cr4
    or ecx, 0x00000010                          ; Set PSE bit in CR4 to enable 4MB pages.
    mov cr4, ecx

    mov ecx, cr0
    or ecx, 0x80000000                          ; Set PG bit in CR0 to enable paging.
    mov cr0, ecx

    ; Start fetching instructions in kernel space.
    ; Since eip at this point holds the physical address of this command (approximately 0x00100000)
    ; we need to do a long jump to the correct virtual address of StartInHigherHalf which is
    ; approximately 0xC0100000.
    lea ecx, [StartInHigherHalf]
    jmp ecx                                                     ; NOTE: Must be absolute jump!

StartInHigherHalf:
    ; Unmap the identity-mapped first 4MB of physical address space. It should not be needed
    ; anymore.
    mov dword [BootPageDirectory], 0
    invlpg [0]

    ; NOTE: From now on, paging should be enabled. The first 4MB of physical address space is
    ; mapped starting at KERNEL_VIRTUAL_BASE. Everything is linked to this address, so no more
    ; position-independent code or funny business with virtual-to-physical address translation
    ; should be necessary. We now have a higher-half kernel.
    mov esp, stack+STACKSIZE           ; set up the stack
    push eax                           ; pass Multiboot magic number

    ; pass Multiboot info structure -- WARNING: This is a physical address and may not be
    ; in the first 4MB!
    push ebx

    call  k_main                 ; call kernel proper
    hlt                          ; halt machine should kernel return


section .bss
align 32
stack:
    resb STACKSIZE      ; reserve 16k stack on a uint64_t boundary

This was loader.asm

I tried to solve this transforming the ASM block in an advanced ASM block and parsing gdtr as an argument but I don't understand this last method How can I solve the error?

Your error:

kc.o: In function `k_enter_protected_mode':
kernel.c:(.text+0x1e1): undefined reference to `gdtr'

Is being generated because of this line of assembly code:

"lgdt (gdtr);"

gdtr is a memory operand and represents a label to a memory address where a GDT record can be found. You don't have such a structure defined with that name. That causes the undefined reference.

You need to create GDT record that contains the size and length of a GDT table. This record is what will get loaded into the GDT register by the LGDT instruction. You also haven't created a GDT table. gdtr should be a 6 byte structure consisting of the length of a GDT minus 1 (stored in a 16-bit word) and a 32-bit linear address where the GDT table can be found.

Rather than doing what you want in C I recommend just doing this in your assembly code prior to call k_main but after paging is set up.

Remove your k_enter_protected_mode function altogether in the C code. Then in the assembly file loader.asm place this code to load a new GDT at the start of your StartInHigherHalf code. So it would look like:

StartInHigherHalf:
    ; Set our own GDT, can't rely GDT register being valid after bootloader
    ; transfers control to our entry point
    lgdt [gdtr]         ; Load GDT Register with GDT record
    mov eax, DATA_SEG
    mov ds, eax         ; Reload all the data descriptors with Data selector (2nd argument)
    mov es, eax
    mov gs, eax
    mov fs, eax
    mov ss, eax

    jmp CODE_SEG:.setcs
                        ; Do the FAR JMP to next instruction to set CS with Code selector, and
                        ;    set the EIP (instruction pointer) to offset of setcs
.setcs:

The only thing left is to define the GDT table. A simple one with a required NULL descriptor and a flat 32-bit code and data descriptor can be placed in your .data section by changing it to this:

section .data
align 0x1000
BootPageDirectory:
    ; This page directory entry identity-maps the first 4MB of the 32-bit physical address space.
    ; All bits are clear except the following:
    ; bit 7: PS The kernel page is 4MB.
    ; bit 1: RW The kernel page is read/write.
    ; bit 0: P  The kernel page is present.
    ; This entry must be here -- otherwise the kernel will crash immediately after paging is
    ; enabled because it can't fetch the next instruction! It's ok to unmap this page later.
    dd 0x00000083
    times (KERNEL_PAGE_NUMBER - 1) dd 0                 ; Pages before kernel space.
    ; This page directory entry defines a 4MB page containing the kernel.
    dd 0x00000083
    times (1024 - KERNEL_PAGE_NUMBER - 1) dd 0  ; Pages after the kernel image.


; 32-bit GDT to replace one created by multiboot loader
; Per the multiboot specification we Can't rely on GDTR
; being valid so we need our own if we ever intend to
; reload any of the segment registers (this may be an
; issue with protected mode interrupts).
align 8
gdt_start:
    dd 0                ; null descriptor
    dd 0

gdt32_code:
    dw 0FFFFh           ; limit low
    dw 0                ; base low
    db 0                ; base middle
    db 10011010b        ; access
    db 11001111b        ; 32-bit size, 4kb granularity, limit 0xfffff pages
    db 0                ; base high

gdt32_data:
    dw 0FFFFh           ; limit low (Same as code)
    dw 0                ; base low
    db 0                ; base middle
    db 10010010b        ; access
    db 11001111b        ; 32-bit size, 4kb granularity, limit 0xfffff pages
    db 0                ; base high
end_of_gdt:

gdtr:
    dw end_of_gdt - gdt_start - 1
                        ; limit (Size of GDT - 1)
    dd gdt_start        ; base of GDT

CODE_SEG equ gdt32_code - gdt_start
DATA_SEG equ gdt32_data - gdt_start

We've now added the required GDT structure and created a record called gdtr that can be loaded with the LGDT instruction.

Since you are using OSDev as a resource, I recommend looking at the GDT tutorial for information on creating a GDT. The Intel manuals are also an excellent source of information.

Other Observations

Your loader.asm sets up a Multiboot header so it is a good bet you are using a Multiboot compliant bootloader. When you use a Multiboot compliant bootloader your CPU will be placed into 32-bit protected mode before it starts running your code starting at _loader. Your question suggests that you think you are in real mode, but you are actually already in protected mode. With a Mulitboot loader it isn't necessary to set CR0 bit 0 to a value of 1. It is guaranteed to already be 1 (set). In my code above I have removed it after setting up the GDT.