GDT Flush fails the entire kernel

Question

I am writing an x86_32 bit kernel from scratch in C, after writing a VGA driver, I have been trying to implement the GDT as well as the interrupts for a keyboard driver. However, my GDT_flush() function that is being called from assembly fails and it results in a bootloop. commenting that function, and it boots just fine( but without the GDT obviously ).

I am simulating the kernel on Qemu, and using a target specific toolchain to compile and link all the files together.

GDT.c:

struct gdt_entry{
  unsigned short limit_low;
  unsigned short base_low;
  unsigned char base_middle;
  unsigned char access;
  unsigned char granularity; 
  unsigned char base_high;
}__attribute__((packed));


struct gdt_pointer{
  unsigned short limit;
  unsigned int base;
}__attribute__((packed));

struct gdt_entry gdt[3];
struct gdt_pointer gdtptr;

//extern void flush_gdt(unsigned char gdtptr);
extern void flush_gdt();

void set_gdt_gate(int num, unsigned long base, unsigned long limit,unsigned char access,unsigned char gran){
  gdt[num].base_low = (base & 0xFFFF);
 gdt[num].base_middle = (base >> 16) & 0xFF;
  gdt[num].base_high = (base >> 24) & 0xFF;

  gdt[num].limit_low = (limit & 0xFFFF);
  gdt[num].granularity = (limit >> 16) & 0x0F;
  gdt[num].granularity |= (gran & 0x0F);

  gdt[num].access = access;
}

void init_gdt(){
  gdtptr.limit = (sizeof(struct gdt_entry) * 3) - 1;
  gdtptr.base = (uint32_t)&gdt;

  print_string("setting up kernel segments \n");
  set_gdt_gate(0, 0, 0, 0, 0); //Null segment
  set_gdt_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF); //Code segment
  set_gdt_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF); //data segment
  print_string("kernel segments have been setup hehehhehehe\n");
  //flush_gdt((unsigned char)&gdtptr);
  flush_gdt();
}

descriptor.asm:

global flush_gdt
[extern gdtptr]

flush_gdt:
  ;mov eax, [esp + 4]
  ;lgdt[eax]
  lgdt[gdtptr]
  mov ax, 0x10
  mov ds, ax
  mov es, ax
  mov fs, ax 
  mov gs, ax
  mov ss, ax
  jmp 0x08: .flush_two 

.flush_two:
  ret

I looked up other similar projects and my implementation isn't very different. I've been trying to solve it for a couple of weeks now, any help is very appreciated.

thanks :)

I have tried passing the descriptor pointer as an argument to the function being called, but that also results in a bootloop.

Here is the QEMU log file:

0: v=0d e=0010 i=0 cpl=0 IP=0010:0000000000100d3b pc=0000000000100d3b SP=0018:0000000000104f1c env->regs[R_EAX]=0000000000100010
EAX=00100010 EBX=001066c8 ECX=000003d5 EDX=000003d5
ESI=00000000 EDI=00000000 EBP=00104f38 ESP=00104f1c
EIP=00100d3b EFL=00000002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
ES =0018 00000000 ffffffff 00cf9300 DPL=0 DS   [-WA]
CS =0010 00000000 ffffffff 00cf9a00 DPL=0 CS32 [-R-]
SS =0018 00000000 ffffffff 00cf9300 DPL=0 DS   [-WA]
DS =0018 00000000 ffffffff 00cf9300 DPL=0 DS   [-WA]
FS =0018 00000000 ffffffff 00cf9300 DPL=0 DS   [-WA]
GS =0018 00000000 ffffffff 00cf9300 DPL=0 DS   [-WA]
LDT=0000 00000000 0000ffff 00008200 DPL=0 LDT
TR =0000 00000000 0000ffff 00008b00 DPL=0 TSS32-busy
GDT=     696c6261 00006e65
IDT=     00000000 00000000
CR0=00000011 CR2=00000000 CR3=00000000 CR4=00000000
DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 DR3=0000000000000000 
DR6=00000000ffff0ff0 DR7=0000000000000400
CCS=00000010 CCD=00104f10 CCO=ADDL
EFER=0000000000000000
check_exception old: 0xd new 0xd
     1: v=08 e=0000 i=0 cpl=0 IP=0010:0000000000100d3b pc=0000000000100d3b SP=0018:0000000000104f1c env->regs[R_EAX]=0000000000100010
EAX=00100010 EBX=001066c8 ECX=000003d5 EDX=000003d5
ESI=00000000 EDI=00000000 EBP=00104f38 ESP=00104f1c
EIP=00100d3b EFL=00000002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
ES =0018 00000000 ffffffff 00cf9300 DPL=0 DS   [-WA]
CS =0010 00000000 ffffffff 00cf9a00 DPL=0 CS32 [-R-]
SS =0018 00000000 ffffffff 00cf9300 DPL=0 DS   [-WA]
DS =0018 00000000 ffffffff 00cf9300 DPL=0 DS   [-WA]
FS =0018 00000000 ffffffff 00cf9300 DPL=0 DS   [-WA]
GS =0018 00000000 ffffffff 00cf9300 DPL=0 DS   [-WA]
LDT=0000 00000000 0000ffff 00008200 DPL=0 LDT
TR =0000 00000000 0000ffff 00008b00 DPL=0 TSS32-busy
GDT=     696c6261 00006e65
IDT=     00000000 00000000
CR0=00000011 CR2=00000000 CR3=00000000 CR4=00000000
DR0=0000000000000000 DR1=0000000000000000 DR2=0000000000000000 DR3=0000000000000000 
DR6=00000000ffff0ff0 DR7=0000000000000400
CCS=00000010 CCD=00104f10 CCO=ADDL
EFER=0000000000000000
check_exception old: 0x8 new 0xd

edit:

here is the github repo:

https://github.com/MadhavBh/FilterCoffee

apologies for the messy code :') Still very new at this.

Answer 1

gdt[num].granularity = (limit >> 16) & 0x0F;
gdt[num].granularity |= (gran & 0x0F);

You have the the granularity bits and limit bits being mashed into the same 4 bits. The second line should possibly read:

gdt[num].granularity |= (gran & 0xF0);