%sp register doesn't point to stack

问题

I'm writing on a very basic kernel. I tried to write a function, with parameters passed through the stack. The kernel is compiled with nasm (like described in this question) and run with QEMU. I'm using gdb for debugging.

After a long while having problems I wrote this to test some basic stack operations:

BITS 16

global start
start:
    mov ax, 0x7C00
    add ax, 288
    mov ss, ax
    mov sp, 4096
    mov ax, 0x7C00
    mov ds, ax

test:
    push 42
    push 43
    push "T"
    pop ax
    pop ax
    push 44
    pop ax
    pop ax
    jmp $

Going through this step by step and looking what sp contains and looking what at the pointed address is reveals that sp is de-/incremented right, but the address it's pointing to always contains 0x0000.

I thought this could be related to the mov sp, 4096 line. So I commented it out. This didn't work either. The only difference was that the values sp points to are now some others but not the ones I pushed there.

Is there something I have to do to initialize the stack or something similar?

回答1:

Explanation

You want to look at 16*$ss + $esp in GDB. (Like Jester suggested in his comment)
This is explained in x86 Segmentation. Note the same applies to data memory access and the DS register.
You set SS to 0x7C00 + 288 and SP to 4096. Thus the physical stack pointer address is ((0x7c00+0x0120)<<4) + 0x1000 giving 0x7e200.
Writing all memory addresses and offsets in your code in hexadecimal may help with the arithmetic.

Scripting GDB to Examine the Stack

boot.asm

BITS 16

global start
start:
    mov ax, 0x7C00
    add ax, 0x0120
    mov ss, ax
    mov sp, 0x1000
    mov ax, 0x7C00
    mov ds, ax

test:
    push 42
    push 43
    push 'T'
    pop ax
    pop ax
    push 44
    pop ax
    pop ax
    hlt

examine-stack.gdb

set confirm 0
set pagination 0
set architecture i8086
target remote localhost:1234

file boot

set disassemble-next-line 1

define hook-stop
    printf "Stack Pointer: 0x%04x, AX: 0x%04x\n", ($ss*16 + $esp), $ax
    # after stack setup, the linear stack pointer address is 0x7e200
    set variable $sp_linear = 0x7e200
    x/8xb ($sp_linear - 8)
end

break test
continue

set variable $i = 0
while $i < 8
    stepi
    set variable $i = $i + 1
end

monitor quit
disconnect
quit

x86-boot.ld

ENTRY(start);
SECTIONS
{
    . = 0x7C00;
    .text : AT(0x7C00)
    {
        _text = .;
        *(.text);
        _text_end = .;
    }
    .data :
    {
        _data = .;
        *(.bss);
        *(.bss*);
        *(.data);
        *(.rodata*);
        *(COMMON)
        _data_end = .;
    }
    .sig : AT(0x7DFE)
    {
        SHORT(0xaa55);
    }
    /DISCARD/ :
    {
        *(.note*);
        *(.iplt*);
        *(.igot*);
        *(.rel*);
        *(.comment);
/* add any unwanted sections spewed out by your version of gcc and flags here */
    }
}

Build with:

nasm -g -f elf -F dwarf boot.asm -o boot.o
cc -nostdlib -m32 -T x86-boot.ld -Os -Wall -g3 -I. -Wl,--build-id=none  boot.o -o boot
objcopy -O binary boot boot.good.bin

Sample Session

$ qemu-system-x86_64 -s -S boot.good.bin &
$ gdb -q -x examine-stack.gdb

The target architecture is assumed to be i8086
0x0000fff0 in ?? ()
Breakpoint 1 at 0x7c10: file boot.asm, line 13.
Stack Pointer: 0x7e200, AX: 0x7c00
0x7e1f8:    0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00

Breakpoint 1, test () at boot.asm:13
13      push 42
=> 0x00007c10 : 6a 2a   push   $0x2a
Stack Pointer: 0x7e1fe, AX: 0x7c00
0x7e1f8:    0x00    0x00    0x00    0x00    0x00    0x00    0x2a    0x00
14      push 43
=> 0x00007c12 : 6a 2b   push   $0x2b
Stack Pointer: 0x7e1fc, AX: 0x7c00
0x7e1f8:    0x00    0x00    0x00    0x00    0x2b    0x00    0x2a    0x00
15      push 'T'
=> 0x00007c14 : 6a 54   push   $0x54
Stack Pointer: 0x7e1fa, AX: 0x7c00
0x7e1f8:    0x00    0x00    0x54    0x00    0x2b    0x00    0x2a    0x00
16      pop ax
=> 0x00007c16 : 58  pop    %ax
Stack Pointer: 0x7e1fc, AX: 0x0054
0x7e1f8:    0x00    0x00    0x54    0x00    0x2b    0x00    0x2a    0x00
17      pop ax
=> 0x00007c17 : 58  pop    %ax
Stack Pointer: 0x7e1fe, AX: 0x002b
0x7e1f8:    0x00    0x00    0x54    0x00    0x2b    0x00    0x2a    0x00
18      push 44
=> 0x00007c18 : 6a 2c   push   $0x2c
Stack Pointer: 0x7e1fc, AX: 0x002b
0x7e1f8:    0x00    0x00    0x54    0x00    0x2c    0x00    0x2a    0x00
19      pop ax
=> 0x00007c1a :    58  pop    %ax
Stack Pointer: 0x7e1fe, AX: 0x002c
0x7e1f8:    0x00    0x00    0x54    0x00    0x2c    0x00    0x2a    0x00
20      pop ax
=> 0x00007c1b :    58  pop    %ax
Stack Pointer: 0x7e200, AX: 0x002a
0x7e1f8:    0x00    0x00    0x54    0x00    0x2c    0x00    0x2a    0x00
21      hlt
=> 0x00007c1c :    f4  hlt

来源：https://stackoverflow.com/questions/22353268/sp-register-doesnt-point-to-stack

标签

assembly

gdb

stack

qemu

osdev