重定位被截断以适应 r_386_8 反对 .bss'

Question

当我输入 ld -m elf_i386 -o loop loop.asm 时，我得到标题中所述的错误，知道是什么原因造成的吗？对不起，如果代码看起来很糟糕，对汇编来说还很新。

cr equ 13 
lf equ 10 

section .bss
numA resb 1

section .text

global _start:

mov [numA],byte 0
call loop1
jmp endend
loop1:
xor cx,cx
mov al, $numA
cmp cx, 0x0A
jle else 
inc al
jmp end
else:
dec al
jmp end
end:
mov [$numA], al
inc cx
cmp cx,20
jle loop1

endend:
mov dl,$numA
mov ah,2
int 21h

Answer 1

在 NASM 中，$numA 与 numA 相同。 A leading $ stops the assembler from considering it as a register name。因此，您可以编写mov eax, [$eax] 以从名为eax 的符号加载eax 寄存器。 （所以你可以链接到使用int eax = 123;的C）

所以mov [$numA], al 看起来很奇怪，但实际上它只是mov [numA], al，并不是错误的根源。

---

您收到来自mov dl,$numA 的错误，它对地址的低字节执行mov dl, imm8。

链接器会警告您，因为 numA 的地址不适合 1 个字节，因此 r_386_8 重定位将不得不截断地址。

_8 告诉您这是一个重定位，它要求链接器填写 8 位（1 个字节）作为绝对地址。 （8 位相对分支位移具有不同的重定位类型，尽管通常您会使用 32 位位移来跳转到另一个文件中的符号。）

r_386 告诉您它是 i386 重定位，而不是某种类型的 r_x86_64 重定位（可以是绝对的或 RIP 相对的），或 MIPS 跳转目标重定位（需要右移偏移 2)。可能相关：Relocations in the System V gABI（通用 ABI，i386 SysV psABI 是其“处理器补充”）。

Answer 2

cmets 开头的固定代码;* 关于我修改了什么：

;* build commands used to test:
;* nasm -f elf32 -F dwarf -g loop.asm -l loop.lst -w+all
;* ld -m elf_i386 -o loop loop.o

cr equ 13
lf equ 10

section .bss
numA resb 1

section .text

global _start   ;* global directive takes symbol name (without colon)

_start:
;* the actual label you defined as global, and want to start from.

    ;* set memory at numA address to byte zero
    mov [numA],byte 0
    ;* try to call subroutine with loop
    call loop1
    jmp endend

loop1:
    xor cx,cx       ;* loop counter = 0
.real_loop: 
;* you don't want to loop to "loop1" as that will reset CX!
    mov al, [$numA] ; load AL with value from memory at numA address
;* in NASM you must use [] to indicate memory load/store
;* the mov al, $numA tried to put the memory address numA into AL
;* but memory address in x86-32 is 32 bit value, and AL is 8 bit only
;* and you didn't want address, but value any way.
    cmp cx, 0x0A
    jle .else 
    inc al
    jmp .end
.else:
;* I modified all subroutine labels to be "local" starting with dot
;* i.e. ".else" is full label "loop1.else". This practice will allow
;* you to use also ".else" in different subroutines, while global
;* "else:" can be used only once per source file.
    dec al
    jmp .end
.end:
    mov [$numA], al
    inc cx
    cmp cx,20
    jle .real_loop      ;* fix of loop jump (to not reset CX)
    ;* after CX will reach value 21, the CPU will continue here
    ret         ;* so added return from subroutine

endend:
    ;* call linux 32b sys_exit(numA value) to terminate
    ;* return value will be equal to zero-extended [numA] to 32bits
    ;* 8bit -1 = 0xFF -> return value is 255
    movzx   ebx,byte [$numA]
    mov     eax,1
    int     80h

运行后：

nasm -f elf32 -F dwarf -g loop.asm -l loop.lst -w+all
ld -m elf_i386 -o loop loop.o
./loop ; echo $?

预期的输出：

255