重定位被截断以适应 r_386_8 对 .bss'

Question

当我尝试使用

将我的源构建为 Linux 的 32 位静态可执行文件时

 nasm -f elf -F dwarf -g loop.asm
 ld -m elf_i386 -o loop loop.o

我收到这个 R_386_8 错误，知道是什么原因造成的吗？

foo.o: in function `loop1':
foo.asm:(.text+0x12): relocation truncated to fit: R_386_8 against `.bss'
foo.o: in function `endend':
foo.asm:(.text+0x2f): relocation truncated to fit: R_386_8 against `.bss'

loop.asm

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           ; note: DOS system calls won't work in Linux

（编者注：此代码有多个错误；此问答主要是关于阻止它链接的错误。但修复该错误不会使 Linux 程序正常工作。）

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] 从名为 eax 的符号加载 eax 寄存器。（所以你可以 link 和使用 int eax = 123; 的 C）

所以 mov [$numA], al 看起来很奇怪，但它实际上只是 mov [numA], al 而不是错误的来源。

你从 mov dl,$numA 得到错误，它对地址的低字节执行 mov dl, imm8。

大多数时候，这是 Basic use of immediates vs. square brackets in YASM/NASM x86 assembly 的情况 - 您打算从该地址的内存加载，例如 movzx edx, byte [numA] 或 mov dl, [numA]。

linker 警告您，因为 numA 的地址不适合 1 个字节，因此 r_386_8 重定位必须截断地址。

_8告诉你这是一个重定位，要求linker填写8位（1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 告诉您这是一个 i386 重定位，而不是某种类型的 r_x86_64 relocation (which could be absolute or RIP-relative), or a MIPS jump-target relocation (which would need to right-shift the offset by 2). Possibly related: Relocations in the System V gABI（通用 ABI，i386 SysV psABI 是“处理器补充”）。

Answer 2

固定代码，注释以 ;* 开头，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

在运行之后：

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

预期输出：

重定位被截断以适应 r_386_8 对 .bss'

relocation truncated to fit r_386_8 against .bss'

x86

assembly

nasm

relocation

ld