assembly - 汇编程序中的重定位截断错误

Question

我对汇编语言完全陌生。我尝试编写简单的程序来破译凯撒密码。问题是链接后我收到这些错误:

cezar.o: In function `loop':
(.text+0xbf): relocation truncated to fit: R_X86_64_8 against `.data'
cezar.o: In function `check_letter': 
(.text+0xd5): relocation truncated to fit: R_X86_64_8 against `.data'

老实说 - 我不知道这意味着什么。有谁能够帮助我？相反，#explainMeLikeI'm5 ;P

如果需要我的代码:

.data
STDIN = 0
STDOUT = 1
SYSWRITE = 0
SYSREAD = 1
SYSEXIT = 60
EXIT_SUCCESS = 0
BUFLEN = 512
BASE = 10
BASE_NUM = 10
BASE_CHAR = 26
NUM_BEG = 48

error_msg: .ascii "Bad characters in key\n"
error_msg_len = .-error_msg

key: .ascii "-128"
key_len = .-key

.bss
.comm textin, 512
#.comm keyin, 512
.comm textout, 512

.text
.globl _start

_start:
#check if key is good
movq $0, %rdi
movb key(, %rdi, 1), %bl    #load first char to bl
cmp $'-', %bl           #check if it's -
jne error
movq $1, %rdi           #counter
movb key(, %rdi, 1), %bl 
cmp $'0', %bl           
jl error                
cmp $'9', %bl           
jg error

mov $key_len, %rdi
mov $1, %r11            #powers of 10
mov $0, %r8             #calculated key goes to r8
sub $1, %rdi            #because I want numbers only, "-" doesn't interest me now

ascii_to_num:
cmp $1, %rdi            #at rdi=0 jump out of loop
jl load_text        

mov $0, %rax            #clearing rax
mov key(, %rdi, 1), %al 
sub $NUM_BEG, %al       
mul %r11                #multiplying al by power of it's position
add %rax, %r8           

mov %r11, %rax          
mov $10, %r12       
mul %r12                
mov %rax, %r11          

dec %rdi                #decrementation
jmp ascii_to_num        


load_text:
movq $SYSREAD, %rax     
movq $STDIN, %rdi       
movq $textin, %rsi      
movq $BUFLEN, %rdx      

dec %rax                #delete '\n'
movq %rax, %r10         

movq $0, %rdi           #counter
loop:
movb textin(, %rdi, 1), %bh 
                        #check if number
movb $'0', %bl          #I'm using bl for comparing
cmp %bl, %bh            
jl ending_loop          
movb $'9', %bl
cmp %bl, %bh            
jg check_letter         
add $key, %bh           
cmp $'0', %bh
jl correct_num
jmp ending_loop

check_letter:
movb $'A', %bl
cmp %bl, %bh            
jl ending_loop          
movb $'Z', %bl
cmp %bl, %bh            
jg ending_loop          
add $key, %bh           
cmp $'A', %bh   
jl correct_char 
jmp ending_loop

correct_num:            
add $BASE_NUM, %bh
cmp $'0', %bh
jl correct_num
jmp ending_loop

correct_char:       
add $BASE_CHAR, %bh
cmp $'A', %bh
jl correct_char
jmp ending_loop

ending_loop:
movb %bh, textout(, %rdi, 1)
inc %rdi
cmp %r10, %rdi
jle loop
jmp show

error:                  #error message
mov $SYSWRITE, %rax
mov $STDOUT, %rdi
mov $error_msg, %rsi
mov $error_msg_len, %rdx
syscall

show:                   #display message and close programm
add $'\n', textout
mov $SYSWRITE, %rax
mov $STDOUT, %rdi
mov $textout, %rsi
mov $BUFLEN, %rdx
syscall

mov $SYSEXIT, %rax
mov $EXIT_SUCCESS, %rdi
syscall

Answer 1

错误指的是您编写的两行:

add $key, %bh

这会尝试将key的值(即字符串-128的地址)添加到bh。由于地址大于 255，因此无法放入 bh 等字节寄存器中。因此，您会收到链接器指示的错误。

我还没有分析过你尝试将key的地址加载到bh中来做什么，但这样做是行不通的。如果要加载key处数据的第一个字节，请写入

add key(%rip), %bh

如果你想加载key的地址，你需要使用lea将其加载到64位寄存器中，如下所示:

lea key(%rip), %rbx