CODEWITHSUNDEEP
linuxassemblygccsystem-calls
Martin.H
Martin.H

Reputation: 3

Assembly , syscall not work as expected. Ubuntu Linux x86_64 , using AT&T syntax

I am testing use of .bss for allocation of a memory area to hold a single number. Then print that number to console. The output is not as expected. I am supposed to get e number (12), but get a newline.

System config:

$ uname -a
Linux 5.8.0-48-generic #54~20.04.1-Ubuntu SMP Sat Mar 20 13:40:25 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

description: CPU
product: Intel(R) Core(TM) i5-8350U CPU @ 1.70GHz

The code:

# compile with: gcc -ggdb -nostdlib -no-pie  test.s -o test

.bss
.lcomm          output,1

.global _start
.text

_start:
        # test .bss and move numer 12 to rbx where memory are allocated in .bss
        mov     $output, %rbx    # rbx to hold address of allocated space
        mov     $12,%rdx          # Move a number to rdx
        mov     %rdx,(%rbx)       # Move content in rdx to the address where rbx points to (e.g ->output)

        # setup for write syscall:  
        mov     $1,%rax          # system call for write, according to syscall table (http://blog.rchapman.org/posts/Linux_System_Call_Table_for_x86_64/)
        mov     $1,%rdi          # fd = 1, stdout
        mov     $output,%rsi     # adress of string to output moved to rsi
        mov     $1,%rdx          # number of bytes to be written

        syscall                  # should write 12 in console

        mov     $60,%rax
        xor     %rdi,%rdi
        syscall                 # exit normally

I have set a breakpoint with the first syscall (using GDB), to look into the registers:

i r rax rbx rdx rdi rsi

rax            0x1                 1
rbx            0x402000            4202496
rdx            0x1                 1
rdi            0x1                 1
rsi            0x402000            4202496

x/1 0x402000
0x402000 <output>:  12

The output after syscall is blank, was expected to get the number "12":

:~/Dokumenter/ASM/dec$ gcc -ggdb -nostdlib -no-pie  test.s -o test
:~/Dokumenter/ASM/dec$ ./test

:~/Dokumenter/ASM/dec$ ./test

:~/Dokumenter/ASM/dec$

So, my question is, are there any obvious explanation of why I am getting blank and not 12 ?

Upvotes: 0

Views: 214

Answers (1)

paxdiablo
paxdiablo

Reputation: 881093

mov     $output,%rsi     # address of string to output moved to rsi
                                      ^^^^^^

Address of string. The value $12 is not the character sequence "12". If you wanted to print the string 12, you would need to load 0x31 and 0x32 ('1' and '2') into the memory area (making it big enough) the use 2 as the length.

For example, movw $0x3231, output or better movw $0x3231, output(%rip) to use RIP-relative addressing for static data, like normal for x86-64. (Unlike NASM, GAS syntax doesn't $'12' as a way to write the same integer constant.)

If you want to print an integer as a string, you'll probably want to manipulate it mathematically so you can do it one digit at a time. (Printing an integer as a string with AT&T syntax, with Linux system calls instead of printf)

Upvotes: 2

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