[solved] print to STDOUT acts strangely.

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ntos

Joined: 06 Jul 2023
Posts: 18

ntos 23 Jul 2023, 13:48

Hi,
Here is the code:

Code:

 format ELF64 executable 3

segment readable executable
entry main
main:
    ;write your code here
    mov rax, 1
    mov rdi, 1
    mov rsi, msg
    xor rdx, rdx
    mov rdx, var2
    syscall
    
    mov rdi, 0
    mov rax, 60
    syscall   
    
segment readable writable
msg db 'hello world!', 10, 0
var2 dw 2

AFAIK rdx contains the length of the string, but I loaded it with var2 and the code still produced the correct string hello world!. Why?

23 Jul 2023, 13:48

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20571
Location: In your JS exploiting you and your system

revolution 23 Jul 2023, 14:28

RDX contains the address of var2, so it might be 0x4010ef or similar.

Try this;

Code:

;...
    mov rdx, var2 - msg
    syscall

23 Jul 2023, 14:28

Flier-Mate

Joined: 26 May 2023
Posts: 88

Flier-Mate 23 Jul 2023, 15:37

By the way, can I ask a related question? Can I use exit_group syscall next time instead of exit syscall, since :

https://man7.org/linux/man-pages/man2/exit_group.2.html wrote:

Since glibc 2.3, this is the system call invoked when the
_exit(2) wrapper function is called.

I saw @revolution used exit_group in his programs.

23 Jul 2023, 15:37

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20571
Location: In your JS exploiting you and your system

revolution 23 Jul 2023, 15:46

I recommend to always use exit_group unless you really know what you are doing and actually want the obscure non-exiting behaviour of exit.

23 Jul 2023, 15:46

Flier-Mate

Joined: 26 May 2023
Posts: 88

Flier-Mate 23 Jul 2023, 15:52

revolution wrote:

I recommend to always use exit_group unless you really know what you are doing and actually want the obscure non-exiting behaviour of exit.

Thanks for the recommendation.

23 Jul 2023, 15:52

ntos

Joined: 06 Jul 2023
Posts: 18

ntos 23 Jul 2023, 22:07

revolution wrote:

RDX contains the address of var2, so it might be 0x4010ef or similar.

Try this;
Code:
;...
    mov rdx, var2 - msg
    syscall    

thanks.

23 Jul 2023, 22:07

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