nasm，64，Linux，分段故障核心已转储

Question

this is foo.asm

extern choose;
[section .data]
num1st dq 3
num2nd dq 4
[section .text]
global main
global myprint
main:
  push qword [num2nd]
  push qword [num1st]
  call choose
  add esp,8
  mov ebx,0
  mov eax,1
  int 0x80
  ;  pop qword [num1st]
  ;  pop qword [num2nd]
myprint:
  mov edx,[esp+8]
  mov ecx,[esp+4]
  mov ebx,1
  mov eax,4
  int 0x80
  ;  pop qword [num1st]
  ;  pop qword [num2nd]
  ret

it is a C-asm-program

this is bar.c

void myprint(char * msg ,int len);
int choose(int a,int b) 
{ 
  if (a>=b){
    myprint("the 1st one\n",13);}
  else {
    myprint("the 2nd one\n",13);}
  return 0;
}

nasm -f elf64 foo.asm

gcc -c bar.c

gcc -s -o foobar bar.o foo.o

./foobar ，it says segmentation fault core dumped

I use gdb to debug ,but it says missing debuginfo-install, I am also trying to install it.

maybe the problem has sth to do with the 86_64 arch...

Segmentation fault when pushing on stack (NASM) after watched this link,I add some 'pop' into it but it doesn't work

Answer 1

Arguments are not passed on the stack in 64-bit mode , unless you have more than 6 of them. The first two arguments will be in RDI and RSI .

There's also a difference in how you should use system calls in 64-bit mode. The syscall number and arguments should be placed in the following registers ( source ):

syscall nr  rax
arg 1       rdi
arg 2       rsi
arg 3       rdx
arg 4       r10
arg 5       r9
arg 6       r8

And the sys_write syscall number in 64-bit mode is 1, not 4 . Also, instead of int 0x80 you should use syscall . Performing syscalls with int 0x80 might work in 64-bit mode depending on how your kernel has been configured, but you still need to consider how function arguments are passed.