CODEWITHSUNDEEP
macosassemblynasm
carloabelli
carloabelli

Reputation: 4349

Nasm Hello World Bus Error OS X

I have the following assembly code (copied from here):

hello.asm:

section .data
    msg: db  'hello, world!', 0

section .text

global _start
_start:
    mov     rax, 4
    mov     rdi, 1
    mov     rsi, qword msg ; I added qword because the compiler complained
    mov     rdx, 13
    syscall

    mov     rax, 1
    xor     rdi, rdi
    syscall

As explained in the comment above I added qword because I recieved the following error message:

$ yasm -f macho64 hello.asm
hello.asm:10: error: macho: sorry, cannot apply 32 bit absolute relocations in 64 bit mode, consider "[_symbol wrt rip]" for mem access, "qword" and "dq _foo" for pointers.

After making that change, both compiling and linking succeed, but I get a bus error while running:

$ yasm -f macho64 hello.asm
$ ld -o hello -e _start hello.o
$ ./hello
bus error  ./hello

Is the addition of qword incorrect, or is there something else I need to change?

Upvotes: 1

Views: 986

Answers (1)

Michael
Michael

Reputation: 58427

64-bit system calls on OSX need to have a class specifier in bits 24..31 of the register that holds the system call number (rax in your case). You've implicitly used the class specifier 0, which is invalid.

You probably want the UNIX/BSD class, which means that you should be adding 2<<24 (0x2000000). So these lines:

mov     rax, 4
mov     rax, 1

should be changed to:

mov     rax, 0x2000004
mov     rax, 0x2000001

Also, I don't see what purpose that sub rsp, 8 at the end is serving, so you can probably remove that line.

Upvotes: 4

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