I have a small example program written in NASM(2.11.08) targeting the macho64 architecture. I'm running OSX 10.10.3:
bits 64
section .data
msg1 db 'Message One', 10, 0
msg1len equ $-msg1
msg2 db 'Message Two', 10, 0
msg2len equ $-msg2
section .text
global _main
extern _printf
_main:
sub rsp, 8 ; align
lea rdi, [rel msg1]
xor rax, rax
call _printf
lea rdi, [rel msg2]
xor rax, rax
call _printf
add rsp, 8
ret
I'm compiling and linking using the following command line:
/usr/local/bin/nasm -f macho64 test2.s
ld -macosx_version_min 10.10.0 -lSystem -o test2 test2.o
When I do an object dump on the test2 executable, this is the relevant snippet(I can post more if I'm wrong!):
0000000000001fb7 <_main>:
1fb7: 48 83 ec 08 sub $0x8,%rsp
1fbb: 48 8d 3d 56 01 00 00 lea 0x156(%rip),%rdi # 2118 <msg2+0xf3>
1fc2: 48 31 c0 xor %rax,%rax
1fc5: e8 14 00 00 00 callq 1fde <_printf$stub>
1fca: 48 8d 3d 54 00 00 00 lea 0x54(%rip),%rdi # 2025 <msg2>
1fd1: 48 31 c0 xor %rax,%rax
1fd4: e8 05 00 00 00 callq 1fde <_printf$stub>
1fd9: 48 83 c4 08 add $0x8,%rsp
1fdd: c3 retq
...
0000000000002018 <msg1>:
0000000000002025 <msg2>:
And, finally, the output:
$ ./test2
Message Two
$
My question is, what happened to msg1?
I'm assuming msg1 isn't printed because 0x14f(%rip)
is not the correct address (just nulls).
Why is lea edi, [rel msg2]
pointing to the correct address, while lea edi, [rel msg1]
is pointing past msg2, into NULLs?
It looks like the 0x14f(%rip)
offset is exactly 0x100 beyond where msg1 lies in memory (this is true throughout many tests of this problem).
What am I missing here?
Edit: Whichever message (msg1 or msg2) appears last in the .data section is the only message that gets printed.