Keep getting e8 00 00 00 00 as the machine code to call a function in assembly

Question

I know when using objdump -dr in my file call shows up in machine code as e8 00 00 00 00 because it has not yet been linked. But I need to find out what the 00 00 00 00 will turn into after the linker has done it's job. I know it should calculate the offset, but I'm a little confused about that.

As an example with the code below, after the linker part is done, how should the e8 00 00 00 00 be? And how do I get to that answer?

I'm testing out with this sample code: (I'm trying to call moo)

Disassembly of section .text:

0000000000000000 <foo>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   89 7d fc                mov    %edi,-0x4(%rbp)
   7:   8b 45 fc                mov    -0x4(%rbp),%eax
   a:   83 e8 0a                sub    $0xa,%eax
   d:   5d                      pop    %rbp
   e:   c3                      retq   

000000000000000f <moo>:
   f:   55                      push   %rbp
  10:   48 89 e5                mov    %rsp,%rbp
  13:   89 7d fc                mov    %edi,-0x4(%rbp)
  16:   b8 01 00 00 00          mov    $0x1,%eax
  1b:   5d                      pop    %rbp
  1c:   c3                      retq   

000000000000001d <main>:
  1d:   55                      push   %rbp
  1e:   48 89 e5                mov    %rsp,%rbp
  21:   48 83 ec 10             sub    $0x10,%rsp
  25:   c7 45 fc 8e 0c 00 00    movl   $0xc8e,-0x4(%rbp)
  2c:   8b 45 fc                mov    -0x4(%rbp),%eax
  2f:   89 c7                   mov    %eax,%edi
  31:   e8 00 00 00 00          callq  36 <main+0x19>
            32: R_X86_64_PC32   moo-0x4
  36:   89 45 fc                mov    %eax,-0x4(%rbp)
  39:   b8 00 00 00 00          mov    $0x0,%eax
  3e:   c9                      leaveq 
  3f:   c3                      retq

Answer 1

With objdump -r you have Relocations printed with your disassembly -d:

  31:   e8 00 00 00 00          callq  36 <main+0x19>
            32: R_X86_64_PC32   moo-0x4

ld-linux.so.2 loader will relocate objects (in modern world it will relocate even executable to random address) and fill the relocations with correct address.

Check with gdb by adding breakpoint at main and starting program (linker works before main function is started):

gdb ./program
(gdb) start
(gdb) disassemble main

If you want to compile the code without relocations, show source code and compilation options.