details of address relocation procedure

[sean yang]

I want to know some details about symbol address relocation procedure. 
Thanks for your patience to read.

Here I use a simple example to make the question clear:
we have two files:
-----------------------
//in main.c
extern int foo();
extern int foo2();
extern int bar;
int main(){
        bar++;
        foo();
        foo2();
}

--------
//in foo.c
int bar =1;
int foo2(){
        bar>>2;
}
int foo(){
        bar++;
}
------------------------

after >gcc -o *.c, I think the symbol table in main.o should include "foo" 
and "bar" with type of Undefined, which in turn should be resolved during 
the linking procedure.
But how can a linker backpatch the resolved address to the instruction "e8 
fc ff ff ff          call   17 <main+0x17>", after it figures out the 
address of foo()? As both call foo and call foo2 are represented by "e8 fc 
ff ff ff ", how can the linker tell them apart and `backpatch' correctly? a 
related question is: does opcode fc ff ff ff (following e8) means "address 
to be solved"? does opcode  00 00 00 00 (following ff 05) means variable 
address to be solved"?

----------------------------------------
//objdump -D main.o
Disassembly of section .text:
00000000 <main>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   83 ec 08                sub    $0x8,%esp
   6:   83 e4 f0                and    $0xfffffff0,%esp
   9:   b8 00 00 00 00          mov    $0x0,%eax
   e:   29 c4                   sub    %eax,%esp
  10:   ff 05 00 00 00 00       incl   0x0
  16:   e8 fc ff ff ff          call   17 <main+0x17>
  1b:   e8 fc ff ff ff          call   1c <main+0x1c>
  20:   c9                      leave
  21:   c3                      ret
Disassembly of section .data:

---------------------------------
//objdump -d foo.o
Disassembly of section .text:
00000000 <foo2>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   c9                      leave
   4:   c3                      ret
00000005 <foo>:
   5:   55                      push   %ebp
   6:   89 e5                   mov    %esp,%ebp
   8:   ff 05 00 00 00 00       incl   0x0
   e:   c9                      leave
   f:   c3                      ret
Disassembly of section .data:
00000000 <bar>:
   0:   01 00                   add    %eax,(%eax)
        ...
-------------------------------------------------

now, we do >gcc -o a.out main.o foo.o, which drives the ld to do the 
relocation as one of its job.

My final question is about the ld source code: which functions(and .c file) 
in the ./binutils-2.16.1/  are in charge of the relocation procedure? In 
other words, I'd like to know (1) the function that enables ld to find the 
location where an address needs to be resolved (e.g.  16:   e8 fc ff ff ff   
        call   17 <main+0x17> in main.o); (2) the function actually get the 
resolved address and write the address to the corresponding place.

Please give a hint that where I should start to look at. Thanks,
-----------------------------------------------
//objdump -d a.out
....
08048310 <main>:
8048310:       55                      push   %ebp
8048311:       89 e5                   mov    %esp,%ebp
8048313:       83 ec 08                sub    $0x8,%esp
8048316:       83 e4 f0                and    $0xfffffff0,%esp
8048319:       b8 00 00 00 00          mov    $0x0,%eax
804831e:       29 c4                   sub    %eax,%esp
8048320:       ff 05 28 94 04 08       incl   0x8049428
8048326:       e8 0e 00 00 00          call   8048339 <foo>
804832b:       e8 04 00 00 00          call   8048334 <foo2>
8048330:       c9                      leave
8048331:       c3                      ret
8048332:       90                      nop
8048333:       90                      nop

08048334 <foo2>:
8048334:       55                      push   %ebp
8048335:       89 e5                   mov    %esp,%ebp
8048337:       c9                      leave
8048338:       c3                      ret

08048339 <foo>:
8048339:       55                      push   %ebp
804833a:       89 e5                   mov    %esp,%ebp
804833c:       ff 05 28 94 04 08       incl   0x8049428
8048342:       c9                      leave
8048343:       c3                      ret
......
====================================

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