GOT error in gas

[Mikulas Patocka]

Hi

I found the following issue. This program:

#include <stdio.h>
__asm__ (".global number; number = 0x12345678");
extern void number;
int main()
{
        printf("%p\n", &number);
        return 0;
}

works when compiled without -fPIC and segfaults when compiled with -fPIC. 
When the program is broken to two files (one file containing __asm__ line 
and the second file containing main), it works fine even with -fPIC --- 
even if I break it into two files and link them dynamically with each 
other, it works and prints correct number --- it just doesn't work when 
all program is contained in one file.

It seems like an error in gas --- gas replaced address@hidden with constant 
0x12345678 when number was constant --- it probably errorneously thought 
that address@hidden is equivalent to number for costant numbers.

(tested on i386)

Mikulas


Output of gcc:
        .file   "got.c"
#APP
        .global number; number = 0x12345678
#NO_APP
        .section        .rodata.str1.1,"aMS",@progbits,1
.LC0:
        .string "%p\n"
        .text
        .p2align 4,,15
.globl main
        .type   main, @function
main:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ebx
        subl    $20, %esp
        andl    $-16, %esp
        call    __i686.get_pc_thunk.bx
        addl    $_GLOBAL_OFFSET_TABLE_, %ebx
        movl    address@hidden(%ebx), %eax
        subl    $16, %esp
        movl    %eax, 4(%esp)
        leal    address@hidden(%ebx), %eax
        movl    %eax, (%esp)
        call    address@hidden
        movl    -4(%ebp), %ebx
        xorl    %eax, %eax
        leave
        ret
        .size   main, .-main
        .section 
.gnu.linkonce.t.__i686.get_pc_thunk.bx,"ax",@progbits
.globl __i686.get_pc_thunk.bx
        .hidden __i686.get_pc_thunk.bx
        .type   __i686.get_pc_thunk.bx, @function
__i686.get_pc_thunk.bx:
        movl    (%esp), %ebx
        ret
        .section        .note.GNU-stack,"",@progbits
        .ident  "GCC: (GNU) 3.4.6"


objdump -d:
00000000 <main>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   53                      push   %ebx
   4:   83 ec 14                sub    $0x14,%esp
   7:   83 e4 f0                and    $0xfffffff0,%esp
   a:   e8 fc ff ff ff          call   b <main+0xb>
   f:   81 c3 02 00 00 00       add    $0x2,%ebx
  15:   8b 83 78 56 34 12       mov    0x12345678(%ebx),%eax
                                !!!!!!!!!!!!!!!!!!
This causes crash!!!
  1b:   83 ec 10                sub    $0x10,%esp
  1e:   89 44 24 04             mov    %eax,0x4(%esp)
  22:   8d 83 00 00 00 00       lea    0x0(%ebx),%eax
  28:   89 04 24                mov    %eax,(%esp)
  2b:   e8 fc ff ff ff          call   2c <main+0x2c>
  30:   8b 5d fc                mov    0xfffffffc(%ebp),%ebx
  33:   31 c0                   xor    %eax,%eax
  35:   c9                      leave
  36:   c3                      ret
Disassembly of section .gnu.linkonce.t.__i686.get_pc_thunk.bx:

00000000 <__i686.get_pc_thunk.bx>:
   0:   8b 1c 24                mov    (%esp),%ebx
   3:   c3                      ret

objdump -r:
got.o:     file format elf32-i386

RELOCATION RECORDS FOR [.text]:
OFFSET   TYPE              VALUE
0000000b R_386_PC32        __i686.get_pc_thunk.bx
00000011 R_386_GOTPC       _GLOBAL_OFFSET_TABLE_
00000024 R_386_GOTOFF      .LC0
0000002c R_386_PLT32       printf
^^^^^ note that there is no R_386_GOT32 relocation for offset 00000017, 
although there should be.