Hind Forsum Hind Forsum - 1 year ago 90
Linux Question

AT&T assembly try to "movb" a character constant to register, core dump

I'm using ubuntu64 + gas
I use AT&T assembly, I tried to assign al register with a 'e', and compile it into 32 bit program

$ cat c2.s

.globl _start
movb 'e',%al # Problem here!!!!
mov $1,%eax
mov $0,%ebx
int $0x80

$ as -g c2.s -o c2.o && ld c2.o -o c2

$ c2

Segmentation fault(SIGSEGV)

I used gdb to debug c2, and found it crash at movb 'e',%al. So weird, how could a "movb" crash?

Then I switched my syntax to use intel same content:

$ cat b2.s

.intel_syntax noprefix
.section .text
.global _start
mov al,'e'
mov eax,1
mov ebx,0
int 0x80

$ as -g b2.s -o b2.o && ld b2.o -o b2

$ b2

This time, no problem. But why, is my usage of AT&T assembly has something wrong?

Answer Source

Isn't that interesting. AS produces this object code for AT&T syntax

  400078:   a0 65 00 00 00 b8 01    movabs 0x1b800000065,%al
  40007f:   00 00 
  400081:   00 bb 00 00 00 00       add    %bh,0x0(%rbx)
  400087:   cd 80                   int    $0x80

obviously, location 0x1b800000065 is not mapped, but Intel;

  400078:   b0 65                   mov    $0x65,%al
  40007a:   b8 01 00 00 00          mov    $0x1,%eax
  40007f:   bb 00 00 00 00          mov    $0x0,%ebx
  400084:   cd 80                   int    $0x80

remove .code32 from AT&T and you get this.

  400078:   8a 04 25 65 00 00 00    mov    0x65,%al
  40007f:   b8 01 00 00 00          mov    $0x1,%eax
  400084:   bb 00 00 00 00          mov    $0x0,%ebx
  400089:   cd 80                   int    $0x80

Notice how it wants to move the contents of memory location 0x64 into AL.

movb   $'e',%al

fixes that problem. In any event, developing 32 bit code on a 64 bit system will probably give you grief at some point in time, especially when you start dealing with stack.

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