gistfile1.txt

# Little program for testing return from interrupt privilege check.
.intel_syntax noprefix

.text
.global main
main:

    # Print the CS
    mov eax, cs
    push eax
    lea eax, cs_is
    push eax
    call printf
    add esp, 8

    # Set up the interrupt stack stuff, that iret restores.
    mov eax, ss
    push  eax
    mov eax, esp    # ss:esp
    push eax
    pushfd          # EFLAGS
    mov eax, cs
    # Clear the CPL bits in cs (ring 0) 
    and eax, 0xFFFFFFFC
    push eax
    lea eax, rofl
    push eax        # cs:eip
    # The intel manual does not mention a privilege check when returning from
    # a ring 3 interrupt handler, so maybe this will actually work?
    # nope - generates a segfault on my machine.
    iret

    mov eax, 0
    ret

rofl:
    mov eax, 1
    mov ebx, 3
    int 0x80

cs_is: .asciz "cs is %d\n"
.data
gdtr: .byte 0x00, 0x00, 0x00, 0x00, 0x00, 0x00