Question

I have a sample code run in kernel(2.6.30 x86_64) mode(r0),trying to simulate a iret.I push variables under intel manual’s guide.but it turns out a runtime fault right in the pos of iret instruction:

general protection fault:fffc[#] SMP

asm volatile(
    "mov %%ss,%%ax \n\t"
    "push %%rax \n\t"/*ss*/
    "push %%rsp \n\t"/*rsp*/
    "pushfq \n\t"/*rflags*/
    "mov %%cs,%%ax \n\t"
    "push %%rax \n\t"/*cs*/
    "mov $._restart_code,%%rax \n\t"
    "push %%rax \n\t"/*rip*/
    "iret \n\t"/*here is the fault rip!!!!!!*/
    "._restart_code:"
    "nop" :);

Answer 1

the bug is that rsp is changed after push.

asm volatile(
    "mov %%ss,%%ax \n\t"
    "push %%rax \n\t"/*ss*/
    "push %%rsp \n\t"/*rsp ##########error here!!!!!!! */ 
    "pushfq \n\t"/*rflags*/
    "mov %%cs,%%ax \n\t"
    "push %%rax \n\t"/*cs*/
    "mov $._restart_code,%%rax \n\t"
    "push %%rax \n\t"/*rip*/
    "iret \n\t"/*###iretq should be used under 64bit mode*/
    "._restart_code:"
    "nop" :);

so,save the rsp before all push instructions. and that correct code is:

asm volatile(
        "mov %%rsp,%%rbx \n\t"
        "mov %%ss,%%ax \n\t"
        "push %%rax \n\t"/*ss*/
        "push %%rbx \n\t"/*rsp*/ 
        "pushfq \n\t"/*rflags*/
        "mov %%cs,%%ax \n\t"
        "push %%rax \n\t"/*cs*/
        "mov $._restart_code,%%rax \n\t"
        "push %%rax \n\t"/*rip*/
        "iretq \n\t"
        "._restart_code:"
        "nop" :);

Thanks!!!