Code a simple thread with C and assembly

Question

I'm trying to code a simple user-level thread library as an exercise for my OS course. As the first step, I am trying to run a program and jump to a function leaving the first program. The code so far is this:

The initial program:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <assert.h>

#define STACK_SIZE (sizeof(void *) * 512)


void proc2() //This is the function that should run as the thread.
{
    int i;
    for(i=0;i<30;i++)
    {
        printf("Here I am!\n");
        sleep(0.5);
    }
    exit(0);
}

void* malloc_stack() //used to malloc the stack for the new thread. 
{
    void *ptr = malloc(STACK_SIZE + 16);
    if (!ptr) return NULL;
        ptr = (void *)(((unsigned long)ptr & (-1 << 4)) + 0x10); //size align
    return ptr;
}

int main()
{
    int *bp, *sp; 
    sp = malloc_stack();
    bp  = (int*) ((unsigned long)sp + STACK_SIZE);
    proc1(&proc2,sp,bp); //the actual code that runs the thread. Written in assembly
    assert(0);
}

And then I wrote a simple assembly code called proc1 that takes three arguments, pointer to a function (used as the instruction pointer), stack pointer and base pointer and replace the current registers with these values. The code I wrote is:

.globl  proc1
proc1:   
movq    %rdx, %rbp        #store the new base pointer
movq    %rsi,%rsp         #store the new stack pointer  
jmp     %rdi              #jump to the new instruction pointer.

But when I run this code, what I get is a segmentation fault. Please help me to find the error here.

Well it's working correctly when I ran it under GDB using the following commands:

gcc -g test.c switch.s
gdb a.out
run

but when it rus alone as in ./a.out, it doesn't work!!!! Please help.

Thanks in advance.

Answer 1

Try changing your code to include the assembly instructions directly in the C source as follow:

void proc1(void (*fun)(), int *sp, int *bp){
    register int *sptr asm ("%rsi") = sp;
    register int *bptr asm ("%rdx") = bp;
    register void (*fptr)() asm ("%rdi") = fun;

    asm (
        "mov %rdx, %ebp\n"
        "mov %rsi, %esp\n"
        "jmp *%rdi\n"
    );
}

The above code ensures that the parameters to proc1 are in the right registers (although your code seems to be correct wrt the abi). Note the * in front of the jmp argument, which my version of gnu as warned about when I first tried your code.

With the above function, and code compiled with -g you should be able to debug it properly (use the breakpoint instruction on proc1 and info registers to check the content of the cpu).

---

The issue is actually on the %rsp pointer, which must always be equal to or greater than %rbp (the stack grows downward). Simply passing bp instead of sp to proc1 in main should fix the issue:

 proc1(&proc2, bp, bp);

2 small remarks:

• don't forget to give proc1 prototype in the C code for the asm version:

  extern void proc1(void (*)(), int *, int *);
  

• the sleep libc function only take unsigned long, not float.

  sleep(1);
  

Answer 2

Your movq at the top of your assembly are (well, "were" before you edited :-) ) written as

movq dst,src

but your movq before the jmp is written movq %rax,%rsp and %rsp is clearly the desired dst. That's obviously wrong, not sure about anything else.