Not sure why the stack is unaligned there. The way I solved this in liblinux is to just pass the stack pointer to a C function and compute all the other pointers there. That way the compiler deals with the stack stuff.

  _start:
      xorq %rbp,%rbp    /* user code should mark the deepest stack frame
                         * by setting the frame pointer to zero
                         */

      movq %rsp,%rdi

      call liblinux_start

      movq %rax,%rdi
      movq $__NR_exit,%rax
      syscall

  static void *after(void *vector) {
      void **pointer = (void **) vector;
      while (*pointer++ != 0);
      return pointer;
  }

  int liblinux_start(void *stack_pointer) {
      long count;
      char *arguments;
      char *environment;
      struct auxiliary *values;

      count = *((long *) stack_pointer);
      arguments = ((char **) stack_pointer) + 1;
      environment = arguments + count + 1;
      values = after(environment);

      /* start is the main function */
      return start(count, arguments, environment, values);
  }

> Can you share this code? If not, could you document how exactly would you have done this?

Unfortunately I didn't make it that far. I was planning to study how glibc and musl do it just like how I studied their system call implementations. If I start working on this again I'm gonna look this up. I'll need to learn this linking stuff in order to support the kernel vDSO anyway.