Ensure your stack is aligned correctly on aarch64 clone()

I got bitten this week with the clone() system call returning -EINVAL on aarch64 on code that worked fine on x86.  After re-reading the manual several times and looking at my code, I resorted to shoving in debug into the kernel to track down where the -EINVAL was occurring.

The answer to my issue is in arch/arm64/kernel/process.c, copy_thread():

         if (stack_start) {  
             if (is_compat_thread(task_thread_info(p)))  
                 childregs->compat_sp = stack_start;  
             /* 16-byte aligned stack mandatory on AArch64 */  
             else if (stack_start & 15)  
                 return -EINVAL;  
             else  
                 childregs->sp = stack_start;  
         }  

Ahah! The stack being passed into clone() has to be 16 byte aligned.  With this simple fix to my code, clone() worked.   Pity this was not in the documentation.