Re: [PATCH 02/14] sched: add extended scheduling interface. (new ABI)

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Nov 28, 2013 at 12:14:03PM +0100, Juri Lelli wrote:
> +SYSCALL_DEFINE2(sched_getattr, pid_t, pid, struct sched_attr __user *, attr)
>  {
> -	struct sched_param2 lp;
> +	struct sched_attr lp;
>  	struct task_struct *p;
>  	int retval;
>  
> -	if (!param2 || pid < 0)
> +	if (!attr || pid < 0)
>  		return -EINVAL;
>  
> +	memset(&lp, 0, sizeof(struct sched_attr));
> +
>  	rcu_read_lock();
>  	p = find_process_by_pid(pid);
>  	retval = -ESRCH;
> @@ -3427,7 +3495,7 @@ SYSCALL_DEFINE2(sched_getparam2, pid_t, pid, struct sched_param2 __user *, param
>  	lp.sched_priority = p->rt_priority;
>  	rcu_read_unlock();
>  
> -	retval = copy_to_user(param2, &lp, sizeof(lp)) ? -EFAULT : 0;
> +	retval = copy_to_user(attr, &lp, sizeof(lp)) ? -EFAULT : 0;
>  	return retval;
>  
>  out_unlock:


So this side needs a bit more care; suppose the kernel has a larger attr
than userspace knows about.

What would make more sense; add another syscall argument with the
userspace sizeof(struct sched_attr), or expect userspace to initialize
attr->size to the right value before calling sched_getattr() ?

To me the extra argument makes more sense; that is:

  struct sched_attr attr;

  ret = sched_getattr(0, &attr, sizeof(attr));

seems like a saner thing than:

  struct sched_attr attr = { .size = sizeof(attr), };

  ret = sched_getattr(0, &attr);

Mostly because the former has a clear separation between input and
output arguments, whereas for the second form the attr argument is both
input and output.

Ingo?