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

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> 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?

I suppose so - in the sys_perf_event_open() case we ran out of 
arguments, so attr::size was the only sane way to do it.

[ Btw., perf events side note: for completeness and for debugging we 
  probably want to add a sys_perf_event_get() method as well, to 
  recover the attributes of an existing event. Unidirectional APIs are 
  not very nice. ]

Thanks,

	Ingo