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Subject: Re: [RFC 12/12][PATCH] SCHED_DEADLINE: modified sched_*_ex API
From: Raistlin <raistlin@linux.it>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
       michael trimarchi <michael@evidence.eu.com>,
       Fabio Checconi <fabio@gandalf.sssup.it>, Ingo Molnar <mingo@elte.hu>,
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On Tue, 2009-12-29 at 13:15 +0100, Peter Zijlstra wrote:
> >  	if (!param_ex || pid < 0)
> >  		return -EINVAL;
> > +	if (len < sizeof(struct sched_param_ex))
> > +		return -EINVAL;
> > =20
> >  	read_lock(&tasklist_lock);
> >  	p =3D find_process_by_pid(pid);
> > @@ -6837,7 +6844,7 @@ SYSCALL_DEFINE2(sched_getparam_ex, pid_t, pid,
> >  	/*
> >  	 * This one might sleep, we cannot do it with a spinlock held ...
> >  	 */
> > -	retval =3D copy_to_user(param_ex, &lp, sizeof(*param_ex)) ? -EFAULT :=
 0;
> > +	retval =3D copy_to_user(param_ex, &lp, len) ? -EFAULT : 0;
> > =20
> >  	return retval;
> > =20
>=20
> I think this doesn't even do what it claims to do, namely provide a
> flexible ABI, since you fail the operation when there is not enough room
> provided. Hence, when we grow the struct an older program that was
> compiled against the smaller one will become an insta-fail.
>=20
> What this should do is deal with smaller structs by ensuring the tail is
> 0 and simply copying out the head.
>=20
Yep... As said in the previous mail I wanted to do so, and I'll do it
now that I see how odd was what I wrote! :-P

> New bits in the flags field are also an interesting challenge.
>=20
Right... I think that a (partial?) solution could be to properly choose
default values for newcomer flags, could that be right?

Thanks and regards,
Dario

--=20
<<This happens because I choose it to happen!>> (Raistlin Majere)
----------------------------------------------------------------------
Dario Faggioli, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa  (Italy)

http://blog.linux.it/raistlin / raistlin@ekiga.net /
dario.faggioli@jabber.org

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