Messages in this thread Patch in this message | | | Date | Thu, 10 Jun 2021 21:15:45 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 2/3] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS |
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On Thu, Jun 10, 2021 at 03:13:05PM +0000, Quentin Perret wrote: > SCHED_FLAG_KEEP_PARAMS can be passed to sched_setattr to specify that > the call must not touch scheduling parameters (nice or priority). This > is particularly handy for uclamp when used in conjunction with > SCHED_FLAG_KEEP_POLICY as that allows to issue a syscall that only > impacts uclamp values. > > However, sched_setattr always checks whether the priorities and nice > values passed in sched_attr are valid first, even if those never get > used down the line. This is useless at best since userspace can > trivially bypass this check to set the uclamp values by specifying low > priorities. However, it is cumbersome to do so as there is no single > expression of this that skips both RT and CFS checks at once. As such, > userspace needs to query the task policy first with e.g. sched_getattr > and then set sched_attr.sched_priority accordingly. This is racy and > slower than a single call. > > As the priority and nice checks are useless when SCHED_FLAG_KEEP_PARAMS > is specified, simply inherit them in this case to match the policy > inheritance of SCHED_FLAG_KEEP_POLICY. > > Reported-by: Wei Wang <wvw@google.com> > Signed-off-by: Quentin Perret <qperret@google.com> > --- > kernel/sched/core.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 3b213402798e..1d4aedbbcf96 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -6585,6 +6585,10 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr, > rcu_read_unlock(); > > if (likely(p)) { > + if (attr.sched_flags & SCHED_FLAG_KEEP_PARAMS) { > + attr.sched_priority = p->rt_priority; > + attr.sched_nice = task_nice(p); > + } > retval = sched_setattr(p, &attr); > put_task_struct(p); > }
I don't like this much... afaict the KEEP_PARAMS clause in __setscheduler() also covers the DL params, and you 'forgot' to copy those.
Can't we short circuit the validation logic?
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