linux-kernel - Re: [PATCH v4 14/16] sched/core: uclamp: request CAP_SYS

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Date:   Thu, 6 Sep 2018 18:21:45 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v4 14/16] sched/core: uclamp: request CAP_SYS_ADMIN by
 default

On 06-Sep 16:59, Juri Lelli wrote:
> On 06/09/18 15:40, Patrick Bellasi wrote:
> > On 04-Sep 15:47, Juri Lelli wrote:
> 
> [...]
> 
> > > Wondering if you want to fold the check below inside the
> > > 
> > >  if (user && !capable(CAP_SYS_NICE)) {
> > >    ...
> > >  }
> > > 
> > > block. It would also save you from adding another parameter to the
> > > function.
> > 
> > So, there are two reasons for that:
> > 
> > 1) _I think_ we don't want to depend on capable(CAP_SYS_NICE) but
> >    instead on capable(CAP_SYS_ADMIN)
> > 
> >    Does that make sense ?
> > 
> >    If yes, the I cannot fold it in the block you reported above
> >    because we will not check for users with CAP_SYS_NICE.
> 
> Ah, right, not sure though. Looks like CAP_SYS_NICE is used for settings
> that relates to priorities, affinity, etc.: CPU related stuff. Since
> here you are also dealing with something that seems to fall into the
> same realm, it might actually fit more than CAP_SYS_ADMIN?

Yes and no... from the functional standpoint if a task is running in
the root cgroup, or cgroups are not in use at all, with this API a
task can always ask for the max OPP. Which is what CAP_SYS_NICE is
there for AFAIU... but...

... this check was meant also to fix the issue of the limited number
of clamp groups. That's why I'm now asking for CAP_SYS_ADMIN.

However, I would say that if we condsider to get in also the
discretization support introduced in:

   [PATCH v4 15/16] sched/core: uclamp: add clamp group discretization support
   https://lore.kernel.org/lkml/20180828135324.21976-16-patrick.bellasi@arm.com/

then yes, we remain with the "nice" semantics to cover, and
CAP_SYS_NICE could be just enough.

> Now that I think more about it, would it actually make sense to allow
> unpriviledged users to lower their assigned umin/umax properties if they
> want? Something alike what happens for nice values or RT priorities.

Yes... if we fix the issue with the limited clamp groups, i.e. we take
discretization in.

> > 2) Then we could move it after that block, where there is another
> >    set of checks with just:
> > 
> >       if (user) {
> > 
> >    We can potentially add the check there yes... but when uclamp is
> >    not enabled we will still perform those checks or we have to add
> >    some compiler guards...
> > 
> > 3) ... or at least check for:
> > 
> >      if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
> > 
> >    Which is what I'm doing right after the block above (2).
> > 
> >    But, at this point, by passing in the parameter to the
> >    __setscheduler_uclamp() call, I get the benefits of:
> > 
> >    a) reducing uclamp specific code in the caller
> >    b) avoiding the checks on !CONFIG_UCLAMP_TASK build
> > 
> > > >  {
> > > >  	int group_id[UCLAMP_CNT] = { UCLAMP_NOT_VALID };
> > > >  	int lower_bound, upper_bound;
> > > >  	struct uclamp_se *uc_se;
> > > >  	int result = 0;
> > > >  
> > > > +	if (!capable(CAP_SYS_ADMIN) &&
> > > > +	    user && !uclamp_user_allowed) {
> > > > +		return -EPERM;
> > > > +	}
> > > > +
> > 
> > Does all the above makes sense ?
> 
> If we agree on CAP_SYS_ADMIN, however, your approach looks cleaner yes.

Cheers Patrick

-- 
#include <best/regards.h>

Patrick Bellasi