linux-kernel - Re: [PATCH v2 3/3] sched: Make uclamp changes depend on CAP_SYS

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Message-ID: <YMN2ljLMUikvCBXk@google.com>
Date:   Fri, 11 Jun 2021 14:43:34 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rickyiu@...gle.com, wvw@...gle.com,
        patrick.bellasi@...bug.net, xuewen.yan94@...il.com,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2 3/3] sched: Make uclamp changes depend on CAP_SYS_NICE

On Friday 11 Jun 2021 at 15:17:37 (+0100), Qais Yousef wrote:
> On 06/11/21 13:49, Quentin Perret wrote:
> > Thinking about it a bit more, a more involved option would be to have
> > this patch as is, but to also introduce a new RLIMIT_UCLAMP on top of
> > it. The semantics could be:
> > 
> >   - if the clamp requested by the non-privileged task is lower than its
> >     existing clamp, then allow;
> >   - otherwise, if the requested clamp is less than UCLAMP_RLIMIT, then
> >     allow;
> >   - otherwise, deny,
> > 
> > And the same principle would apply to both uclamp.min and uclamp.max,
> > and UCLAMP_RLIMIT would default to 0.
> > 
> > Thoughts?
> 
> That could work. But then I'd prefer your patch to go as-is. I don't think
> uclamp can do with this extra complexity in using it.

Sorry I'm not sure what you mean here?

> We basically want to specify we want to be paranoid about uclamp CAP or not. In
> my view that is simple and can't see why it would be a big deal to have
> a procfs entry to define the level of paranoia the system wants to impose. If
> it is a big deal though (would love to hear the arguments);

Not saying it's a big deal, but I think there are a few arguments in
favor of using rlimit instead of a sysfs knob. It allows for a much
finer grain configuration  -- constraints can be set per-task as well as
system wide if needed, and it is the standard way of limiting resources
that tasks can ask for.

> requiring apps that
> want to self regulate to have CAP_SYS_NICE is better approach.

Rlimit wouldn't require that though, which is also nice as CAP_SYS_NICE
grants you a lot more power than just clamps ...