linux-kernel - Re: [PATCH v3 2/3] sched: Skip priority checks with SCHED_FLAG_KEEP

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Message-ID: <20210630160156.wemzedjg6wuaw7zw@e107158-lin.cambridge.arm.com>
Date:   Wed, 30 Jun 2021 17:01:56 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Quentin Perret <qperret@...gle.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 v3 2/3] sched: Skip priority checks with
 SCHED_FLAG_KEEP_PARAMS

On 06/23/21 12:34, 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@...gle.com>
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---

LGTM.

Reviewed-by: Qais Yousef <qais.yousef@....com>

Cheers

--
Qais Yousef