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    SubjectRe: [PATCH v2 2/3] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS
    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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