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From: Quentin Perret <>
Subject: [PATCH v5 2/2] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS
Date: Thu,  5 Aug 2021 11:21:54 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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 <>
Reviewed-by: Dietmar Eggemann <>
Reviewed-by: Qais Yousef <>
Signed-off-by: Quentin Perret <>
 kernel/sched/core.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index df0480ad59b0..433b4001e71e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7328,6 +7328,16 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
 	return -E2BIG;
+static void get_params(struct task_struct *p, struct sched_attr *attr)
+	if (task_has_dl_policy(p))
+		__getparam_dl(p, attr);
+	else if (task_has_rt_policy(p))
+		attr->sched_priority = p->rt_priority;
+	else
+		attr->sched_nice = task_nice(p);
  * sys_sched_setscheduler - set/change the scheduler policy and RT priority
  * @pid: the pid in question.
@@ -7389,6 +7399,8 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr,
 	if (likely(p)) {
+		if (attr.sched_flags & SCHED_FLAG_KEEP_PARAMS)
+			get_params(p, &attr);
 		retval = sched_setattr(p, &attr);
@@ -7537,12 +7549,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
 	kattr.sched_policy = p->policy;
 	if (p->sched_reset_on_fork)
 		kattr.sched_flags |= SCHED_FLAG_RESET_ON_FORK;
-	if (task_has_dl_policy(p))
-		__getparam_dl(p, &kattr);
-	else if (task_has_rt_policy(p))
-		kattr.sched_priority = p->rt_priority;
-	else
-		kattr.sched_nice = task_nice(p);
+	get_params(p, &kattr);
 	kattr.sched_flags &= SCHED_FLAG_ALL;

  parent reply	other threads:[~2021-08-05 10:22 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-05 10:21 [PATCH v5 0/2] A couple of uclamp fixes Quentin Perret
2021-08-05 10:21 ` [PATCH v5 1/2] sched: Fix UCLAMP_FLAG_IDLE setting Quentin Perret
2021-08-06 12:57   ` [tip: sched/core] " tip-bot2 for Quentin Perret
2021-08-05 10:21 ` Quentin Perret [this message]
2021-08-06 12:57   ` [tip: sched/core] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS tip-bot2 for Quentin Perret

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