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Commit ca94c442 authored by Lennart Poettering's avatar Lennart Poettering Committed by Ingo Molnar
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sched: Introduce SCHED_RESET_ON_FORK scheduling policy flag 



This patch introduces a new flag SCHED_RESET_ON_FORK which can be passed
to the kernel via sched_setscheduler(), ORed in the policy parameter. If
set this will make sure that when the process forks a) the scheduling
priority is reset to DEFAULT_PRIO if it was higher and b) the scheduling
policy is reset to SCHED_NORMAL if it was either SCHED_FIFO or SCHED_RR.

Why have this?

Currently, if a process is real-time scheduled this will 'leak' to all
its child processes. For security reasons it is often (always?) a good
idea to make sure that if a process acquires RT scheduling this is
confined to this process and only this process. More specifically this
makes the per-process resource limit RLIMIT_RTTIME useful for security
purposes, because it makes it impossible to use a fork bomb to
circumvent the per-process RLIMIT_RTTIME accounting.

This feature is also useful for tools like 'renice' which can then
change the nice level of a process without having this spill to all its
child processes.

Why expose this via sched_setscheduler() and not other syscalls such as
prctl() or sched_setparam()?

prctl() does not take a pid parameter. Due to that it would be
impossible to modify this flag for other processes than the current one.

The struct passed to sched_setparam() can unfortunately not be extended
without breaking compatibility, since sched_setparam() lacks a size
parameter.

How to use this from userspace? In your RT program simply replace this:

  sched_setscheduler(pid, SCHED_FIFO, &param);

by this:

  sched_setscheduler(pid, SCHED_FIFO|SCHED_RESET_ON_FORK, &param);

Signed-off-by: default avatarLennart Poettering <lennart@poettering.net>
Acked-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090615152714.GA29092@tango.0pointer.de>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 45e3e193

include/linux/sched.h

+6 −0
Original line number Original line Diff line number Diff line
@@ -38,6 +38,8 @@ 
#define SCHED_BATCH		3

#define SCHED_BATCH		3

/* SCHED_ISO: reserved but not implemented yet */

/* SCHED_ISO: reserved but not implemented yet */

#define SCHED_IDLE		5

#define SCHED_IDLE		5

/* Can be ORed in to make sure the process is reverted back to SCHED_NORMAL on fork */

#define SCHED_RESET_ON_FORK     0x40000000





#ifdef __KERNEL__

#ifdef __KERNEL__
@@ -1209,6 +1211,10 @@ struct task_struct { 
	unsigned did_exec:1;

	unsigned did_exec:1;

	unsigned in_execve:1;	/* Tell the LSMs that the process is doing an

	unsigned in_execve:1;	/* Tell the LSMs that the process is doing an

				 * execve */

				 * execve */



	/* Revert to default priority/policy when forking */

	unsigned sched_reset_on_fork:1;



	pid_t pid;

	pid_t pid;

	pid_t tgid;

	pid_t tgid;

kernel/sched.c

+40 −9
Original line number Original line Diff line number Diff line
@@ -2613,12 +2613,28 @@ void sched_fork(struct task_struct *p, int clone_flags) 
	set_task_cpu(p, cpu);

	set_task_cpu(p, cpu);





	/*

	/*

	 * Make sure we do not leak PI boosting priority to the child:

	 * Revert to default priority/policy on fork if requested. Make sure we

	 * do not leak PI boosting priority to the child.

	 */

	 */

	if (current->sched_reset_on_fork &&

			(p->policy == SCHED_FIFO || p->policy == SCHED_RR))

		p->policy = SCHED_NORMAL;



	if (current->sched_reset_on_fork &&

			(current->normal_prio < DEFAULT_PRIO))

		p->prio = DEFAULT_PRIO;

	else

		p->prio = current->normal_prio;

		p->prio = current->normal_prio;



	if (!rt_prio(p->prio))

	if (!rt_prio(p->prio))

		p->sched_class = &fair_sched_class;

		p->sched_class = &fair_sched_class;





	/*

	 * We don't need the reset flag anymore after the fork. It has

	 * fulfilled its duty:

	 */

	p->sched_reset_on_fork = 0;



#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)

#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)

	if (likely(sched_info_on()))

	if (likely(sched_info_on()))

		memset(&p->sched_info, 0, sizeof(p->sched_info));

		memset(&p->sched_info, 0, sizeof(p->sched_info));
@@ -6094,17 +6110,25 @@ static int __sched_setscheduler(struct task_struct *p, int policy, 
	unsigned long flags;

	unsigned long flags;

	const struct sched_class *prev_class = p->sched_class;

	const struct sched_class *prev_class = p->sched_class;

	struct rq *rq;

	struct rq *rq;

	int reset_on_fork;





	/* may grab non-irq protected spin_locks */

	/* may grab non-irq protected spin_locks */

	BUG_ON(in_interrupt());

	BUG_ON(in_interrupt());

recheck:

recheck:

	/* double check policy once rq lock held */

	/* double check policy once rq lock held */

	if (policy < 0)

	if (policy < 0) {

		reset_on_fork = p->sched_reset_on_fork;

		policy = oldpolicy = p->policy;

		policy = oldpolicy = p->policy;

	else if (policy != SCHED_FIFO && policy != SCHED_RR &&

	} else {

		reset_on_fork = !!(policy & SCHED_RESET_ON_FORK);

		policy &= ~SCHED_RESET_ON_FORK;



		if (policy != SCHED_FIFO && policy != SCHED_RR &&

				policy != SCHED_NORMAL && policy != SCHED_BATCH &&

				policy != SCHED_NORMAL && policy != SCHED_BATCH &&

				policy != SCHED_IDLE)

				policy != SCHED_IDLE)

			return -EINVAL;

			return -EINVAL;

	}



	/*

	/*

	 * Valid priorities for SCHED_FIFO and SCHED_RR are

	 * Valid priorities for SCHED_FIFO and SCHED_RR are

	 * 1..MAX_USER_RT_PRIO-1, valid priority for SCHED_NORMAL,

	 * 1..MAX_USER_RT_PRIO-1, valid priority for SCHED_NORMAL,
@@ -6148,6 +6172,10 @@ recheck: 
		/* can't change other user's priorities */

		/* can't change other user's priorities */

		if (!check_same_owner(p))

		if (!check_same_owner(p))

			return -EPERM;

			return -EPERM;



		/* Normal users shall not reset the sched_reset_on_fork flag */

		if (p->sched_reset_on_fork && !reset_on_fork)

			return -EPERM;

	}

	}





	if (user) {

	if (user) {
@@ -6191,6 +6219,8 @@ recheck: 
	if (running)

	if (running)

		p->sched_class->put_prev_task(rq, p);

		p->sched_class->put_prev_task(rq, p);





	p->sched_reset_on_fork = reset_on_fork;



	oldprio = p->prio;

	oldprio = p->prio;

	__setscheduler(rq, p, policy, param->sched_priority);

	__setscheduler(rq, p, policy, param->sched_priority);
@@ -6307,14 +6337,15 @@ SYSCALL_DEFINE1(sched_getscheduler, pid_t, pid) 
	if (p) {

	if (p) {

		retval = security_task_getscheduler(p);

		retval = security_task_getscheduler(p);

		if (!retval)

		if (!retval)

			retval = p->policy;

			retval = p->policy

				| (p->sched_reset_on_fork ? SCHED_RESET_ON_FORK : 0);

	}

	}

	read_unlock(&tasklist_lock);

	read_unlock(&tasklist_lock);

	return retval;

	return retval;

}

}





/**

/**

 * sys_sched_getscheduler - get the RT priority of a thread

 * sys_sched_getparam - get the RT priority of a thread

 * @pid: the pid in question.

 * @pid: the pid in question.

 * @param: structure containing the RT priority.

 * @param: structure containing the RT priority.

 */

 */