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: Lennart Poettering <lennart@poettering.net>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20090615152714.GA29092@tango.0pointer.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4896fdf..d4a2c66 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -38,6 +38,8 @@
 #define SCHED_BATCH            3
 /* SCHED_ISO: reserved but not implemented yet */
 #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__
 
@@ -1209,6 +1211,10 @@ struct task_struct {
        unsigned did_exec:1;
        unsigned in_execve:1;   /* Tell the LSMs that the process is doing an
                                 * execve */
+
+       /* Revert to default priority/policy when forking */
+       unsigned sched_reset_on_fork:1;
+
        pid_t pid;
        pid_t tgid;
 

diff --git a/kernel/sched.c b/kernel/sched.c
index 8ec9d13..32e6ede 100644 (file)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2613,12 +2613,28 @@ void sched_fork(struct task_struct *p, int clone_flags)
        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.
         */
-       p->prio = current->normal_prio;
+       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;
+
        if (!rt_prio(p->prio))
                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 (likely(sched_info_on()))
                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;
        const struct sched_class *prev_class = p->sched_class;
        struct rq *rq;
+       int reset_on_fork;
 
        /* may grab non-irq protected spin_locks */
        BUG_ON(in_interrupt());
 recheck:
        /* 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;
-       else if (policy != SCHED_FIFO && policy != SCHED_RR &&
-                       policy != SCHED_NORMAL && policy != SCHED_BATCH &&
-                       policy != SCHED_IDLE)
-               return -EINVAL;
+       } 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_IDLE)
+                       return -EINVAL;
+       }
+
        /*
         * Valid priorities for SCHED_FIFO and SCHED_RR are
         * 1..MAX_USER_RT_PRIO-1, valid priority for SCHED_NORMAL,
@@ -6148,6 +6172,10 @@ recheck:
                /* can't change other user's priorities */
                if (!check_same_owner(p))
                        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) {
@@ -6191,6 +6219,8 @@ recheck:
        if (running)
                p->sched_class->put_prev_task(rq, p);
 
+       p->sched_reset_on_fork = reset_on_fork;
+
        oldprio = p->prio;
        __setscheduler(rq, p, policy, param->sched_priority);
 
@@ -6307,14 +6337,15 @@ SYSCALL_DEFINE1(sched_getscheduler, pid_t, pid)
        if (p) {
                retval = security_task_getscheduler(p);
                if (!retval)
-                       retval = p->policy;
+                       retval = p->policy
+                               | (p->sched_reset_on_fork ? SCHED_RESET_ON_FORK : 0);
        }
        read_unlock(&tasklist_lock);
        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.
  * @param: structure containing the RT priority.
  */