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>

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;