Messages in this thread Patch in this message | | | Date | Thu, 28 May 2009 16:09:53 +0200 | From | Lennart Poettering <> | Subject | [PATCH 3/3] scheduler: introduce SCHED_RESET_ON_FORK scheduling policy flag |
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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, ¶m);
by this:
sched_setscheduler(pid, SCHED_FIFO|SCHED_RESET_ON_FORK, ¶m);
This current patch compiles but is not otherwise tested. For now, I am sending this primarily as a request for comments. So please, I'd love to hear your comments!
Lennart --- include/linux/sched.h | 4 ++++ kernel/sched.c | 43 ++++++++++++++++++++++++++++++++++--------- 2 files changed, 38 insertions(+), 9 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h index b4c38bc..c28c81b 100644 --- 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__ @@ -1429,6 +1431,8 @@ struct task_struct { /* state flags for use by tracers */ unsigned long trace; #endif + /* Revert to default priority/policy when forking */ + int sched_reset_on_fork; }; /* Future-safe accessor for struct task_struct's cpus_allowed. */ diff --git a/kernel/sched.c b/kernel/sched.c index 26efa47..35a63ff 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -2529,9 +2529,19 @@ 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; @@ -5807,17 +5817,25 @@ 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 = 0; /* 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) { policy = oldpolicy = p->policy; - else if (policy != SCHED_FIFO && policy != SCHED_RR && - policy != SCHED_NORMAL && policy != SCHED_BATCH && - policy != SCHED_IDLE) - return -EINVAL; + reset_on_fork = p->sched_reset_on_fork; + } 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, @@ -5861,6 +5879,10 @@ 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) { @@ -5904,6 +5926,8 @@ 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); @@ -6020,14 +6044,15 @@ 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. */ -- 1.6.2.2
Lennart
-- Lennart Poettering Red Hat, Inc. lennart [at] poettering [dot] net ICQ# 11060553 http://0pointer.net/lennart/ GnuPG 0x1A015CC4
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