[PATCH] nice and rt-prio rlimits

Add a pair of rlimits for allowing non-root tasks to raise nice and rt
priorities. Defaults to traditional behavior. Originally written by
Chris Wright.

The patch implements a simple rlimit ceiling for the RT (and nice) priorities
a task can set.  The rlimit defaults to 0, meaning no change in behavior by
default.  A value of 50 means RT priority levels 1-50 are allowed.  A value of
100 means all 99 privilege levels from 1 to 99 are allowed.  CAP_SYS_NICE is
blanket permission.

(akpm: see http://www.uwsg.iu.edu/hypermail/linux/kernel/0503.1/1921.html for
tips on integrating this with PAM).

Signed-off-by: Matt Mackall <mpm@selenic.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 19 insertions, 6 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 9bb7489ee645..5dadcc6df7dd 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3223,6 +3223,19 @@ out_unlock:
 
 EXPORT_SYMBOL(set_user_nice);
 
+/*
+ * can_nice - check if a task can reduce its nice value
+ * @p: task
+ * @nice: nice value
+ */
+int can_nice(const task_t *p, const int nice)
+{
+        /* convert nice value [19,-20] to rlimit style value [0,39] */
+        int nice_rlim = 19 - nice;
+        return (nice_rlim <= p->signal->rlim[RLIMIT_NICE].rlim_cur ||
+                capable(CAP_SYS_NICE));
+}
+
 #ifdef __ARCH_WANT_SYS_NICE
 
 /*
@@ -3242,12 +3255,8 @@ asmlinkage long sys_nice(int increment)
          * We don't have to worry. Conceptually one call occurs first
          * and we have a single winner.
          */
-        if (increment < 0) {
-                if (!capable(CAP_SYS_NICE))
-                        return -EPERM;
-                if (increment < -40)
-                        increment = -40;
-        }
+        if (increment < -40)
+                increment = -40;
         if (increment > 40)
                 increment = 40;
 
@@ -3257,6 +3266,9 @@ asmlinkage long sys_nice(int increment)
         if (nice > 19)
                 nice = 19;
 
+        if (increment < 0 && !can_nice(current, nice))
+                return -EPERM;
+
         retval = security_task_setnice(current, nice);
         if (retval)
                 return retval;
@@ -3372,6 +3384,7 @@ recheck:
                 return -EINVAL;
 
         if ((policy == SCHED_FIFO || policy == SCHED_RR) &&
+            param->sched_priority > p->signal->rlim[RLIMIT_RTPRIO].rlim_cur &&
             !capable(CAP_SYS_NICE))
                 return -EPERM;
         if ((current->euid != p->euid) && (current->euid != p->uid) &&