[PATCH] RLIMIT_CPU: fix handling of a zero limit

At present the kernel doesn't honour an attempt to set RLIMIT_CPU to zero
seconds.  But the spec says it should, and that's what 2.4.x does.

Fixing this for real would involve some complexity (such as adding a new
it-has-been-set flag to the task_struct, and testing that everwhere, instead
of overloading the value of it_prof_expires).

Given that a 2.4 kernel won't actually send the signal until one second has
expired anyway, let's just handle this case by treating the caller's
zero-seconds as one second.

Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Ulrich Weigand <uweigand@de.ibm.com>
Cc: Cliff Wickman <cpw@sgi.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sys.c

@@ -1661,8 +1661,19 @@ asmlinkage long sys_setrlimit(unsigned int resource, struct rlimit __user *rlim)
 
 	it_prof_secs = cputime_to_secs(current->signal->it_prof_expires);
 	if (it_prof_secs == 0 || new_rlim.rlim_cur <= it_prof_secs) {
-		cputime_t cputime = secs_to_cputime(new_rlim.rlim_cur);
+		unsigned long rlim_cur = new_rlim.rlim_cur;
+		cputime_t cputime;
 
+		if (rlim_cur == 0) {
+			/*
+			 * The caller is asking for an immediate RLIMIT_CPU
+			 * expiry.  But we use the zero value to mean "it was
+			 * never set".  So let's cheat and make it one second
+			 * instead
+			 */
+			rlim_cur = 1;
+		}
+		cputime = secs_to_cputime(rlim_cur);
 		read_lock(&tasklist_lock);
 		spin_lock_irq(&current->sighand->siglock);
 		set_process_cpu_timer(current, CPUCLOCK_PROF, &cputime, NULL);