Off-by-one error for SIGXCPU / RLIMIT_CPU checking

Off-by-one error for SIGXCPU / RLIMIT_CPU checking

[Michael Kerrisk]

Hello Andrew,

[Apologies -- resend with fixed subject line for benefit of LKML]

There is a lonstanding off-by-one error that results from an incorrect 
comparison when checking whether a process has consumed CPU time in 
excess of its RLIMIT_CPU limits.  

This means, for example, that if we use setrlimit() to set the soft CPU 
limit (rlim_cur) to 5 seconds and the hard limit (rlim_max) to 10 seconds, 
then the process only receives a SIGXCPU signal after consuming 6 seconds 
of CPU time, and, if it continues consuming CPU after handling that 
signal, only receives SIGKILL after consuming 11 seconds of CPU time.

The fix is trivial, and included below.

Cheers,

Michael


--- /spare/KERNEL/linux-2.6.7/kernel/timer.c	2004-06-16 07:19:52.000000000 +0200
+++ /spare/KERNEL/linux-2.6.7-xcpu/kernel/timer.c	2004-08-02 09:34:10.000000000 +0200
@@ -792,12 +792,12 @@
 
 	psecs = (p->utime += user);
 	psecs += (p->stime += system);
-	if (psecs / HZ > p->rlim[RLIMIT_CPU].rlim_cur) {
+	if (psecs / HZ >= p->rlim[RLIMIT_CPU].rlim_cur) {
 		/* Send SIGXCPU every second.. */
 		if (!(psecs % HZ))
 			send_sig(SIGXCPU, p, 1);
 		/* and SIGKILL when we go over max.. */
-		if (psecs / HZ > p->rlim[RLIMIT_CPU].rlim_max)
+		if (psecs / HZ >= p->rlim[RLIMIT_CPU].rlim_max)
 			send_sig(SIGKILL, p, 1);
 	}
 }