Off by one error for SIGXCPURLMIC_CPU checking

Michael Kerrisk
2004-08-02T13:11:15Z

Hello Andrew,

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);
}
}