Hi,
When we recompute the scaling factor during tc_windup() there is an
opportunity for arithmetic overflow/underflow when we add the NTP
adjustment into the scale:
649 scale = (u_int64_t)1 << 63;
650 scale += \
651 ((th->th_adjustment + th->th_counter->tc_freq_adj) / 1024)
* 2199;
652 scale /= th->th_counter->tc_frequency;
653 th->th_scale = scale * 2;
At lines 650 and 651, you will overflow/underflow if
th->th_counter->tc_freq_adj is sufficiently positive/negative.
I don't like the idea of checking for that overflow during
tc_windup(). We can pick a reasonable adjustment range and check for
it during adjfreq(2) and that should be good enough.
My strawman proposal is a range of -500000000 to 500000000 parts per
billion. We could push the limits a bit, but half a billion seems
like a nice round number to me.
On a perfect clock, this means you can effect a 0.5x slowdown or a
1.5x speedup via adjfreq(2), but no slower/faster.
I don't *think* ntpd(8) would ever reach such extreme adjustments
through its algorithm. I don't think this will break anyone's working
setup.
(Maybe I'm wrong, though. otto@?)
Just so we're all clear that the math is sound, here's the result at
the upper limit of the input range. Note that adjtime(2) is capped at
5000PPM in ntp_update_second(), hence its value here.
int64_t th_adjustment = (5000 * 1000) << 32; /* 21474836480000000 */
int64_t tc_freq_adj = 500000000LL << 32; /* 2147483648000000000
*/
scale = (u_int64_t)1 << 63 /* 9223372036854775808
*/
scale += (th_adjustment + tc_freq_adj) / 1024 * 2199;
/* += (2168958484480000000) / 1024 * 2199; */
/* += 4657753620480000000; */
9223372036854775808 + 4657753620480000000 = 13881125657334775808,
which less than 18446744073709551616, so we don't have overflow.
At the negative end of the input range, i.e.
int64_t th_adjustment = (-5000 * 1000) << 32;
int64_t tc_freq_adj = -500000000LL << 32;
you have 9223372036854775808 - 4657753620480000000 = 4565618416374775808,
so no underflow either.
Thoughts?
What is the best way to express this range in the documentation? Do I
say "parts per billion", or something else?
Index: sys/kern/kern_time.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_time.c,v
retrieving revision 1.131
diff -u -p -r1.131 kern_time.c
--- sys/kern/kern_time.c 22 Jun 2020 18:25:57 -0000 1.131
+++ sys/kern/kern_time.c 3 Jul 2020 00:57:49 -0000
@@ -391,6 +391,9 @@ sys_settimeofday(struct proc *p, void *v
return (0);
}
+#define ADJFREQ_MAX (500000000LL << 32)
+#define ADJFREQ_MIN (-500000000LL << 32)
+
int
sys_adjfreq(struct proc *p, void *v, register_t *retval)
{
@@ -408,6 +411,8 @@ sys_adjfreq(struct proc *p, void *v, reg
return (error);
if ((error = copyin(freq, &f, sizeof(f))))
return (error);
+ if (f < ADJFREQ_MIN || f > ADJFREQ_MAX)
+ return (EINVAL);
}
rw_enter(&tc_lock, (freq == NULL) ? RW_READ : RW_WRITE);
Index: lib/libc/sys/adjfreq.2
===================================================================
RCS file: /cvs/src/lib/libc/sys/adjfreq.2,v
retrieving revision 1.7
diff -u -p -r1.7 adjfreq.2
--- lib/libc/sys/adjfreq.2 10 Sep 2015 17:55:21 -0000 1.7
+++ lib/libc/sys/adjfreq.2 3 Jul 2020 00:57:49 -0000
@@ -60,6 +60,10 @@ The
.Fa freq
argument is non-null and the process's effective user ID is not that
of the superuser.
+.It Bq Er EINVAL
+.Fa freq
+is less than -500000000 parts-per-billion or greater than 500000000
+parts-per-billion.
.El
.Sh SEE ALSO
.Xr date 1 ,
