Re: [PATCH net] ptp: netc: fix period truncation and potential divide-by-zero in PEROUT

[Simon Horman]

On Tue, Aug 18, 2026 at 11:33:41AM +0800, wei.fang@xxxxxxxxxxx wrote:
> From: Wei Fang <wei.fang@xxxxxxx>
> 
> The max_period bound in net_timer_enable_perout() was computed as:
> 
>   max_period = (u64)NETC_TMR_DEFAULT_FIPER + integral_period;
> 
> which exceeds U32_MAX when integral_period > 0 (e.g. 0x100000002 for
> the default 333333333 Hz clock). A period_ns that passes this check but
> exceeds U32_MAX is then silently truncated when stored into the u32
> struct netc_pp::period field.
> 
> A truncated value of zero can reach netc_timer_set_perout_alarm(), where
> the local u32 period variable would also be 0, causing a divide-by-zero
> in roundup_u64(delta, period) whenever the stime < min_time branch is
> taken (which always happens for a start time of {0, 0}).
> 
> Additionally, netc_timer_enable_periodic_pulse() and
> netc_timer_enable_fiper() both compute:
> 
>   fiper = pp->period - integral_period;
> 
> A zero pp->period results in an unsigned wraparound to 0xFFFFFFFD,
> mis-programming the FIPER hardware register.
> 
> Fix all three issues by capping max_period at NETC_TMR_DEFAULT_FIPER
> (0xFFFFFFFF). This ensures that any period_ns passing the range check
> fits in a u32 without truncation, so the stored value is always valid
> and non-zero. The accepted range is reduced by integral_period ns
> (typically only a few nanoseconds), which is negligible in practice.
> 
> Fixes: 671e266835b8 ("ptp: netc: add periodic pulse output support")
> Signed-off-by: Wei Fang <wei.fang@xxxxxxx>

If so, then I think NETC_TMR_DEFAULT_FIPER can simply be used in
place of max_period.

Something like this (completely untested!):

diff --git a/drivers/ptp/ptp_netc.c b/drivers/ptp/ptp_netc.c
index 28889b7b3e15..66d1f0532585 100644
--- a/drivers/ptp/ptp_netc.c
+++ b/drivers/ptp/ptp_netc.c
@@ -434,22 +434,22 @@
 	}

 	if (on) {
-		u64 period_ns, gclk_period, max_period, min_period;
+		u64 period_ns, gclk_period, min_period;
 		struct timespec64 period, stime;
 		u32 integral_period;
 		int alarm_id;

 		period.tv_sec = rq->perout.period.sec;
 		period.tv_nsec = rq->perout.period.nsec;
 		period_ns = timespec64_to_ns(&period);

 		integral_period = netc_timer_get_integral_period(priv);
-		max_period = NETC_TMR_DEFAULT_FIPER;
 		gclk_period = netc_timer_get_gclk_period(priv);
 		min_period = gclk_period * 4 + integral_period;
-		if (period_ns > max_period || period_ns < min_period) {
+		if (period_ns > NETC_TMR_DEFAULT_FIPER ||
+                   period_ns < min_period) {
 			dev_err(dev, "The period range is %llu ~ %lu\n",
-				min_period, max_period);
+				min_period, NETC_TMR_DEFAULT_FIPER);
 			err = -EINVAL;
 			goto unlock_spinlock;
 		}