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authorThomas Gleixner <tglx@linutronix.de>2013-09-24 21:50:23 +0200
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2013-11-13 12:05:32 +0900
commitb5b02b140665d016236918cb88b88f6249050605 (patch)
treecefd0a045b7451325e4368d7961f704b03e66da4 /kernel/time
parent722b78bc8a42b6910c291266e87abe2e63dc750f (diff)
downloadlinaro-lsk-b5b02b140665d016236918cb88b88f6249050605.tar.gz
clockevents: Sanitize ticks to nsec conversion
commit 97b9410643475d6557d2517c2aff9fd2221141a9 upstream. Marc Kleine-Budde pointed out, that commit 77cc982 "clocksource: use clockevents_config_and_register() where possible" caused a regression for some of the converted subarchs. The reason is, that the clockevents core code converts the minimal hardware tick delta to a nanosecond value for core internal usage. This conversion is affected by integer math rounding loss, so the backwards conversion to hardware ticks will likely result in a value which is less than the configured hardware limitation. The affected subarchs used their own workaround (SIGH!) which got lost in the conversion. The solution for the issue at hand is simple: adding evt->mult - 1 to the shifted value before the integer divison in the core conversion function takes care of it. But this only works for the case where for the scaled math mult/shift pair "mult <= 1 << shift" is true. For the case where "mult > 1 << shift" we can apply the rounding add only for the minimum delta value to make sure that the backward conversion is not less than the given hardware limit. For the upper bound we need to omit the rounding add, because the backwards conversion is always larger than the original latch value. That would violate the upper bound of the hardware device. Though looking closer at the details of that function reveals another bogosity: The upper bounds check is broken as well. Checking for a resulting "clc" value greater than KTIME_MAX after the conversion is pointless. The conversion does: u64 clc = (latch << evt->shift) / evt->mult; So there is no sanity check for (latch << evt->shift) exceeding the 64bit boundary. The latch argument is "unsigned long", so on a 64bit arch the handed in argument could easily lead to an unnoticed shift overflow. With the above rounding fix applied the calculation before the divison is: u64 clc = (latch << evt->shift) + evt->mult - 1; So we need to make sure, that neither the shift nor the rounding add is overflowing the u64 boundary. [ukl: move assignment to rnd after eventually changing mult, fix build issue and correct comment with the right math] Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Russell King - ARM Linux <linux@arm.linux.org.uk> Cc: Marc Kleine-Budde <mkl@pengutronix.de> Cc: nicolas.ferre@atmel.com Cc: Marc Pignat <marc.pignat@hevs.ch> Cc: john.stultz@linaro.org Cc: kernel@pengutronix.de Cc: Ronald Wahl <ronald.wahl@raritan.com> Cc: LAK <linux-arm-kernel@lists.infradead.org> Cc: Ludovic Desroches <ludovic.desroches@atmel.com> Link: http://lkml.kernel.org/r/1380052223-24139-1-git-send-email-u.kleine-koenig@pengutronix.de Signed-off-by: Uwe Kleine-K├Ânig <u.kleine-koenig@pengutronix.de> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'kernel/time')
-rw-r--r--kernel/time/clockevents.c65
1 files changed, 50 insertions, 15 deletions
diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index c6d6400ee13..6a23c6c556c 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -30,29 +30,64 @@ static RAW_NOTIFIER_HEAD(clockevents_chain);
/* Protection for the above */
static DEFINE_RAW_SPINLOCK(clockevents_lock);
-/**
- * clockevents_delta2ns - Convert a latch value (device ticks) to nanoseconds
- * @latch: value to convert
- * @evt: pointer to clock event device descriptor
- *
- * Math helper, returns latch value converted to nanoseconds (bound checked)
- */
-u64 clockevent_delta2ns(unsigned long latch, struct clock_event_device *evt)
+static u64 cev_delta2ns(unsigned long latch, struct clock_event_device *evt,
+ bool ismax)
{
u64 clc = (u64) latch << evt->shift;
+ u64 rnd;
if (unlikely(!evt->mult)) {
evt->mult = 1;
WARN_ON(1);
}
+ rnd = (u64) evt->mult - 1;
+
+ /*
+ * Upper bound sanity check. If the backwards conversion is
+ * not equal latch, we know that the above shift overflowed.
+ */
+ if ((clc >> evt->shift) != (u64)latch)
+ clc = ~0ULL;
+
+ /*
+ * Scaled math oddities:
+ *
+ * For mult <= (1 << shift) we can safely add mult - 1 to
+ * prevent integer rounding loss. So the backwards conversion
+ * from nsec to device ticks will be correct.
+ *
+ * For mult > (1 << shift), i.e. device frequency is > 1GHz we
+ * need to be careful. Adding mult - 1 will result in a value
+ * which when converted back to device ticks can be larger
+ * than latch by up to (mult - 1) >> shift. For the min_delta
+ * calculation we still want to apply this in order to stay
+ * above the minimum device ticks limit. For the upper limit
+ * we would end up with a latch value larger than the upper
+ * limit of the device, so we omit the add to stay below the
+ * device upper boundary.
+ *
+ * Also omit the add if it would overflow the u64 boundary.
+ */
+ if ((~0ULL - clc > rnd) &&
+ (!ismax || evt->mult <= (1U << evt->shift)))
+ clc += rnd;
do_div(clc, evt->mult);
- if (clc < 1000)
- clc = 1000;
- if (clc > KTIME_MAX)
- clc = KTIME_MAX;
- return clc;
+ /* Deltas less than 1usec are pointless noise */
+ return clc > 1000 ? clc : 1000;
+}
+
+/**
+ * clockevents_delta2ns - Convert a latch value (device ticks) to nanoseconds
+ * @latch: value to convert
+ * @evt: pointer to clock event device descriptor
+ *
+ * Math helper, returns latch value converted to nanoseconds (bound checked)
+ */
+u64 clockevent_delta2ns(unsigned long latch, struct clock_event_device *evt)
+{
+ return cev_delta2ns(latch, evt, false);
}
EXPORT_SYMBOL_GPL(clockevent_delta2ns);
@@ -317,8 +352,8 @@ void clockevents_config(struct clock_event_device *dev, u32 freq)
sec = 600;
clockevents_calc_mult_shift(dev, freq, sec);
- dev->min_delta_ns = clockevent_delta2ns(dev->min_delta_ticks, dev);
- dev->max_delta_ns = clockevent_delta2ns(dev->max_delta_ticks, dev);
+ dev->min_delta_ns = cev_delta2ns(dev->min_delta_ticks, dev, false);
+ dev->max_delta_ns = cev_delta2ns(dev->max_delta_ticks, dev, true);
}
/**