[LTP] [PATCH v3 1/9] Add fuzzy synchronisation library for triggering races

[Richard Palethorpe]

Provide some functions for starting two actions at approximately the same time
on different threads or processes. This is intended to amalgamate
functionality built into at least three CVE tests. As well as conforming to
DRY it is intended to help make reasoning about each test easier by
abstracting away the synchronisation mechanism.

Signed-off-by: Richard Palethorpe <rpalethorpe@suse.com>
---
 include/tst_fuzzy_sync.h | 184 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 184 insertions(+)
 create mode 100644 include/tst_fuzzy_sync.h

diff --git a/include/tst_fuzzy_sync.h b/include/tst_fuzzy_sync.h
new file mode 100644
index 000000000..122a6d736
--- /dev/null
+++ b/include/tst_fuzzy_sync.h
@@ -0,0 +1,184 @@
+/*
+ * Copyright (c) 2017 Richard Palethorpe <rpalethorpe@suse.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+/*
+ * Fuzzy Synchronisation - abreviated to fzsync
+ *
+ * This library is intended to help reproduce race conditions while running in
+ * a loop. You can use it to measure the time at which two functions are
+ * called in different threads. Then calculate the average time gap between
+ * the function calls and introduce a delay in one thread to synchronise the
+ * calls.
+ *
+ * It is called 'fuzzy' synchronisation because the time gap will naturally vary
+ * due to environmental factors. It is not a 'hard' synchronisation mechanism
+ * like lockstepping.
+ *
+ * For a usage example see testcases/cve/cve-2017-2671.c
+ */
+
+#include <sys/time.h>
+#include <time.h>
+
+#ifndef CLOCK_MONOTONIC_RAW
+#define CLOCK_MONOTONIC_RAW CLOCK_MONOTONIC
+#endif
+
+/**
+ * struct tst_fzsync_pair - the state of a two way synchronisation
+ * @avg_diff: The average time difference over multiple iterations
+ * @avg_diff_trgt: The desired average time difference, defaults to 0
+ * @avg_alpha: The rate at which old diff samples are forgotten,
+ *             defaults to 0.25
+ * @a: The time at which call site A was last passed
+ * @b: The time at which call site B was last passed
+ * @delay: The size of the delay, positive to delay A, negative to delay B
+ * @delay_inc: The step size of a delay increment, defaults to 10
+ * @update_gap: The number of iterations between recalculating the delay.
+ *              Defaults to 0xF and must be of the form $2^n - 1$
+ *
+ * This contains all the necessary state for synchronising two points A and
+ * B. Where A is the time of an event in one process and B is the time of an
+ * event in another process.
+ */
+struct tst_fzsync_pair {
+	double avg_diff;
+	double avg_diff_trgt;
+	double avg_alpha;
+	struct timespec a;
+	struct timespec b;
+	long delay;
+	long delay_inc;
+	int update_gap;
+};
+
+/**
+ * TST_FZSYNC_PAIR_INIT - Default values for struct tst_fzysnc_pair
+ */
+#define TST_FZSYNC_PAIR_INIT {	\
+	.avg_alpha = 0.25,	\
+	.delay_inc = 10,	\
+	.update_gap = 0xF	\
+}
+
+static void tst_fzsync_pair_info(struct tst_fzsync_pair *pair)
+{
+	tst_res(TINFO, "avg_diff = %.5gns, delay = %05ld loops",
+		pair->avg_diff, pair->delay);
+}
+
+/**
+ * tst_fzsync_delay_a - Perform spin delay for A, if needed
+ *
+ * Usually called just before the point you want to synchronise.
+ */
+static inline void tst_fzsync_delay_a(struct tst_fzsync_pair *pair)
+{
+	volatile long spin_delay = pair->delay;
+
+	while (spin_delay > 0)
+		spin_delay--;
+}
+
+/**
+ * tst_fzsync_delay_b - Perform spin delay for B, if needed
+ *
+ * Usually called just before the point you want to synchronise.
+ */
+static inline void tst_fzsync_delay_b(struct tst_fzsync_pair *pair)
+{
+	volatile long spin_delay = pair->delay;
+
+	while (spin_delay < 0)
+		spin_delay++;
+}
+
+static inline void tst_fzsync_time(struct timespec *t)
+{
+	clock_gettime(CLOCK_MONOTONIC_RAW, t);
+}
+
+/**
+ * tst_fzsync_time_a - Set A's time to now.
+ *
+ * Called at the point you want to synchronise.
+ */
+static inline void tst_fzsync_time_a(struct tst_fzsync_pair *pair)
+{
+	tst_fzsync_time(&pair->a);
+}
+
+/**
+ * tst_fzsync_time_b - Set B's call time to now.
+ *
+ * Called at the point you want to synchronise.
+ */
+static inline void tst_fzsync_time_b(struct tst_fzsync_pair *pair)
+{
+	tst_fzsync_time(&pair->b);
+}
+
+/**
+ * tst_exp_moving_avg - Exponential moving average
+ * @alpha: The preference for receant samples over old ones.
+ * @sample: The current sample
+ * @prev_avg: The average of the all the previous samples
+ *
+ * Returns average including the current sample.
+ */
+static inline double tst_exp_moving_avg(double alpha, long sample,
+					double prev_avg)
+{
+	return alpha * sample + (1.0 - alpha) * prev_avg;
+}
+
+/**
+ * tst_fzsync_pair_update - Recalculate the delay
+ * @loop_index: The i in "for(i = 0;..." or zero to ignore update_gap
+ * @pair: The state necessary for calculating the delay
+ *
+ * This should be called at the end of each loop to update the average
+ * measured time difference (between A and B) and update the delay. It is
+ * assumed that A and B are less than a second apart.
+ *
+ * The values of update_gap, avg_alpha and delay_inc decide the speed at which
+ * the algorithm approaches the optimum delay value and whether it is
+ * stable. If your test is behaving strangely, it could be because this
+ * algorithm is behaving chaotically and flip-flopping between large positve
+ * and negative delay values. You can call tst_fzysync_pair_info every few
+ * loops to check whether the average difference and delay values are stable.
+ */
+static void tst_fzsync_pair_update(int loop_index, struct tst_fzsync_pair *pair)
+{
+	long diff;
+	long inc = pair->delay_inc;
+	long delay = pair->delay;
+	double target = pair->avg_diff_trgt;
+	double avg = pair->avg_diff;
+
+	diff = pair->a.tv_nsec - pair->b.tv_nsec;
+	avg = tst_exp_moving_avg(pair->avg_alpha, diff, avg);
+
+	if (!(loop_index & pair->update_gap)) {
+		if (avg > target)
+			delay -= inc;
+		else if (avg < target)
+			delay += inc;
+	}
+
+	pair->avg_diff = avg;
+	pair->delay = delay;
+}
-- 
2.12.2