BUG/MINOR: freq_ctr: Prevent possible signed overflow in freq_ctr_overshoot_period

All of the other bandwidth-limiting code stores limits and intermediate
(byte) counters as unsigned integers. The exception here is
freq_ctr_overshoot_period which takes in unsigned values but returns a
signed value. While this has the benefit of letting the caller know how
far away from overshooting they are, this is not currently leveraged
anywhere in the codebase, and it has the downside of halving the positive
range of the result.

More concretely though, returning a signed integer when all intermediate
values are unsigned (and boundaries are not checked) could result in an
overflow, producing values that are at best unexpected. In the case of
flt_bwlim (the only usage of freq_ctr_overshoot_period in the codebase at
the time of writing), an overflow could cause the filter to wait for a
large number of milliseconds when in fact it shouldn't wait at all.

This is a niche possibility, because it requires that a bandwidth limit is
defined in the range [2^31, 2^32). In this case, the raw limit value would
not fit into a signed integer, and close to the end of the period, the
`(elapsed * freq)/period` calculation could produce a value which also
doesn't fit into a signed integer.

If at the same time `curr` (the number of events counted so far in the
current period) is small, then we could get a very large negative value
which overflows. This is undefined behaviour and could produce surprising
results. The most obvious outcome is flt_bwlim sometimes waiting for a
large amount of time in a case where it shouldn't wait at all, thereby
incorrectly slowing down the flow of data.

Converting just the return type from signed to unsigned (and checking for
the overflow) prevents this undefined behaviour. It also makes the range
of valid values consistent between the input and output of
freq_ctr_overshoot_period and with the input and output of other freq_ctr
functions, thereby reducing the potential for surprise in intermediate
calculations: now everything supports the full 0 - 2^32 range.

include/haproxy/freq_ctr.h

@@ -31,7 +31,7 @@
 ullong _freq_ctr_total_from_values(uint period, int pend, uint tick, ullong past, ullong curr);
 ullong freq_ctr_total(const struct freq_ctr *ctr, uint period, int pend);
 ullong freq_ctr_total_estimate(const struct freq_ctr *ctr, uint period, int pend);
-int freq_ctr_overshoot_period(const struct freq_ctr *ctr, uint period, uint freq);
+uint freq_ctr_overshoot_period(const struct freq_ctr *ctr, uint period, uint freq);
 uint update_freq_ctr_period_slow(struct freq_ctr *ctr, uint period, uint inc);
 
 /* Only usable during single threaded startup phase. */

src/flt_bwlim.c

@@ -79,9 +79,9 @@ static int bwlim_apply_limit(struct filter *filter, struct channel *chn, unsigne
 	struct bwlim_state *st = filter->ctx;
 	struct freq_ctr *bytes_rate;
 	uint64_t remain;
-	unsigned int period, limit, tokens, users, factor;
+	unsigned int period, limit, tokens, users, factor, overshoot;
 	unsigned int wait = 0;
-	int overshoot, ret = 0;
+	int ret = 0;
 
 	/* Don't forward anything if there is nothing to forward or the waiting
 	 * time is not expired

src/freq_ctr.c

@@ -184,17 +184,17 @@ ullong freq_ctr_total_estimate(const struct freq_ctr *ctr, uint period, int pend
 	return _freq_ctr_total_from_values(period, pend, tick, past, curr);
 }
 
-/* Returns the excess of events (may be negative) over the current period for
+/* Returns the excess of events over the current period for target frequency
- * target frequency <freq>. It returns 0 if the counter is in the future or if
+ * <freq>. It returns 0 if the counter is in the future or if the counter is
- * the counter is empty. The result considers the position of the current time
+ * empty. The result considers the position of the current time within the
- * within the current period.
+ * current period.
  *
- * The caller may safely add new events if result is negative or null.
+ * The caller may safely add new events if result is zero.
  */
-int freq_ctr_overshoot_period(const struct freq_ctr *ctr, uint period, uint freq)
+uint freq_ctr_overshoot_period(const struct freq_ctr *ctr, uint period, uint freq)
 {
 	ullong curr, old_curr;
-	uint tick, old_tick;
+	uint tick, old_tick, linear_usage;
 	int elapsed;
 
 	tick = HA_ATOMIC_LOAD(&ctr->curr_tick);
@@ -245,7 +245,12 @@ int freq_ctr_overshoot_period(const struct freq_ctr *ctr, uint period, uint freq
 		return 0;
 	}
 
-	return curr - div64_32((uint64_t)elapsed * freq, period);
+	linear_usage = div64_32((uint64_t)elapsed * freq, period);
+	if (curr < linear_usage) {
+		/* The counter is below a uniform linear increase across the period, no overshoot */
+		return 0;
+	}
+	return curr - linear_usage;
 }
 
 /*