token_bucket_ctr: replace 32-bit wallclock time with monotime

This started as a response to ticket #40792 where Coverity is
complaining about a potential year 2038 bug where we cast time_t from
approx_time() to uint32_t for use in token_bucket_ctr.

There was a larger can of worms though, since token_bucket really
doesn't want to be using wallclock time here. I audited the call sites
for approx_time() and changed any that used a 32-bit cast or made
inappropriate use of wallclock time. Things like certificate lifetime,
consensus intervals, etc. need wallclock time. Measurements of rates
over time, however, are better served with a monotonic timer that does
not try and sync with wallclock ever.

Looking closer at token_bucket, its design is a bit odd because it was
initially intended for use with tick units but later forked into
token_bucket_rw which uses ticks to count bytes per second, and
token_bucket_ctr which uses seconds to count slower events. The rates
represented by either token bucket can't be lower than 1 per second, so
the slower timer in 'ctr' is necessary to represent the slower rates of
things like connections or introduction packets or rendezvous attempts.

I considered modifying token_bucket to use 64-bit timestamps overall
instead of 32-bit, but that seemed like an unnecessarily invasive change
that would grant some peace of mind but probably not help much. I was
more interested in removing the dependency on wallclock time. The
token_bucket_rw timer already uses monotonic time. This patch converts
token_bucket_ctr to use monotonic time as well. It introduces a new
monotime_coarse_absolute_sec(), which is currently the same as nsec
divided by a billion but could be optimized easily if we ever need to.

This patch also might fix a rollover bug.. I haven't tested this
extensively but I don't think the previous version of the rollover code
on either token bucket was correct, and I would expect it to get stuck
after the first rollover.

Signed-off-by: Micah Elizabeth Scott <beth@torproject.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/src/feature/hs/hs_circuit.c b/src/feature/hs/hs_circuit.c
index 4c27f417c5..4904f3ddf9 100644
--- a/src/feature/hs/hs_circuit.c
+++ b/src/feature/hs/hs_circuit.c
@@ -35,6 +35,7 @@
 #include "lib/crypt_ops/crypto_dh.h"
 #include "lib/crypt_ops/crypto_rand.h"
 #include "lib/crypt_ops/crypto_util.h"
+#include "lib/time/compat_time.h"
 
 /* Trunnel. */
 #include "trunnel/ed25519_cert.h"
@@ -794,7 +795,7 @@ handle_rend_pqueue_cb(mainloop_event_t *ev, void *arg)
 
     if (pow_state->using_pqueue_bucket) {
       token_bucket_ctr_refill(&pow_state->pqueue_bucket,
-                              (uint32_t) approx_time());
+                              (uint32_t) monotime_coarse_absolute_sec());
 
       if (token_bucket_ctr_get(&pow_state->pqueue_bucket) > 0) {
         token_bucket_ctr_dec(&pow_state->pqueue_bucket, 1);