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, 3 insertions, 2 deletions

diff --git a/src/feature/hs/hs_dos.c b/src/feature/hs/hs_dos.c
index 6323dbeeac..80ad3b1daa 100644
--- a/src/feature/hs/hs_dos.c
+++ b/src/feature/hs/hs_dos.c
@@ -28,6 +28,7 @@
 #include "feature/relay/routermode.h"
 
 #include "lib/evloop/token_bucket.h"
+#include "lib/time/compat_time.h"
 
 #include "feature/hs/hs_dos.h"
 
@@ -143,7 +144,7 @@ hs_dos_setup_default_intro2_defenses(or_circuit_t *circ)
   token_bucket_ctr_init(&circ->introduce2_bucket,
                         consensus_param_introduce_rate_per_sec,
                         consensus_param_introduce_burst_per_sec,
-                        (uint32_t) approx_time());
+                        (uint32_t) monotime_coarse_absolute_sec());
 }
 
 /** Called when the consensus has changed. We might have new consensus
@@ -188,7 +189,7 @@ hs_dos_can_send_intro2(or_circuit_t *s_intro_circ)
 
   /* Refill INTRODUCE2 bucket. */
   token_bucket_ctr_refill(&s_intro_circ->introduce2_bucket,
-                          (uint32_t) approx_time());
+                          (uint32_t) monotime_coarse_absolute_sec());
 
   /* Decrement the bucket for this valid INTRODUCE1 cell we just got. Don't
    * underflow else we end up with a too big of a bucket. */