tor

commit 28413e75605cc2d05a2a3e4c766bfbe0a47d848d
parent 5080a4ff671da572afc0744d79402447ba57096f
Author: David Goulet <dgoulet@torproject.org>
Date:   Fri, 14 Oct 2022 09:12:23 -0400

Merge branch 'maint-0.4.7'

Diffstat:
A
changes/bug40684
 | 
6
++++++
M
src/lib/time/compat_time.c
 | 
11
+++++++----

2 files changed, 13 insertions(+), 4 deletions(-)
diff --git a/changes/bug40684 b/changes/bug40684
@@ -0,0 +1,6 @@
+  o Major bugfixes (OSX):
+    - Fix coarse-time computation on Apple platforms (like Mac M1) where
+      the Mach absolute time ticks do not correspond directly to
+      nanoseconds.  Previously, we computed our shift value wrong, which
+      led us to give incorrect timing results.
+      Fixes bug 40684; bugfix on 0.3.3.1-alpha.
diff --git a/src/lib/time/compat_time.c b/src/lib/time/compat_time.c
@@ -253,11 +253,14 @@ monotime_init_internal(void)
   tor_assert(mach_time_info.denom != 0);
 
   {
-    // approximate only.
-    uint64_t ns_per_tick = mach_time_info.numer / mach_time_info.denom;
-    uint64_t ms_per_tick = ns_per_tick * ONE_MILLION;
+    // We want to compute this, approximately:
+    //   uint64_t ns_per_tick = mach_time_info.numer / mach_time_info.denom;
+    //   uint64_t ticks_per_ms = ONE_MILLION / ns_per_tick;
+    // This calculation multiplies first, though, to improve accuracy.
+    uint64_t ticks_per_ms = (ONE_MILLION * mach_time_info.denom)
+      / mach_time_info.numer;
     // requires that tor_log2(0) == 0.
-    monotime_shift = tor_log2(ms_per_tick);
+    monotime_shift = tor_log2(ticks_per_ms);
   }
   {
     // For converting ticks to milliseconds in a 32-bit-friendly way, we