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commit e531d4d1b9753e80b56a28805b01c014a1fe5d51 Author: Nick Mathewson nickm@torproject.org AuthorDate: Thu Oct 13 13:40:10 2022 -0400

Fix a completely wrong calculation in mach monotime_init_internal()

Bug 1: We were purporting to calculate milliseconds per tick, when we *should* have been computing ticks per millisecond.

Bug 2: Instead of computing either one of those, we were _actually_ computing femtoseconds per tick.

These two bugs covered for one another on x86 hardware, where 1 tick == 1 nanosecond. But on M1 OSX, 1 tick is about 41 nanoseconds, causing surprising results.

Fixes bug 40684; bugfix on 0.3.3.1-alpha. --- changes/bug40684 | 6 ++++++ src/lib/time/compat_time.c | 11 +++++++---- 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/changes/bug40684 b/changes/bug40684 new file mode 100644 index 0000000000..8c751ede2c --- /dev/null +++ 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 index 6bbad4f98a..9a5ce062d7 100644 --- 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