[tor-bugs] #29500 [Core Tor/Tor]: Broken circuitpadding unittests on appveyor

[Tor Bug Tracker & Wiki]

#29500: Broken circuitpadding unittests on appveyor
-------------------------------------------------+-------------------------
 Reporter:  asn                                  |          Owner:  (none)
     Type:  defect                               |         Status:
                                                 |  needs_revision
 Priority:  High                                 |      Milestone:  Tor:
                                                 |  0.4.0.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  wtf-pad, tor-relay, tor-cell,        |  Actual Points:
  padding, 041-proposed, 040-must                |
Parent ID:  #28631                               |         Points:
 Reviewer:                                       |        Sponsor:
-------------------------------------------------+-------------------------

Comment (by teor):

 Hi Mike,

 I noticed your question during the network team meeting:

 {{{
 mikeperry: the #29500 one is annoying. I think we need another flag or
 something. I also only barely understand how this could ever happen in a
 real scenario
 mikeperry: it seems almost impossible
 nickm: mikeperry: maybe add the appropriate assertions that should never
 be hit, so that we can figure out why it would be happening sometime
 anyway?
 nickm: diagnosis is the next best thing to a fix
 }}}

 Here are two scenarios where the monotime_diff or monotime_absolute
 functions can return zero:
 (I'll use monotime_absolute below, because I think that's what you're
 using. But monotime_absolute just calls monotime_diff with the previous
 monotime value.)

 The Ratchet Returns Zero

 1. Tor is running on a platform which has monotime API bugs (Windows), or
 Tor is compiled without support for monotime functions (old Linux, old
 macOS, other platforms where we haven't implemented monotime support)
 2. Tor calls a monotime_absolute() function, and stores the last monotime
 value
 3. Any amount of time elapses
 4. Tor calls a monotime_absolute() function, and Windows experiences its
 monotime bug, or the user has changed the wall clock time backwards
 5. Tor emulates monotonic time using a ratchet. The ratchet stores an
 internal correction factor for future monotonic times, and returns the
 same value as it previously returned
 6. The monotime_absolute() function gets the same value for the previous
 and current times. It returns zero.

 Low Resolution Timers in the OS

 1. Tor is running on a platform with low-resolution timers
 2. Tor calls a monotime_absolute() function, and stores the last monotime
 value
 3. A small amount of time elapses, which is lower than the timer
 resolution
 4. Tor calls a monotime_absolute() function, which gets the same monotime
 value as the previous call to the low-resolution timer
 6. The monotime_absolute() function gets the same value for the previous
 and current times. It returns zero.

 Low Resolution Timer APIs in Tor

 1. Tor is running on any platform
 2. Tor calls a monotime_absolute() function with low-resolution units
 (seconds, milliseconds, or microseconds) and stores the last monotime
 value
 3. A small amount of time elapses, which is less than one unit
 4. Tor calls a monotime_absolute() function, which gets a monotime value
 which is greater than the previous call to the low-resolution timer, but
 less than one unit away from the previous timer.
 6. The monotime_absolute() function gets less than one unit's difference
 between the previous and current times. It divides by the unit conversion
 factor, and returns zero. (These functions truncate, rather than
 rounding.)

 I hope that explains the scenarios that Tor code needs to deal with when
 calling Tor's monotime APIs.

 If you have any more questions, please feel free to ask on this ticket.
 Sometimes I miss questions on IRC or in meetings.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/29500#comment:20>
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