Commits on Nov 29, 2018

1. fix: Strip top bits from pointers before reading memory contents …

   In the new Xcode 10 build system, Swift object register values have the
   top bit used as a flag. This change strips the flag while not losing
   anything relevant to us in our quest to see error messages for assertion
   failures.
   
   This technique does not capture messages which are less than 16
   characters, as short strings are stored as raw char arrays on the stack
   rather than being allocated. (See WWDC 2018 #401 for more info on new
   string optimizations)
   
   While it is possible to check for char arrays as well as pointers when
   searching for notable address values, sweeping up local variables has a
   likely chance of capturing unintended data as well from the surrounding
   code, some of which may be sensitive. It is also not guaranteed that the
   value would still be on the stack after the message is logged, so it is
   possible to get only unrelated string values as the message.
   
   In the current Swift stdlib, the following messages passed to
   fatalError, preconditionFailure, and precondition (and their internal
   func counterparts) are less than 16 characters:
   
   * empty string
   * `unavailable`
   * `not implemented`
   * `abstract method`
   * `unknown value`
   * `invalid count` (where a dictionary contains < 0 items(?))
   * `invalid index` (where a dictionary ceases to be a dictionary)
   * `don't touch me` (from SpriteKit)
   * `close() failed` (from the private Subprocess implementation)
   
   The vast majority have more meaningful messages.
   
   Reference:
   * https://asciiwwdc.com/2018/sessions/401
   
   Fixes #318

   

   kattrali committed Nov 29, 2018
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