Be more strict about using invalid `pthread_t`s.

Another release, another attempt to remove the global thread list.

But this time, let's admit that it's not going away. We can switch to using
a read/write lock for the global thread list, and to aborting rather than
quietly returning ESRCH if we're given an invalid pthread_t.

This change affects pthread_detach, pthread_getcpuclockid,
pthread_getschedparam/pthread_setschedparam, pthread_join, and pthread_kill:
instead of returning ESRCH when passed an invalid pthread_t, if you're
targeting O or above, they'll abort with the message "attempt to use
invalid pthread_t".

Note that this doesn't change behavior as much as you might think: the old
lookup only held the global thread list lock for the duration of the lookup,
so there was still a race between that and the dereference in the caller,
given that callers actually need the tid to pass to some syscall or other,
and sometimes update fields in the pthread_internal_t struct too.

(This patch replaces such users with calls to pthread_gettid_np, which
at least makes the TOCTOU window smaller.)

We can't check thread->tid against 0 to see whether a pthread_t is still
valid because a dead thread gets its thread struct unmapped along with its
stack, so the dereference isn't safe.

Taking the affected functions one by one:

    * pthread_getcpuclockid and pthread_getschedparam/pthread_setschedparam
      should be fine. Unsafe calls to those seem highly unlikely.

    * Unsafe pthread_detach callers probably want to switch to
      pthread_attr_setdetachstate instead, or using
      pthread_detach(pthread_self()) from the new thread's start routine
      rather than doing the detach in the parent.

    * pthread_join calls should be safe anyway, because a joinable thread
      won't actually exit and unmap until it's joined. If you're joining an
      unjoinable thread, the fix is to stop marking it detached. If you're
      joining an already-joined thread, you need to rethink your design.

    * Unsafe pthread_kill calls aren't portably fixable. (And are obviously
      inherently non-portable as-is.) The best alternative on Android is to
      use pthread_gettid_np at some point that you know the thread to be
      alive, and then call kill/tgkill directly.

      That's still not completely safe because if you're too late, the tid
      may have been reused, but then your code is inherently unsafe anyway.

Bug: http://b/19636317
Test: ran tests
Change-Id: I0372c4428e8a7f1c3af5c9334f5d9c25f2c73f21

0 files changed, 0 insertions, 0 deletions