CPython needs to be regularly built and vetted for C undefined behavior.

CFLAGS=-fsanitize=undefined LDFLAGS=-fsanitize=undefined CC=clang LD=clang ./configure

is all it takes to start with.

if you do this today, test_ctypes will knock your socks off with errors.  We should detect this type of build and intentionally skip its tests on ubsan, tracking that with another yet to be filed issue.

I'll look into having Google run one.

Perhaps your “ctypes” problems may be helped by my ctypes_v2.patch in Issue 1621. Or perhaps they are already documented in Issue 15119 and/or Issue 28169.

A buildbot is up and running:
  http://buildbot.python.org/all/#/builders?tags=%2Bubsan

in order to see the errors though, you have to manually string search for "runtime error" in the giant test logs stdout.

At the moment, Modules/_ctypes/_ctypes.c dominates the error lists with misaligned access problems - I suggest filtering that out and looking for the others to start with for anyone working on these issues.