Detect pointer math wrap-around (overflow and underflow)

[kees]

Much like issue #26 and issue #27, we must mitigate pointer arithmetic wrap-around (overflow/underflow). This should be possible via -fsanitize=pointer-overflow but it has similar problems as the other issues, namely -fwrapv-pointer.

• [ ] make sanitizer work with -fwrapv-pointer (and -fno-strict-overflow)
• [ ] fix sanitizer to work at all on Clang: https://github.com/llvm/llvm-project/issues/66451
• [ ] create "expected pointer overflow" helper inline functions marked with attribute((no_sanitize("pointer-overflow"))).
• [ ] find all true positives and replace with helper calls.
• [ ] add note to "deprecated.rst" with something like "open coded pointer math wrap around without a helper".
• [ ] add pointer overflow as a UBSan Kconfig

[JustinStitt]

As for bullet point 2: fix sanitizer to work at all on Clang: https://github.com/llvm/llvm-project/issues/66451 it looks like this sanitizer DOES work... just not for void* 😕

demo: https://godbolt.org/z/Ef3Kvqq1x

~What's going on here?~

Update: I triaged and nikic fixed here: https://github.com/llvm/llvm-project/pull/67772