RFR: 8340945: Ubsan: oopStorage.cpp:374:8: runtime error: applying non-zero offset 18446744073709551168 to null pointer

[Kim Barrett]

Please review this change to the OopStorage handling of storage block lookup,
now being more careful about pointer arithmetic to avoid UB.

As an initial cleanup, renamed OopStorage::find_block_or_null to
block_for_ptr, for consistency with the Block function that implements it.
Also moved the precondition assert that the argument is non-null into the
Block function, where the requirement is located.

Changed OopStorage::Block::block_for_ptr to avoid pointer arithmetic that
might invoke UB, instead converting the pointer argument to uintptr_t and
performing arithmetic on it. Also fixed its description in the header file.

Similarly changed OopStorage::Block::active_index_safe to avoid pointer
arithmetic, instead converting to uintptr_t for arithmetic.  This avoids
potential problems when the Block argument is a "false positive" from
block_for_ptr. 

Changed OopStorage::allocation_status to check up front for a null argument,
immediately returning INVALID_ENTRY in that case. This avoids voilating
block_for_ptr's precondition that the argument is non-null. Added a gtest for
this.  Also added a gtest for the potential false-positive case.

While updating gtests, removed #ifndef DISABLE_GARBAGE_ALLOCATION_STATUS_TESTS.
That macro was included when these tests were first added, because some tests
needed to be disabled on Windows, due to SafeFetchN in gtest context not working
on that platform. That was later fixed by JDK-8185734. The conditional #define
of that macro in test_oopStorage.cpp was removed, but the no longer needed
#ifndef was inadvertently not removed.

Testing: mach5 tier1-5
Locally (linux-x64) reproduced the reported ubsan failure, and verified it no
longer reproduces with these changes.

While working on this change I noticed a related issue.  The recently added
OopStorage::print_containing doesn't verify the block is not a false positive
before using it as a block.  I'll file a JBS issue for this.

-------------

Commit messages:
 - be more careful

Changes: https://git.openjdk.org/jdk/pull/21240/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21240&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340945
  Stats: 69 lines in 4 files changed: 37 ins; 4 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/21240.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21240/head:pull/21240

PR: https://git.openjdk.org/jdk/pull/21240