TSan's shadow mappings only support 30-bits of ASLR entropy on x86
Linux, and it is not practical to support the maximum of 32-bits (due to pointer compression and the overhead of shadow mappings). Instead, this patch changes TSan to re-exec without ASLR if it encounters an
incompatible memory layout, as suggested by Dmitry in
https://github.com/google/sanitizers/issues/1716.
If ASLR is already disabled but the memory layout is still incompatible,
it will abort.
This patch involves a bit of refactoring, because the old code is:
1. InitializePlatformEarly()
2. InitializeAllocator()
3. InitializePlatform(): CheckAndProtect()
but it may already segfault during InitializeAllocator() if the memory
layout is incompatible, before we get a chance to check in
CheckAndProtect().
This patch adds CheckAndProtect() during InitializePlatformEarly(), before the allocator is initialized. Naturally, it is necessary to ensure that CheckAndProtect() does *not* allow the heap regions to be occupied here, hence we generalize CheckAndProtect() to optionally check the heap
regions. We keep the original behavior of CheckAndProtect() in InitializePlatform() as a last line of defense.
We need to be careful not to prematurely abort if ASLR is disabled but TSan was going to re-exec for other reasons (e.g., unlimited stack size); we implement this by moving all the re-exec logic into ReExecIfNeeded().
0784b1eefa