ASan poisoning has alignment requirements: the user must add padding such that the padding ends on a byte boundary in the shadow memory. Since each bit in the ASan shadow memory encodes the state of a byte in real memory, this means that the size + padding for each allocation must align on a 8 byte boundary. If this requirement is not satisfied it can lead to incorrect bug reporting, including missed and false-positive reports.
See the included examples for a little background. One is a small program to show what can go wrong with manual shadow memory poisoning. The second is an example implementation of manual poisoning using the std::allocator interface.
To build and run the allocator example:
nmake allocator && allocator.exe
To build the 'bad unpoisoning' example:
nmake unpoison && unpoison.exe
For further information on the shadow bytes and asan see