Until now AArch64 development has been on patched kernels that have an always
on relaxed syscall ABI where tagged pointers are accepted.
The patches that have gone into the mainline kernel rely on each process opting
in to this relaxed ABI.
This commit adds code to choose that ABI into __hwasan_init.
The idea has already been agreed with one of the hwasan developers
The patch ignores failures of EINVAL for Android, since there are older versions of the Android kernel that don't require this prctl or even have the relevant values. Avoiding EINVAL will let the library run on them.
I've tested this on an AArch64 VM running a kernel that requires this
prctl, having compiled both with clang and gcc.