/usr/lib/x86_64-linux-gnu/libdmalloc.so is in libdmalloc-dev 5.5.2-8.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libdmalloc.so.5.5.2
/usr/lib/x86_64-linux-gnu/libdmalloc.so.5.5.2