Note that glibc does provide a malloc_usable_size to query the size of a malloc'd block; not standard though of course.

A problem with just directly exposing such is it makes precise sanitizing impossible, as you'd have to tolerate some out-of-intended-bounds reads/writes. (and making the sanitizer always give exact-size allocations would also be bad as that'd end up not testing code paths that may break when they're not)