Next-gen malloc for musl libc - Working draft

This is a draft of the upcoming next-gen malloc implementation for musl libc. It is essentially feature-complete, but is not yet integrated with musl and lacks some additional hardening and optimizations that the final version is expected to include.

The included Makefile builds static and shared library files that programs can link with, or use with LD_PRELOAD (for the shared version).

High-level design

This allocator organizes memory dynamically into small slab-style groups of up to 32 identical-size allocation units with status controlled by bitmasks, and utilizes a mix of in-band and out-of-band metadata to isolate sensitive state from regions easily accessible through out-of-bounds writes, while avoiding the need for expensive global data structures.

The closest analogue among other well-known allocators is probably OpenBSD's omalloc.

Base allocation granularity and alignment is 16 bytes. Large allocations are made individually by mmap, but they also have out-of-band metadata records treating them as special one-member groups, allowing realloc and free to validate them. Smaller allocations come from groups of one of 48 size classes, spaced linearly up to 128 (the first 8 classes), then roughly geometrically with four steps per doubling, but adjusted to divide powers of two with minimal remainder (waste).