mke2fs built against libarchive populates an ext4 from a tarball in
one pass; Homebrew's bottle is not built that way, so when the found
mke2fs cannot take a tarball the builder unpacks the rootfs with the
library's own tar decoder and populates from the directory — slower,
and the run says so. The mkfs child is exec'd fresh, so it reads
DYLD_FALLBACK_LIBRARY_PATH (libarchive's keg) at startup.
The engine is libkrun.1.dylib, dlopen'd from the Homebrew kegs when
the loader's own search fails (bare sonames never reach the brew
prefix); the probe becomes a per-platform hypervisor probe — KVM on
Linux, sysctl + a real hv_vm_create on macOS, so an entitlement
denial (HV_DENIED) names the re-sign fix instead of surfacing as an
opaque krun_start_enter EINVAL. Tool lookup (mke2fs, debugfs) gains
the same keg fallback, and dyld_fallback_dirs() collects the kegs the
bare-soname dlopens (libkrunfw, libarchive) need in
DYLD_FALLBACK_LIBRARY_PATH. The machine child's parent-death contract
moves to a kqueue NOTE_EXIT watcher: macOS has no PR_SET_PDEATHSIG,
and SIGKILL-on-parent-exit is what keeps a killed caller from
stranding a booted machine.
vhaudiquet