Show HN: Kakehashi – Experimental userspace to run macOS binaries on Linux ARM

Userspace macOS ARM64 → Linux aarch64 translation layer (CLI-first, no JIT).

Load Darwin Mach-O on Linux aarch64, map a freestanding libSystem, translate BSD syscalls, and run real guests (clang probes, 7-Zip 7zz, curl, threads).

Verified on Docker/Colima and UTM (Linux aarch64). Install once:

cargo install kakehashi
# or from a checkout:
cargo install --path crates/kh-cli --force

kh bottle ensure
kh install 7zip    # Darwin 7zz → guest /usr/local/bin/7zz
kh install curl    # Darwin curl → guest /usr/local/bin/curl

Relative -o / archive paths resolve against the host CWD of the kh process (create parent dirs yourself, or rely on auto-mkdir for O_CREAT). Through the bottle, /Volumes/linux/… bridges to the host root (/ → host /).

# Version / help
kh run 7zz --
kh run 7zz -- --help

# Create archive (cwd-relative)
kh run 7zz -- a demo.7z README.md
kh run 7zz -- t demo.7z
kh run 7zz -- l demo.7z
kh run 7zz -- x -o./out demo.7z

# Multi-thread compress (correctness gate)
kh run 7zz -- a -t7z -m0=lzma2 -mx=5 -mmt=4 mt.7z README.md
kh run 7zz -- t mt.7z
# expect: Everything is Ok, exit 0

Docker helpers (artifacts under host .tmp/kh-out/):

./scripts/docker-7zz.sh --help
./scripts/docker-7zz.sh a /Volumes/linux/out/demo.7z /Volumes/linux/src/README.md
ls -lh .tmp/kh-out/demo.7z

KAKEHASHI_HYPERCALL=1 ./scripts/docker-7zz.sh a -t7z -m0=lzma2 -mx=5 -mmt=4 \
  /Volumes/linux/out/mt.7z /Volumes/linux/src/README.md
./scripts/docker-7zz.sh t /Volumes/linux/out/mt.7z

# Banner (G1)
kh run curl -- --version

# HTTP GET → file (G3 / G5). Parents for -o are created when missing.
kh run curl -- -sS -o .tmp/kh-out/body http://example.com/
# expect: exit 0, ~559 bytes, HTML contains "Example Domain"
wc -c .tmp/kh-out/body
head -c 80 .tmp/kh-out/body; echo

# HTTP to stdout
kh run curl -- -sS http://example.com/ | head -c 80; echo

# HTTPS GET (G4) — OpenSSL + bottle CA (from host or curl.se download)
kh run curl -- -sS -o .tmp/kh-out/https-body https://example.com/
wc -c .tmp/kh-out/https-body

# Negative: bad / self-signed cert must fail (rc ≠ 0)
kh run curl -- -sS -o /dev/null https://self-signed.badssl.com/; echo exit:$?

Docker helpers:

./scripts/docker-curl.sh --version
./scripts/docker-curl.sh -sS -o /Volumes/linux/out/body http://example.com/
./scripts/docker-curl.sh -sS -o /Volumes/linux/out/https-body https://example.com/
ls -lh .tmp/kh-out/body .tmp/kh-out/https-body

# Trace-first probe logs → .tmp/kh-curl-probe/
./scripts/docker-curl-probe.sh --version

# Option matrix (large tiers) → .tmp/kh-curl-options/
./scripts/docker-curl-options.sh tier1
./scripts/docker-curl-options.sh tier9-10
./scripts/docker-curl-options.sh all          # tier1..10

Harmless noise on many runs:

• kh: open fail ENOENT(openat) path=/etc/ssl/openssl.cnf — OpenSSL optional config; HTTP/HTTPS still work via the seeded CA bundle.
• WARN … skip dylib … Security/CoreFoundation — Apple frameworks not in the bottle; soft stubs cover the load path.
• unresolved strong symbol; bound to named missing trampoline — symbols not hit on the happy path.

Details and gates: docs/curl.md.

Full curl feature set (POST bodies, proxies, HTTP/3 end-to-end, every scheme), real Apple Security.framework, git / CLT, GUI, codesign. Next product slice: git via kh install xcode-tools — see docs/git.md.

The guest dylib is vendored at crates/kh-runtime/resources/libSystem.B.dylib and compiled into the runtime with include_bytes!. Publishing kh-runtime ships the dylib; end users do not need a separate download.

• Rust 1.88+
• Linux aarch64 for live kh run / kh trace
• Page sizes: 4 KiB (containers) and 16 KiB (Asahi-class)
• Optional: curl/wget + tar for kh install 7zip / kh install curl

cargo install kakehashi
# or from a checkout: cargo install --path crates/kh-cli

kh bottle ensure
kh install 7zip
kh install curl

Default root: ~/.local/share/kakehashi/bottle/ (override with KAKEHASHI_DATA_DIR / KAKEHASHI_ROOT).

Kakehashi runs guest code natively on the CPU. The tax is the syscall boundary (TLS switch, alt stack, NEON save/restore, Rust dispatch) × how chatty the guest is — not an instruction emulator.

On Ubuntu aarch64 bare-metal (UTM), multi-file 7zz archive (-t7z -m0=lzma2 -mx=5 -mmt=4, ~8k files / ~240 MiB tree):

On compression-heavy, few-file samples the gap is often much smaller (~×1.1–1.2). The large multi-file gap is dominated by path walk + per-syscall boundary, not “wrong LZMA”.

Hypercall is on by default for all guest threads. Opt out with KAKEHASHI_HYPERCALL=0 only for debug (residual svc→brk / SIGTRAP).

The product goal for CI is not “as fast as native macOS,” but run Darwin CLI/tools on cheap Linux aarch64 runners instead of scarce, expensive macOS capacity.

GitHub Actions hosted runners (private-repo overage rates, USD per minute; see Actions runner pricing):

macOS standard is roughly ×10–×12 the Linux arm64 minute rate before any wall-time difference. Even if a job runs ×5 slower under kh on Linux arm64 than the same work on a macOS runner, billable cost can still be lower because the macOS minute is an order of magnitude more expensive (illustrative: 5 × $0.005 ≈ $0.025 vs 1 × $0.062). Public-repo free minutes and self-hosted Linux amplify that further; macOS hosted capacity also tends to queue longer and (on GitLab SaaS) is Premium/Ultimate / beta-gated.

When macOS runners still win: GUI, codesign/notarization, Xcode UI tests, or any workload that is not a pure CLI Darwin binary under freestanding libSystem.

Gates, not benches: CI correctness is cargo test / smoke / 7zz -mmt=4, not “match native wall clock.” Perf work is tracked in docs/roadmap.md.

docker build -t kakehashi:dev -f Dockerfile.dev .
docker run --rm -v "$PWD":/src -w /src kakehashi:dev \
  cargo test --workspace --exclude kh-libsystem

# Full smoke (build + clippy + test + micro run)
./scripts/docker-smoke.sh

cargo build -p kakehashi --release
cargo test --workspace --exclude kh-libsystem
cargo clippy --workspace --exclude kh-libsystem --all-targets -- -D warnings

# Maintainers: refresh embed after freestanding ABI changes
cargo build -p kh-libsystem --release --target aarch64-apple-darwin
./scripts/stage-libsystem.sh   # → crates/kh-runtime/resources/libSystem.B.dylib

libSystem discovery: --libsystem → KAKEHASHI_LIBSYSTEM → paths next to kh → crate resources/ → embedded bytes in kh-runtime.

.tmp/, .kh/, and target/ are gitignored.

Bottle bridges the Linux FS as /Volumes/linux/…:

Apache License 2.0. See LICENSE.txt and NOTICE.

This project is not derived from Darling. Do not vendor proprietary Apple SDKs or blobs.