fakelinux

Run unmodified aarch64 Linux ELF binaries directly on macOS (Apple Silicon) — no virtual machine, no Docker, no Rosetta. fakelinux is a user-space Linux emulator: it loads a Linux executable into its own address space, emulates the Linux syscall ABI on top of macOS, and traps the handful of ARM64 instructions that behave differently between the two operating systems.

The host and guest share the same CPU (Apple Silicon is aarch64), so guest code runs natively at full speed. Only syscalls and a few privileged/per-thread instructions are intercepted.

Status: experimental research project. It is good enough to run real, non-trivial software — bash, apt-get update && apt-get install, vim, python3, and even VPP (the Vector Packet Processor, with live packet forwarding) — but it implements ~110 syscalls and is not a complete or hardened Linux. Expect rough edges. See docs/known-issues.md.

How it works

fakelinux uses a trap-and-emulate loop built around macOS signal delivery:

1. ELF loader (src/loader.rs) maps the Linux binary (static or dynamic, PIE or ET_EXEC) and scans .text for instructions that must be intercepted, rewriting them in place to UDF #0xF000 (an undefined instruction). A literal-pool-aware patcher avoids corrupting embedded data.
2. Each guest syscall is an SVC #0, which the patcher has turned into a UDF. Executing it raises SIGILL, which fakelinux catches.
3. The trap handler (src/trap.rs) decodes the faulting instruction and either emulates the Linux syscall (src/syscall.rs) or the privileged instruction, then resumes the guest where it left off.

Why UDF/SIGILL instead of a breakpoint? On macOS the kernel zeroes x18 on every SIGTRAP but preserves it for SIGILL. The guest needs x18, so we trap with an undefined instruction. This is one of many Apple-Silicon-specific discoveries documented in docs/devlog/.

A few things also get virtualized via the same trap mechanism:

• MSR/MRS TPIDR_EL0 — the thread pointer is backed by a pthread key, so guest TLS survives macOS clobbering the real register across signals.
• x18 accesses — x18 is the macOS "platform register"; the guest's value rides in per-thread storage.
• fork/clone and PROT_EXEC mprotect — deferred to a post-signal trampoline because these are unsafe to perform inside a signal handler.

The instruction decoder is generated from ARM's official spec

The decoder is not hand-written. build.rs parses ARM's machine-readable A64 ISA XML and generates a (mask, value, register-fields, emulation-class) table at build time. This makes the decoder a faithful, single-source-of-truth reflection of the architecture rather than a pile of hand-tuned bit masks.

The generated table is committed under src/generated/, so you don't need the ARM XML to build or run fakelinux. The XML itself is not bundled (it is ARM's copyrighted material under ARM's own license); you only need it to regenerate the table. See data/encoding/README.md.

Requirements

• macOS on Apple Silicon (M1 or later — aarch64 host is mandatory).
• Rust (stable, edition 2021) — install via rustup.
• Docker (optional) — only to cross-compile the C test suite in tests/.
• An aarch64 Linux root filesystem if you want to run dynamically-linked programs (see below). None is bundled.

Build

The generated decode table is committed under src/generated/, so the emulator builds and runs out of the box — no ARM spec download required:

cargo build --release             # binary at target/release/fakelinux

You only need ARM's ISA XML if you want to regenerate that table (e.g. to pick up a newer architecture release). Fetch the spec and rebuild with the regeneration flag set:

scripts/fetch-arm-isa-xml.sh                 # downloads the latest A64 ISA XML
FAKELINUX_REGEN_DECODER=1 cargo build        # rewrites src/generated/ from the spec

See data/encoding/README.md for details.

Usage

fakelinux [--container] <aarch64-linux-elf> [args...]

Run a static binary directly:

cargo build --release
./target/release/fakelinux ./my-static-aarch64-binary

Run something from a full Linux userland (dynamic binaries). Fetch an aarch64 root filesystem into roots/ (no Docker needed — it uses the bundled fakelinux-pull to pull the linux/arm64 image and extract it), then point FAKELINUX_ROOT at it and use --container to get a virtual PID namespace (the guest runs as PID 1, virtual root):

scripts/fetch-rootfs.sh ubuntu        # ubuntu:24.04 -> roots/ubuntu

FAKELINUX_ROOT=roots/ubuntu \
  ./target/release/fakelinux --container roots/ubuntu/bin/bash -c "uname -a && apt-get update"

For an interactive poke-around, scripts/rootfs-shell.sh opens a shell inside a rootfs (ROOTFS=roots/alpine scripts/rootfs-shell.sh, default roots/ubuntu).

Pass the guest binary as a host path that includes the roots/<name> prefix (e.g. roots/ubuntu/bin/bash); fakelinux strips the prefix to derive the guest's own view of the path. See roots/README.md for other distros and details.

Key environment variables

(See git grep FAKELINUX_ src/ for the full list, including tracing knobs.)

Tests

Rust unit tests:

cargo test

The C integration suite cross-compiles small Linux programs (in tests/) with Docker and runs each one under fakelinux, checking exit codes and output:

./tests/run_tests.sh

Project layout

src/
  main.rs         entry point; signal-handler setup, CLI parsing
  loader.rs       ELF loader + instruction patcher (literal-pool aware)
  trap.rs         SIGILL handler; instruction emulation + dispatch
  decoder.rs      decode helpers (generated table from data/encoding/)
  syscall.rs      Linux syscall dispatch and translation
  signal.rs       guest signal forwarding; sigframe build/restore
  thread.rs       fork/clone trampolines, wait4, futex
  trampoline.rs   post-signal trampoline pages (fork, mprotect)
  x18_trampoline.rs  x18 virtualization
  net.rs          sockets / networking translation
  procfs.rs       synthetic /proc, /sys
  rootfs.rs       path rebasing into the guest rootfs
  pidns.rs        virtual PID namespace
  creds.rs        virtual uid/gid (unshare -r semantics)
  ...
build.rs          parses ARM A64 ISA XML → generates the decode table
data/encoding/    ARM A64 ISA spec (only needed to regenerate the decoder)
roots/            guest root filesystems (fetched on demand; gitignored)
scripts/          helper scripts (fetch-rootfs.sh, fetch-arm-isa-xml.sh, …)
tests/            C integration tests + run_tests.sh
docs/             design notes, roadmap, known issues
docs/devlog/      development journal (the messy, honest story)

Documentation

• docs/theory.md — fork concurrency & the trampoline pattern.
• docs/compatibility-roadmap.md — path toward broader Linux compatibility.
• docs/known-issues.md — current limitations.
• docs/devlog/ — chronological development diary; the source of most of the hard-won Apple-Silicon trivia in this project.

License

MIT — see LICENSE.

The contents of data/encoding/ are derived from ARM's published Architecture Specification and are subject to ARM's own licensing terms; see data/encoding/README.md.