rsyscall-ng

A modernization of rsyscall, the process-independent, type-safe, low-level interface to Linux system calls for Python. In rsyscall every syscall is a method on a process object, which may be the local interpreter, a child created with clone, or a process on a remote host; new processes start out sharing everything with their parent and are then shaped with ordinary syscalls (unshare, execve, ...) instead of fork/posix_spawn.

Installation

uv add rsyscall-ng        # or: pip install rsyscall-ng

Linux x86_64 and CPython 3.12 or newer. The wheel bundles the native side, so nothing else is needed.

What it looks like

A process object runs system calls in the process it stands for. Here that is a child of the interpreter:

import trio
from rsyscall import local_process

async def main():
    child = await local_process.fork()        # a new process, driven from this one
    # write(2) runs in the child, on a buffer allocated in the child's memory
    await child.stdout.write(await child.ptr(b"Hello from the child!\n"))
    echo = await child.environ.which("echo")  # looked up on the child's PATH
    await (await child.exec(echo.args("...now it is echo"))).check()  # execve(2), then wait

trio.run(main)

The same kind of object can stand for a process on another machine. Below, the Python code runs here, while the process, its fork and its execve run on worker:

import trio
from rsyscall import local_process
from rsyscall.tasks.ssh import make_ssh_host

async def main():
    host = await make_ssh_host(local_process, lambda ssh: ssh.args("worker"))
    _, worker = await host.ssh(local_process)      # a process on worker, driven from here
    child = await worker.fork()                     # the fork happens on worker
    uname = await child.environ.which("uname")      # found on worker's PATH
    await (await child.exec(uname.args("-n"))).check()  # prints worker's hostname

trio.run(main)

worker is any x86_64 Linux machine you can reach with ssh worker that has a POSIX shell and GNU coreutils. Nothing has to be installed there: a small static bootstrap helper travels over the same ssh connection. The function passed to make_ssh_host extends the ssh command line, so ssh options such as a jump host or connection sharing can go there. scripts/hello.py is the first example with more output; make hello and the CI run it.

Relation to upstream

Upstream has been dormant since 2022-07-25 and its PyPI package no longer installs. rsyscall-ng keeps the upstream Python API (imported with its full git history under python/) and brings it to current Python (>= 3.12) and trio. The small native part (a syscall server, a clone trampoline, a futex helper and three bootstrap helpers, C + x86_64 assembly upstream) has been reimplemented in Rust under native/, clean-room from the specification in docs/spec/. The unmodified upstream C is still built locally, only as a differential test oracle (BACKEND=c).

Status

• upstream python/ imported with history (commit 2a36209, 2022-07-25)
• compatibility patches for Python 3.12+ / current trio and outcome
• test harness migrated off trio.MultiError
• baseline of the upstream test-suite recorded against the original C backend (tests/baseline-c.txt)
• clean-room v0 specification of the native side (docs/spec/)
• Rust native side (native/): librsyscall.so and the three helper executables, byte-compatible with the C oracle (make baseline-diff is empty; make probe-diff differs only where the specification leaves behaviour unspecified)
• PEP 517 packaging: pip install . and make wheel build an abi3 wheel that bundles the Rust native side (rsyscall/_native); make wheel-test checks the installed wheel against the recorded baseline on CPython 3.12, 3.13 and 3.14, and make sdist-test does the same for a wheel built from the sdist outside the repository
• bootstrap helpers decoupled from Nix: rsyscall.tasks.{stub,stdin_bootstrap,ssh} take them from rsyscall._native and OpenSSH from PATH; test_stub, test_stdinboot, test_persistent and test_ssh are in the baseline, so the differential test covers the helper executables
• continuous integration on GitHub-hosted Ubuntu 24.04 runners: crate tests, the differential C-vs-Rust baseline recorded on the runner, and the wheel and the sdist tested on CPython 3.12 to 3.14
• aarch64 (later phase)

Layout

Quick start

Requirements: Linux x86_64, git, uv, a C compiler (for the cffi extension), Rust 1.88 or newer with cargo (native/ uses naked functions) and, for the C oracle only, autotools, libtool, pkg-config and the Linux kernel headers.

make oracle     # clone upstream at the pinned commit and build c/ into reference/prefix
make venv       # .venv with an editable install of python/ built against the oracle
make hello      # clone a child process, write to its stdout, exec `echo`
make baseline   # run the suite and (re)generate tests/baseline-c.txt

The Rust native side is selected with BACKEND=rust, which switches the prefix (native/prefix), the virtualenv (.venv-rust) and the recorded baseline:

make native                                       # cargo build, install into native/prefix, ELF checks
make venv BACKEND=rust && make hello BACKEND=rust
make baseline BACKEND=rust && make baseline-diff  # tests/baseline-rust.txt vs tests/baseline-c.txt
make probe-diff                                   # black-box probes of the helper executables, both backends
make native-test                                  # the crate's own tests (cargo test -p rsyscall-core)
python3 scripts/header-check.py                   # rsyscall.h against the spec's layout tables

In this development flow scripts/env.sh exports RSYSCALL_NATIVE=prefix: the cffi extension rsyscall._raw is linked through pkg-config against the selected backend's prefix, with its rpath baked in by make venv (plus LD_LIBRARY_PATH as a belt-and-braces measure, since both venvs share the in-tree extension), and RSYSCALL_LIBEXEC_DIR points rsyscall._native at that prefix's helper executables. Nothing from native/ is copied into python/ in this mode.

Installing and wheels

Outside the development flow the default mode, bundled, applies: setup.py runs cargo build --release in native/, copies librsyscall.so and the three helper executables into rsyscall/_native/ and links the extension with an $ORIGIN/_native rpath, so the installed package is self-contained and needs neither Nix nor a prefix:

uv pip install .        # or pip install .: builds native/ with cargo, nothing else to set up
make wheel              # dist/rsyscall_ng-*.whl, abi3 (CPython 3.12+), from a clean dist/
make wheel-manylinux    # check it (auditwheel, abi3audit) and retag it as manylinux_2_17 (uvx)
make wheel-test         # install the newest wheel into .venv-wheel and exercise it from /tmp:
                        # import, bundled helpers, smoke test, test-suite vs tests/baseline-rust.txt
RSYSCALL_WHEEL_PYTHON=3.12 make wheel-test   # the same on another interpreter (3.12, 3.13, 3.14)
make sdist              # source distribution: python/ plus the native/ sources and docs/spec/
make sdist-test         # unpack it outside the repository, build a wheel as pip would
                        # (isolated PEP 517 build, cargo included) and test it like wheel-test

The wheel is tagged cp312-abi3: the cffi extension uses only the stable ABI (abi3audit finds no violation), so one wheel serves every CPython from 3.12 on. Both the wheel and a wheel built from the sdist give the recorded baseline's per-test results on 3.12, 3.13 and 3.14. Building from the sdist needs, on the installing machine, cargo with Rust 1.88 or newer (the first release with naked functions; 1.87 fails), a C compiler and the Python headers (python3-dev on Debian and Ubuntu).

rsyscall._native.helper(name) returns the path of a bundled helper executable (or of the one under RSYSCALL_LIBEXEC_DIR, when set) and library_path() the bundled librsyscall.so, or None when the extension was linked against a prefix. Wheels bundle only artefacts built from native/; the upstream C is never packaged.

Running tests

make test PYTEST_ARGS='-k clone'          # any pytest arguments
RSYSCALL_TEST_OPTIONAL=1 make test        # collect even the modules whose environment is missing

Modules are ignored at collection time only when what they need is missing on this machine: a Nix store (test_nix.py); ssh, sshd and ssh-keygen on PATH or in /usr/sbin, /usr/local/sbin, /sbin (test_ssh.py, test_persistent.py); /dev/fuse (test_fuse.py); pyroute2 and /dev/net/tun (test_net.py). RSYSCALL_TEST_OPTIONAL=1 collects them regardless, and the fourth header line of a baseline records what was ignored and why. The baseline records the status of every collected test; it is a description of reality, not a promise that everything passes. make baseline runs every test in its own interpreter: the tests share the module-level rsyscall.local_process, whose state does not survive a test being killed by the timeout, so in a single process one hanging test would poison the rest. Tests also leave processes behind (they share the interpreter's memory and hold both ends of their syscall sockets), so make baseline and make wheel-test run each pytest in a session of its own and kill its whole process group afterwards (scripts/pytest_session.py).

Continuous integration

.github/workflows/ci.yml runs on pushes to main, on pull requests, weekly and on demand, on GitHub-hosted Ubuntu 24.04 runners. scripts/ci-setup.sh installs autotools and the OpenSSH server and lifts Ubuntu's AppArmor restriction on unprivileged user namespaces, which the test-suite creates.

• differential runs the crate tests, builds the Rust native side with its ELF checks and the C oracle, runs the specification checks and the smoke tests, and records both baselines on the runner. It fails unless the two are identical. Differences from the committed baselines are only warnings, because a few tests depend on the machine; the recorded baselines and the per-test logs are uploaded as artifacts.
• dist builds the manylinux abi3 wheel, checked by auditwheel and abi3audit, and the sdist, and uploads them as the dist artifact.
• test-dist installs the wheel on CPython 3.12, 3.13 and 3.14, and a wheel built from the sdist, and compares each one's per-test results with the Rust baseline recorded by the differential job.

Actions are pinned to commit SHAs, with the release named in a comment.

Licensing

New code, including all of native/, is MIT, Copyright (c) 2026 Carlos Andrés Planchón Prestes (see LICENSE). python/ is MIT per upstream package metadata, Copyright 2018-2022 Spencer Baugh and contributors. Upstream c/ is intentionally not part of this repository; native/ was written from docs/spec/ alone, without access to it. See NOTICE for details.