tsiry-sandratraina.com / cVisor
Embedded bash sandbox for agents, inspired by gVisor
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cVisor sdks python
2 folders · 5 files
cvisor
tests
hatch_build.py
pyproject.toml
README.md
remote_smoke.py
uv.lock
README.md

cVisor — Python SDK #

A ctypes FFI wrapper over the libcvisor C ABI. Linux-only.

Quick try (Docker) #

Drop into a Python REPL with cvisor installed, from any machine with Docker:

docker run -it --rm \
  --security-opt seccomp=unconfined --security-opt apparmor=unconfined \
  ghcr.io/astral-sh/uv:python3.12-alpine \
  uv run --with cvisor python

>>> from cvisor import Sandbox
>>> sb = Sandbox()
>>> print(sb.run("echo hi; uname -n").stdout)
hi
cvisor

The --security-opt flags are required: cVisor installs its own seccomp filter, which Docker's default profiles block. An Alpine (musl) image is needed — the published wheels are musllinux-tagged.

Install (uv) #

uv add cvisor

Usage #

from cvisor import Sandbox

with Sandbox() as sb:
    out = sb.run("echo hello")
    print(out.stdout)   # "hello\n"
    print(out.stderr)   # ""

Sandbox.run(cmd) blocks until the sandboxed command exits and returns an Output with .stdout / .stderr (str), .stdout_bytes / .stderr_bytes, and .exit_code (int, shell convention: the command's status, or 128+signo if it was killed by a signal).

Timeouts #

Sandbox.run(cmd, timeout_ms=...) SIGKILLs the guest after timeout_ms milliseconds; a timed-out run reports exit code 137:

out = sb.run("sleep 30", timeout_ms=300)
assert out.exit_code == 137

Network policy #

Sandbox.set_allow_network(allow) controls outbound INET/INET6 networking (allowed by default):

sb.set_allow_network(False)  # deny outbound networking

Sandbox.set_allow_listen(allow) controls inbound TCP servers (listen/ accept), denied by default:

sb.set_allow_listen(True)  # allow the guest to run TCP servers
sb.set_env("TOKEN", "xyz")   # environment variable for the guest

Files #

Sandbox.write_file(path, data) writes data (bytes or str) into the sandbox's persistent overlay, and Sandbox.read_file(path) returns the file's bytes as the guest sees it (the overlay copy, else the real host file for cow paths). path must be absolute; /proc and passthrough paths are not writable. Files written are visible to later run calls of the same Sandbox instance:

with Sandbox() as sb:
    sb.write_file("/tmp/data.txt", "seeded\n")
    print(sb.run("cat /tmp/data.txt").stdout)   # "seeded\n"

    sb.run("echo from-run > /tmp/out.txt")
    print(sb.read_file("/tmp/out.txt"))         # b"from-run\n"

write_file raises OSError on failure; read_file returns b"" for an empty file and raises FileNotFoundError for a missing or unreadable path.

Copying files and directories #

Sandbox.copy_into(host_path, guest_path) copies a host file — or a whole directory tree — into the sandbox's persistent overlay, and Sandbox.copy_out(guest_path, host_path) copies one back out to the real filesystem. Directory copies are recursive and respect .gitignore / .dockerignore. Copied files are visible to later run calls:

with Sandbox() as sb:
    sb.copy_into("./my-project", "/work")     # host dir -> overlay
    sb.run("cd /work && make")
    sb.copy_out("/work/build", "./build-out")  # overlay -> host

Both raise OSError on failure.

Cache #

Sandbox.cache_save(sandbox_path, key, backend="", format="gzip") archives a sandbox directory under key to a cache backend, and Sandbox.cache_restore(sandbox_path, key, backend="", format="gzip") unpacks it back into the overlay. This lets one sandbox persist a directory (a build tree, a dependency cache, …) that a later, independent sandbox can restore:

with Sandbox() as sb:
    sb.run("cd /work && npm install")
    sb.cache_save("/work/node_modules", "npm-deps")

with Sandbox() as sb2:
    sb2.cache_restore("/work/node_modules", "npm-deps")  # ready to use

Both the archive and cache_save respect .gitignore / .dockerignore.

backend selects where the archive is stored:

  • "" or "disk" — host disk (the default)
  • "disk:/path" — host disk at an explicit location
  • "s3://bucket/prefix?region=..&endpoint=.." — S3

S3 requires the library to be built with the s3 feature; the bundled libcvisor.so is not, so disk is the working default.

format selects the archive format:

  • "" or "gzip" — gzip (the default)
  • "estargz" — seekable, lazily-pullable gzip
  • "none" — uncompressed
  • "zstd" — zstd, only if the library is built with zstd (the bundled lib is not)

Both methods raise OSError on failure.

Streaming output #

Sandbox.run_streaming(cmd, on_stdout=..., on_stderr=..., poll_ms=15) runs a command and delivers output to callbacks as it is produced, instead of buffering it all until exit. Each callback receives a decoded str chunk. Returns the exit code:

with Sandbox() as sb:
    sb.run_streaming(
        "for i in 1 2 3; do echo line$i; sleep 0.1; done",
        on_stdout=lambda s: print(s, end=""),
    )

Interactive PTY shell #

Sandbox.shell(on_output=..., poll_ms=15) starts an interactive /bin/sh -i session on a PTY (so test -t 1 is true and programs behave as if attached to a terminal). It returns a Session you can drive:

with Sandbox() as sb:
    sh = sb.shell(on_output=lambda s: print(s, end=""))
    sh.write_stdin("echo hello\n")
    sh.resize(40, 120)          # rows, cols
    sh.write_stdin("exit 0\n")
    code = sh.wait()            # blocks for the exit code
    sh.close()

Session methods:

  • write_stdin(data) — feed input (bytes or str); returns bytes written (PTY only).
  • read_stdout() / read_stderr() — drain and return new bytes (a PTY merges the streams, so stderr is empty). on_output above does this for you on a daemon thread.
  • resize(rows, cols) — resize the PTY.
  • exit_code() — the exit code if the session has finished, else None (non-blocking).
  • wait() — block until exit and return the code.
  • kill() — SIGKILL the session.
  • close() — free the session (idempotent; also a context manager and on __del__).

Remote daemon (GraphQL) — works on macOS #

The Sandbox above needs the native libcvisor (Linux only). To use cVisor from any platform, including macOS, talk to a running cvisord over its GraphQL API. This path is pure stdlib (urllib.request + json) — no native library — and importing it never loads the .so:

from cvisor import RemoteSandbox, GraphQLClient

remote = RemoteSandbox("http://127.0.0.1:8080/graphql", token)
out = remote.run("echo hello")
print(out["stdout"])                       # "hello\n"

sb = remote.create_sandbox("my-box")
remote.write_file(sb["id"], "/app/x", "hi")   # base64 handled for you
print(remote.read_file(sb["id"], "/app/x"))   # b"hi"
snap = remote.snapshot(sb["id"])
remote.fork(sb["id"], "clone")
remote.free_sandbox(sb["id"])

RemoteSandbox mirrors the daemon surface: run, create_sandbox / list_sandboxes / free_sandbox / configure, write_file / read_file, cache_save / cache_restore / cache_list, snapshot / rollback / branch / fork / snapshots / delete_snapshot, and health. For raw documents, drop to the client:

gql = GraphQLClient("http://127.0.0.1:8080/graphql", token)
data = gql.query("{ sandboxes { id name } }")
gql.mutate("mutation($c:String!){ run(command:$c){ stdout } }", {"c": "uname -a"})

The daemon prints its bearer token on startup (or set CVISOR_TOKEN). On a non-Linux host, constructing the FFI Sandbox raises a clear "Linux-only" RuntimeError pointing you here.

Interactive console #

Launch an IPython REPL with a live sandbox preloaded:

uv run --extra console cvisor   # or: python -m cvisor

cVisor interactive console
  sb          -> a Sandbox instance
  sh("cmd")   -> run a shell command in the sandbox, printing stdout/stderr
  Sandbox     -> create your own: Sandbox()

In [1]: sh("echo hello; uname -n")
hello
cvisor

Without the console extra (IPython) it falls back to the stdlib REPL.

Development #

The SDK loads libcvisor.so. Build it from the repo root and point the SDK at it via the CVISOR_LIB environment variable, or let the package resolve a copy bundled under cvisor/_native/:

# from the repo root — builds libcvisor.so into cvisor/_native/
cargo xtask ffi

# run the tests with uv
cd sdks/python
uv run pytest