sandlock 0.4.8

Lightweight process sandbox using Landlock, seccomp, and seccomp user notification

sandlock Python SDK

Python bindings for sandlock, a lightweight process sandbox using Landlock and seccomp. No root, no Docker, no namespaces required.

Requires Linux 6.7+ with Landlock ABI v6.

pip install sandlock

Quick start

from sandlock import Sandbox, Policy

policy = Policy(
    fs_readable=["/usr", "/lib", "/lib64", "/bin", "/etc", "/proc", "/dev"],
    fs_writable=["/tmp"],
)
result = Sandbox(policy).run(["echo", "hello"])
assert result.success
print(result.stdout)  # b"hello\n"

API reference

Platform

sandlock.landlock_abi_version() -> int

Return the Landlock ABI version supported by the running kernel. Returns -1 if Landlock is unavailable.

sandlock.min_landlock_abi() -> int

Return the minimum Landlock ABI version required by sandlock (currently 6).

Policy

sandlock.Policy(**kwargs)

An immutable (frozen dataclass) sandbox policy. All fields are optional. Unset fields mean "no restriction" unless noted otherwise.

Filesystem (Landlock)

Parameter	Type	Default	Description
fs_readable	list[str]	[]	Paths the sandbox can read
fs_writable	list[str]	[]	Paths the sandbox can write
fs_denied	list[str]	[]	Paths explicitly denied
workdir	str | None	None	Working directory; enables COW protection
chroot	str | None	None	Path to chroot into before confinement

Network

Parameter	Type	Default	Description
net_allow_hosts	list[str]	[]	Allowed domain names (empty = unrestricted)
net_bind	list[int | str]	[]	TCP ports the sandbox may bind (empty = unrestricted)
net_connect	list[int | str]	[]	TCP ports the sandbox may connect to (empty = unrestricted)
port_remap	bool	False	Transparent TCP port virtualization
no_raw_sockets	bool	True	Block raw IP sockets
no_udp	bool	False	Block UDP sockets

IPC and process isolation

Parameter	Type	Default	Description
isolate_ipc	bool	False	Block abstract UNIX socket connections outside sandbox
isolate_signals	bool	False	Block signals to processes outside sandbox
isolate_pids	bool	False	Hide foreign PIDs in /proc

Resource limits

Parameter	Type	Default	Description
max_memory	str | int | None	None	Memory limit, e.g. "512M" or int bytes
max_processes	int	64	Lifetime fork count limit
max_open_files	int | None	None	Max file descriptors (RLIMIT_NOFILE)
max_cpu	int | None	None	CPU throttle as percentage of one core (1-100)
cpu_cores	list[int] | None	None	CPU cores to pin sandbox to
num_cpus	int | None	None	Visible CPU count in /proc/cpuinfo

Syscall filtering (seccomp)

Parameter	Type	Default	Description
deny_syscalls	list[str] | None	None	Syscall names to block (blocklist mode)
allow_syscalls	list[str] | None	None	Syscall names to allow (allowlist mode)

Set one or neither, not both.

Deterministic execution

Parameter	Type	Default	Description
random_seed	int | None	None	Seed for deterministic getrandom()
time_start	datetime | float | str | None	None	Start timestamp for time virtualization
no_randomize_memory	bool	False	Disable ASLR
no_huge_pages	bool	False	Disable Transparent Huge Pages
deterministic_dirs	bool	False	Sort directory entries lexicographically
hostname	str | None	None	Override hostname via uname()

Environment

Parameter	Type	Default	Description
clean_env	bool	False	Start with minimal environment
env	dict[str, str]	{}	Variables to set/override in the child
close_fds	bool	True	Close inherited file descriptors (3+)

GPU access

Parameter	Type	Default	Description
gpu_devices	list[int] | None	None	GPU device indices to expose ([] = all)

Misc

Parameter	Type	Default	Description
uid	int | None	None	Map to given UID inside a user namespace (e.g. 0 for fake root)
no_coredump	bool	False	Disable core dumps

COW filesystem isolation

Parameter	Type	Default	Description
fs_isolation	FsIsolation	NONE	NONE, BRANCHFS, or OVERLAYFS
fs_storage	str | None	None	Storage directory for BranchFS deltas
max_disk	str | None	None	Disk quota for BranchFS (e.g. "1G")
on_exit	BranchAction	COMMIT	COMMIT, ABORT, or KEEP
on_error	BranchAction	ABORT	COMMIT, ABORT, or KEEP

Sandbox

sandlock.Sandbox(policy, policy_fn=None, init_fn=None, work_fn=None)

Create a sandbox from a Policy.

• policy -- a Policy instance.
• policy_fn -- optional callback for dynamic per-event decisions (see Dynamic policy).
• init_fn / work_fn -- callbacks for COW fork mode (see Fork).

Sandbox is a context manager:

with Sandbox(policy) as sb:
    result = sb.run(["echo", "hello"])

sandbox.run(cmd, timeout=None) -> Result

Run a command, capturing stdout and stderr.

• cmd -- list of strings (command and arguments).
• timeout -- max execution time in seconds (float). None = no timeout.

result = Sandbox(policy).run(["python3", "-c", "print(42)"], timeout=10.0)

sandbox.dry_run(cmd, timeout=None) -> DryRunResult

Run a command in a temporary COW layer, then discard all writes. Returns the list of filesystem changes that would have been made.

result = Sandbox(policy).dry_run(["sh", "-c", "echo hi > /tmp/out.txt"])
for change in result.changes:
    print(change.kind, change.path)  # "A /tmp/out.txt"

sandbox.run_interactive(cmd) -> int

Run with inherited stdio (no capture). Returns the exit code.

sandbox.pid -> int | None

The child PID while running, None otherwise.

sandbox.pause() / sandbox.resume()

Send SIGSTOP / SIGCONT to the sandbox process group. Raises RuntimeError if the sandbox is not running.

sandbox.checkpoint(save_fn=None) -> Checkpoint

Capture a checkpoint of the running sandbox. See Checkpoint.

Result

Returned by sandbox.run().

Attribute	Type	Description
success	bool	True if exit code is 0
exit_code	int	Process exit code
stdout	bytes	Captured standard output
stderr	bytes	Captured standard error
error	str | None	Error message on failure

DryRunResult

Returned by sandbox.dry_run().

Same attributes as Result, plus:

Attribute	Type	Description
changes	list[Change]	Filesystem changes detected

Change

Attribute	Type	Description
kind	str	"A" (added), "M" (modified), or "D" (deleted)
path	str	Path relative to workdir

Stage and Pipeline

Chain sandboxed commands with pipes using the | operator:

result = (
    Sandbox(policy_a).cmd(["echo", "hello"])
    | Sandbox(policy_b).cmd(["tr", "a-z", "A-Z"])
).run()
assert result.stdout == b"HELLO\n"

sandbox.cmd(args) -> Stage

Create a lazy Stage bound to this sandbox.

pipeline.run(stdout=None, timeout=None) -> Result

Run the pipeline. Each stage's stdout feeds the next stage's stdin.

Dynamic policy

Use policy_fn to make per-syscall decisions at runtime:

from sandlock import Sandbox, Policy, SyscallEvent, PolicyContext

def my_policy(event: SyscallEvent, ctx: PolicyContext):
    if event.category == "network" and event.host == "evil.com":
        return True   # deny
    if event.category == "file" and "/secrets" in (event.path or ""):
        ctx.deny_path("/secrets")
        return True   # deny
    return False      # allow

sb = Sandbox(Policy(...), policy_fn=my_policy)

SyscallEvent

Attribute	Type	Description
syscall	str	Syscall name (e.g. "openat", "connect")
category	str	"file", "network", "process", or "memory"
pid	int	Process ID
parent_pid	int	Parent process ID
path	str | None	File path (for file events)
host	str | None	Hostname (for network events)
port	int	Port number (for network events)
argv	tuple[str, ...] | None	Command arguments (for execve)
denied	bool	Whether this event was already denied by static policy

Helper methods:

• event.path_contains(s) -- True if path contains substring s
• event.argv_contains(s) -- True if any argv element contains s

PolicyContext

Methods available inside policy_fn:

Method	Description
ctx.restrict_network(ips)	Restrict to given IP addresses
ctx.grant_network(ips)	Allow additional IP addresses
ctx.restrict_max_memory(bytes)	Lower memory limit
ctx.restrict_max_processes(n)	Lower process limit
ctx.restrict_pid_network(pid, ips)	Per-PID network restriction
ctx.deny_path(path)	Deny access to a path
ctx.allow_path(path)	Remove a previously denied path

Callback return values:

Return	Meaning
None, False, 0	Allow
True, -1	Deny (EPERM)
positive int	Deny with that errno
"audit", -2	Allow but flag for audit

Fork

COW fork for parallel execution with shared initialization:

sb = Sandbox(policy,
    init_fn=lambda: load_model(),
    work_fn=lambda clone_id: process(clone_id),
)
clones = sb.fork(4)  # returns ForkResult with .pids

sandbox.reduce(cmd, fork_result) -> Result

Pipe combined clone output into a reducer command:

result = Sandbox(policy).reduce(["python3", "sum.py"], clones)

Checkpoint

Save and restore sandbox state:

sb = Sandbox(policy)
# ... start a long-running process ...
cp = sb.checkpoint(save_fn=lambda: my_state_bytes())
cp.save("my-snapshot")

# Later:
cp2 = Checkpoint.load("my-snapshot")
Checkpoint.restore("my-snapshot", restore_fn=lambda data: rebuild(data))

Method	Description
cp.save(name, store=None)	Persist checkpoint to disk
Checkpoint.load(name, store=None)	Load from disk
Checkpoint.restore(name, restore_fn, store=None)	Load and call restore_fn with app_state
Checkpoint.list(store=None)	List saved checkpoint names
Checkpoint.delete(name, store=None)	Delete a saved checkpoint

Properties: cp.name (str), cp.app_state (bytes or None).

Default store: ~/.sandlock/checkpoints/.

Profiles

Load policies from TOML files:

from sandlock import load_profile, list_profiles

policy = load_profile("web-scraper")
names = list_profiles()

Exceptions

SandlockError (base)
  +-- PolicyError          invalid policy configuration
  +-- SandboxError         sandbox lifecycle errors
        +-- ForkError          fork failed
        +-- ChildError         child exited abnormally
        +-- BranchError        BranchFS operation failed
        |     +-- BranchConflictError   sibling branch committed (ESTALE)
        +-- ConfinementError   Landlock/seccomp setup failed
              +-- LandlockUnavailableError   no Landlock support
              +-- SeccompError               seccomp filter failed
                    +-- NotifError           notif supervisor error
  +-- MemoryProtectError   mprotect failed

All exceptions are importable from sandlock.exceptions or directly from sandlock:

from sandlock import SandlockError, SandboxError, PolicyError

Enums

FsIsolation

• FsIsolation.NONE -- direct host writes (default)
• FsIsolation.BRANCHFS -- BranchFS COW isolation
• FsIsolation.OVERLAYFS -- OverlayFS COW

BranchAction

• BranchAction.COMMIT -- merge writes on exit
• BranchAction.ABORT -- discard writes
• BranchAction.KEEP -- leave branch as-is

MCP integration

Sandboxed tool execution for AI agents. Each tool runs in a per-call sandbox with deny-by-default permissions.

pip install 'sandlock[mcp]'

McpSandbox

from sandlock.mcp import McpSandbox

mcp = McpSandbox(workspace="/tmp/agent", timeout=30.0)

• workspace -- directory the sandbox can read (default: "/tmp/sandlock").
• timeout -- default timeout in seconds per tool call (default: 30.0).

mcp.add_tool(name, func, *, description="", capabilities=None, input_schema=None)

Register a local tool. The function must be self-contained (imports inside the body) -- it is serialized and executed in a fresh sandbox process.

def read_file(path: str) -> str:
    import os
    workspace = os.environ["SANDLOCK_WORKSPACE"]
    with open(os.path.join(workspace, path)) as f:
        return f.read()

mcp.add_tool("read_file", read_file,
    description="Read a file from the workspace",
    capabilities={"env": {"SANDLOCK_WORKSPACE": "/tmp/agent"}},
)

No capabilities = read-only, clean environment, no network. Grant permissions explicitly:

Capability	Example	Description
fs_writable	["/tmp/agent"]	Paths the tool can write to
net_connect	[443]	TCP ports the tool can connect to
net_allow_hosts	["api.example.com"]	Allowed domains (implies ports 80, 443)
env	{"KEY": "val"}	Environment variables to pass
max_memory	"256M"	Memory limit

Any Policy field name is accepted as a capability key.

await mcp.add_mcp_session(session)

Discover tools from a remote MCP server. Capabilities are read from sandlock:* keys in the tool's annotations or meta dict.

await mcp.call_tool(name, arguments=None, *, timeout=None) -> str

Call a tool by name. Local tools run in a per-call sandbox. MCP tools are forwarded to their server session.

result = await mcp.call_tool("read_file", {"path": "data.txt"})

mcp.get_policy(tool_name) -> Policy

Return the computed Policy for a registered tool.

mcp.tool_definitions_openai() -> list[dict]

Tool definitions in OpenAI function-calling format, for use with chat completion APIs.

mcp.tools -> dict[str, Any]

All registered tools (local and MCP).

MCP server

A standalone MCP server with built-in sandboxed tools (shell, python, read_file, write_file, list_files):

# stdio (for Claude Desktop / Cursor)
sandlock-mcp --workspace /tmp/sandbox

# SSE (remote)
pip install 'sandlock[mcp-remote]'
sandlock-mcp --transport sse --host 0.0.0.0 --port 8080 --workspace /tmp/sandbox

Claude Desktop configuration:

{
  "mcpServers": {
    "sandlock": {
      "command": "sandlock-mcp",
      "args": ["--workspace", "/tmp/sandbox"]
    }
  }
}

policy_for_tool(*, workspace, capabilities=None) -> Policy

Build a deny-by-default Policy from explicit capabilities. Used internally by McpSandbox but available for direct use.

capabilities_from_mcp_tool(tool) -> dict

Extract sandlock:* capabilities from an MCP tool's annotations/meta.

License

Apache-2.0