text-mirror 0.5.0

//! The sandboxed subprocess runner behind every subprocess adapter.
//!
//! The runner enforces the whole sandbox contract from the design
//! document: a wall-clock timeout, separate stdout and stderr caps
//! drained concurrently, no network, no inherited environment, and a
//! temp-dir working jail. Every violation fails closed with a
//! machine-readable reason and no partial result.
//!
//! The spawn is two-stage. The parent builds a `std::process::Command`
//! through the platform [`jail::JailBackend`] with a cleared
//! environment, the jail as working directory, piped stdio, and a
//! fresh process group. The child starts in the worker binary's
//! single-threaded sandbox helper mode, which applies namespaces,
//! resource limits, descriptor hygiene, the filesystem jail, and the
//! syscall filter, then execs the requested adapter mode. No sandbox
//! work runs in a `pre_exec` closure.
//!
//! The wall clock is a parent-owned monotonic deadline. When it fires,
//! or when either output cap is crossed, the parent kills the whole
//! process group with SIGKILL. The same group kill also runs after a
//! normal clean exit, so no descendant outlives the invocation, and
//! the post-exit drain join is deadline-bounded, so the parent never
//! blocks on a pipe an escaped descendant holds open. A response
//! frame is accepted only from a child that wrote exactly one
//! complete frame, nothing after it, and exited cleanly.

use std::io::Read;
use std::path::PathBuf;
use std::process::{Child, Stdio};
use std::sync::Arc;
use std::sync::atomic::{AtomicBool, Ordering};
use std::time::{Duration, Instant};

pub mod helper;
pub mod jail;
pub mod protocol;
pub mod worker;

#[cfg(target_os = "linux")]
pub mod linux;
#[cfg(target_os = "macos")]
pub mod macos;

use jail::{JailBackend, SpawnSpec};
use protocol::{FrameRead, Request, RequestSchema, Response};

/// Resource and output limits for one adapter class.
#[derive(Debug, Clone)]
pub struct Limits {
    /// Wall-clock ceiling enforced by the parent.
    pub wall_timeout: Duration,
    /// Ceiling on the response frame payload, checked against the
    /// declared length before allocation.
    pub max_response_bytes: u32,
    /// Ceiling on stderr bytes, drained and counted concurrently.
    pub max_stderr_bytes: u64,
    /// RLIMIT_AS for the child, in bytes.
    pub address_space_bytes: u64,
    /// RLIMIT_CPU for the child, in seconds.
    pub cpu_seconds: u64,
    /// RLIMIT_FSIZE for the child, in bytes. Caps regular-file growth
    /// inside the jail. Stdout is a pipe, so the response cap above
    /// is enforced by the parent instead.
    pub file_size_bytes: u64,
    /// RLIMIT_NPROC for the child. An absolute cap on the process and
    /// thread count of the mapped real user id, so a compromised
    /// adapter cannot fork-bomb the host. The default is a modest
    /// absolute value: the shipped adapter spawns nothing, so on a
    /// host where the user already runs more tasks than the cap,
    /// every fork from the adapter fails, which is the intent.
    pub max_processes: u64,
}

impl Default for Limits {
    fn default() -> Limits {
        Limits {
            wall_timeout: Duration::from_secs(120),
            max_response_bytes: 192 * 1024 * 1024,
            max_stderr_bytes: 1024 * 1024,
            address_space_bytes: 4 * 1024 * 1024 * 1024,
            cpu_seconds: 300,
            file_size_bytes: 512 * 1024 * 1024,
            max_processes: 16,
        }
    }
}

/// A runner failure with a machine-readable reason.
///
/// The codes are stable: `sandbox_unavailable`, `sandbox_probe_failed`,
/// `worker_not_found`, `adapter_spawn_error`, `adapter_timeout`,
/// `adapter_frame_oversized`, `adapter_output_overflow`,
/// `adapter_protocol_error`, `adapter_drain_timeout`, and
/// `adapter_crash`.
#[derive(Debug, Clone)]
pub struct RunnerError {
    /// Stable reason code.
    pub code: &'static str,
    /// Detail for a human reading the manifest.
    pub message: String,
}

impl std::fmt::Display for RunnerError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}: {}", self.code, self.message)
    }
}

impl std::error::Error for RunnerError {}

/// File name of the worker binary this crate ships.
pub const WORKER_BINARY: &str = "text-mirror-worker";

/// Exit code the sandbox helper uses when jail setup fails, so the
/// parent can tell a refused jail from an adapter crash.
pub const SANDBOX_SETUP_EXIT: i32 = 71;

/// Locates the worker binary beside the current executable, or one
/// directory up, which covers an installed layout and a build tree.
pub fn locate_worker() -> Result<PathBuf, RunnerError> {
    let missing = |detail: String| RunnerError {
        code: "worker_not_found",
        message: detail,
    };
    let current = std::env::current_exe()
        .map_err(|e| missing(format!("cannot resolve the current executable: {e}")))?;
    let Some(dir) = current.parent() else {
        return Err(missing(
            "the current executable has no directory".to_string(),
        ));
    };
    let mut candidates = vec![dir.join(WORKER_BINARY)];
    if let Some(parent) = dir.parent() {
        candidates.push(parent.join(WORKER_BINARY));
    }
    for candidate in &candidates {
        if candidate.is_file() {
            return candidate
                .canonicalize()
                .map_err(|e| missing(format!("cannot canonicalize {}: {e}", candidate.display())));
        }
    }
    Err(missing(format!(
        "no {WORKER_BINARY} beside {}",
        current.display()
    )))
}

/// Runs worker modes under the platform jail.
pub struct Runner {
    backend: Box<dyn JailBackend>,
    worker: PathBuf,
    limits: Limits,
}

impl Runner {
    /// A runner over an explicit backend, worker, and limits.
    pub fn new(backend: Box<dyn JailBackend>, worker: PathBuf, limits: Limits) -> Runner {
        Runner {
            backend,
            worker,
            limits,
        }
    }

    /// The platform backend, the co-located worker binary, and
    /// default limits.
    pub fn with_platform_defaults() -> Result<Runner, RunnerError> {
        Ok(Runner::new(
            jail::platform_backend()?,
            locate_worker()?,
            Limits::default(),
        ))
    }

    /// The backend this runner spawns through.
    pub fn backend(&self) -> &dyn JailBackend {
        self.backend.as_ref()
    }

    /// The limits this runner enforces.
    pub fn limits(&self) -> &Limits {
        &self.limits
    }

    /// The policy digest that keys this runner's probe cache entry.
    pub fn policy_digest(&self) -> Result<String, RunnerError> {
        jail::policy_digest(self.backend.as_ref(), &self.worker)
    }

    /// Runs one adapter mode inside the jail and returns its response.
    ///
    /// `files` are written into a fresh jail directory before the
    /// spawn, so the child reads inputs by bare name from its working
    /// directory. The capability probe must have passed for the
    /// current policy digest, and runs once per process if not.
    pub fn run(
        &self,
        mode: &str,
        payload: serde_json::Value,
        files: &[(&str, &[u8])],
    ) -> Result<Response, RunnerError> {
        let digest = self.policy_digest()?;
        jail::ensure_probed(self, &digest)?;
        self.run_unprobed(mode, payload, files, true)
    }

    /// Runs one mode without the probe gate. Only the probe itself
    /// uses this, and only through the jail backend.
    pub(crate) fn run_unprobed(
        &self,
        mode: &str,
        payload: serde_json::Value,
        files: &[(&str, &[u8])],
        seccomp: bool,
    ) -> Result<Response, RunnerError> {
        let jail_dir = tempfile::tempdir().map_err(|e| RunnerError {
            code: "adapter_spawn_error",
            message: format!("cannot create the jail directory: {e}"),
        })?;
        let jail_path = jail_dir.path().canonicalize().map_err(|e| RunnerError {
            code: "adapter_spawn_error",
            message: format!("cannot canonicalize the jail directory: {e}"),
        })?;
        for (name, bytes) in files {
            if name.contains('/') || name.contains("..") {
                return Err(RunnerError {
                    code: "adapter_spawn_error",
                    message: format!("input name {name:?} is not a bare file name"),
                });
            }
            std::fs::write(jail_path.join(name), bytes).map_err(|e| RunnerError {
                code: "adapter_spawn_error",
                message: format!("cannot stage input {name:?}: {e}"),
            })?;
        }

        let spec = SpawnSpec {
            worker: &self.worker,
            jail: &jail_path,
            mode,
            limits: &self.limits,
            seccomp,
        };
        let mut command = self.backend.command(&spec)?;
        command
            .env_clear()
            .current_dir(&jail_path)
            .stdin(Stdio::piped())
            .stdout(Stdio::piped())
            .stderr(Stdio::piped());
        #[cfg(unix)]
        {
            use std::os::unix::process::CommandExt;
            command.process_group(0);
        }
        let mut child = command.spawn().map_err(|e| RunnerError {
            code: "adapter_spawn_error",
            message: format!("cannot spawn the jailed worker: {e}"),
        })?;

        let request = Request {
            schema: RequestSchema,
            adapter: mode.to_string(),
            payload,
        };
        if let Some(mut stdin) = child.stdin.take() {
            // A child that dies before reading breaks the pipe. That
            // is not a verdict, the exit status below is.
            let _ = protocol::write_frame(&mut stdin, &request);
        }

        drive(&mut child, &self.limits)
    }
}

/// What the stdout drain thread observed.
enum StdoutEnd {
    Frame(Vec<u8>),
    Empty,
    Truncated,
    Oversized { declared: u64 },
    Trailing,
    ReadError(String),
}

/// How long the parent waits for the drains to reach EOF after the
/// final group kill. A drain still open past this bound means a
/// descendant escaped the process group and holds a pipe, and the
/// parent detaches the drain and fails closed instead of hanging.
const DRAIN_JOIN_BOUND: Duration = Duration::from_secs(2);

/// Drains both pipes concurrently, enforces the deadline and caps,
/// and interprets the exit.
///
/// The group-lifetime contract: no descendant outlives the
/// invocation. The parent SIGKILLs the whole process group on every
/// exit path, the normal clean exit included, before it returns. The
/// parent itself never blocks without a bound: the post-exit drain
/// join is deadline-bounded, and a drain that does not finish is
/// detached while the call returns a fail-closed reason.
fn drive(child: &mut Child, limits: &Limits) -> Result<Response, RunnerError> {
    let kill_now = Arc::new(AtomicBool::new(false));

    let stdout = child.stdout.take().expect("stdout was piped");
    let stdout_kill = kill_now.clone();
    let max_response = limits.max_response_bytes;
    let (stdout_done, stdout_result) = std::sync::mpsc::channel();
    std::thread::spawn(move || {
        let mut reader = stdout;
        let end = match protocol::read_frame(&mut reader, max_response) {
            Ok(FrameRead::Complete(payload)) => {
                // One frame is the whole grammar. Any byte after it is
                // a protocol violation and kills the child.
                let mut probe = [0u8; 1];
                match reader.read(&mut probe) {
                    Ok(0) => StdoutEnd::Frame(payload),
                    Ok(_) => StdoutEnd::Trailing,
                    Err(e) => StdoutEnd::ReadError(e.to_string()),
                }
            }
            Ok(FrameRead::Empty) => StdoutEnd::Empty,
            Ok(FrameRead::Truncated) => StdoutEnd::Truncated,
            Ok(FrameRead::Oversized { declared }) => StdoutEnd::Oversized { declared },
            Err(e) => StdoutEnd::ReadError(e.to_string()),
        };
        if matches!(
            end,
            StdoutEnd::Oversized { .. } | StdoutEnd::Trailing | StdoutEnd::ReadError(_)
        ) {
            stdout_kill.store(true, Ordering::SeqCst);
        }
        if matches!(end, StdoutEnd::Oversized { .. } | StdoutEnd::Trailing) {
            // Keep the pipe drained so the child cannot stall on a
            // full pipe between the violation and the kill. The bytes
            // are discarded.
            let mut sink = [0u8; 8192];
            while matches!(reader.read(&mut sink), Ok(n) if n > 0) {}
        }
        let _ = stdout_done.send(end);
    });

    let stderr = child.stderr.take().expect("stderr was piped");
    let stderr_kill = kill_now.clone();
    let max_stderr = limits.max_stderr_bytes;
    let (stderr_done, stderr_result) = std::sync::mpsc::channel();
    std::thread::spawn(move || {
        let mut reader = stderr;
        let mut kept = Vec::new();
        let mut total: u64 = 0;
        let mut chunk = [0u8; 8192];
        loop {
            match reader.read(&mut chunk) {
                Ok(0) => break,
                Ok(n) => {
                    total += n as u64;
                    if kept.len() < 4096 {
                        let take = (4096 - kept.len()).min(n);
                        kept.extend_from_slice(&chunk[..take]);
                    }
                    if total > max_stderr {
                        stderr_kill.store(true, Ordering::SeqCst);
                        // Keep draining so the child cannot stall.
                    }
                }
                Err(_) => break,
            }
        }
        let _ = stderr_done.send((kept, total));
    });

    let deadline = Instant::now() + limits.wall_timeout;
    let mut timed_out = false;
    let status = loop {
        if let Some(status) = child.try_wait().ok().flatten() {
            break status;
        }
        if Instant::now() >= deadline {
            timed_out = true;
            kill_group(child);
            break wait_after_kill(child);
        }
        if kill_now.load(Ordering::SeqCst) {
            kill_group(child);
            break wait_after_kill(child);
        }
        std::thread::sleep(Duration::from_millis(2));
    };

    // The final group kill runs on every exit path, the normal clean
    // exit included, so a descendant that closed its stdio and
    // lingered past its leader is still killed. The group id stays
    // reserved while any member lives, and ESRCH on an empty group is
    // fine.
    kill_group(child);

    // Bounded drain join. On Linux the group kill guarantees EOF,
    // because a descendant cannot leave the group. On macOS a
    // descendant that changed its session can survive the group kill
    // holding a pipe, and this bound is what keeps the parent from
    // hanging on it: the drain is detached and the call fails closed.
    let join_deadline = Instant::now() + DRAIN_JOIN_BOUND;
    let stdout_end = stdout_result.recv_timeout(DRAIN_JOIN_BOUND).ok();
    let stderr_end = stderr_result
        .recv_timeout(join_deadline.saturating_duration_since(Instant::now()))
        .ok();
    let (Some(stdout_end), Some((stderr_kept, stderr_total))) = (stdout_end, stderr_end) else {
        return Err(RunnerError {
            code: "adapter_drain_timeout",
            message: format!(
                "a pipe stayed open {DRAIN_JOIN_BOUND:?} after the group kill, so a descendant escaped the process group; the drain is detached and all output is discarded"
            ),
        });
    };

    if timed_out {
        return Err(RunnerError {
            code: "adapter_timeout",
            message: format!(
                "killed the process group after the {:?} wall-clock ceiling, partial output discarded",
                limits.wall_timeout
            ),
        });
    }
    if let StdoutEnd::Oversized { declared } = stdout_end {
        return Err(RunnerError {
            code: "adapter_frame_oversized",
            message: format!(
                "response frame declared {declared} bytes over the {} byte ceiling, refused before allocation",
                limits.max_response_bytes
            ),
        });
    }
    if stderr_total > limits.max_stderr_bytes {
        return Err(RunnerError {
            code: "adapter_output_overflow",
            message: format!(
                "stderr wrote {stderr_total} bytes over the {} byte cap",
                limits.max_stderr_bytes
            ),
        });
    }
    if matches!(stdout_end, StdoutEnd::Trailing) {
        return Err(RunnerError {
            code: "adapter_output_overflow",
            message: "bytes after the response frame, one frame is the whole stdout budget"
                .to_string(),
        });
    }
    if !status.success() {
        let detail = exit_detail(&status);
        let stderr_text = String::from_utf8_lossy(&stderr_kept);
        let stderr_text = stderr_text.trim();
        #[cfg(unix)]
        let setup_failed = status.code() == Some(SANDBOX_SETUP_EXIT);
        #[cfg(not(unix))]
        let setup_failed = false;
        if setup_failed {
            return Err(RunnerError {
                code: "sandbox_unavailable",
                message: format!("the sandbox helper refused to engage the jail: {stderr_text}"),
            });
        }
        return Err(RunnerError {
            code: "adapter_crash",
            message: if stderr_text.is_empty() {
                format!("worker {detail}, any response frame is discarded")
            } else {
                format!("worker {detail}: {stderr_text}")
            },
        });
    }
    let payload = match stdout_end {
        StdoutEnd::Frame(payload) => payload,
        StdoutEnd::Empty => {
            return Err(RunnerError {
                code: "adapter_protocol_error",
                message: "worker exited without a response frame".to_string(),
            });
        }
        StdoutEnd::Truncated => {
            return Err(RunnerError {
                code: "adapter_protocol_error",
                message: "worker exited with a truncated response frame".to_string(),
            });
        }
        StdoutEnd::ReadError(detail) => {
            return Err(RunnerError {
                code: "adapter_protocol_error",
                message: format!("reading the response failed: {detail}"),
            });
        }
        StdoutEnd::Oversized { .. } | StdoutEnd::Trailing => unreachable!("handled above"),
    };
    let response: Response = serde_json::from_slice(&payload).map_err(|e| RunnerError {
        code: "adapter_protocol_error",
        message: format!("response frame did not parse: {e}"),
    })?;
    response.validate().map_err(|detail| RunnerError {
        code: "adapter_protocol_error",
        message: detail,
    })?;
    Ok(response)
}

/// Kills the child's whole process group with SIGKILL.
fn kill_group(child: &mut Child) {
    #[cfg(unix)]
    {
        if let Ok(pid) = i32::try_from(child.id()) {
            // The child is its own group leader via process_group(0),
            // and ESRCH after the group is gone is fine.
            let _ = nix::sys::signal::killpg(
                nix::unistd::Pid::from_raw(pid),
                nix::sys::signal::Signal::SIGKILL,
            );
        }
    }
    #[cfg(not(unix))]
    {
        let _ = child.kill();
    }
}

fn wait_after_kill(child: &mut Child) -> std::process::ExitStatus {
    child.wait().unwrap_or_else(|_| {
        // The child was already reaped by try_wait.
        child
            .try_wait()
            .ok()
            .flatten()
            .expect("killed child has an exit status")
    })
}

fn exit_detail(status: &std::process::ExitStatus) -> String {
    #[cfg(unix)]
    {
        use std::os::unix::process::ExitStatusExt;
        if let Some(signal) = status.signal() {
            return format!("died on signal {signal}");
        }
    }
    match status.code() {
        Some(code) => format!("exited with code {code}"),
        None => "exited without a status".to_string(),
    }
}