fix(sandbox): route console logs to stderr (#949)

Signed-off-by: John Myers <johntmyers@users.noreply.github.com>
Co-authored-by: John Myers <johntmyers@users.noreply.github.com>

Author: johntmyers

architecture/sandbox-custom-containers.md

@@ -96,7 +96,7 @@ openshell sandbox create --from ./my-sandbox/  # directory with Dockerfile
 
 The `openshell-sandbox` supervisor adapts to arbitrary environments:
 
-- **Log file fallback**: Attempts to open `/var/log/openshell.log` for append; silently falls back to stdout-only logging if the path is not writable.
+- **Log file fallback**: Attempts to open `/var/log/openshell.log` for append; if the path is not writable, the supervisor keeps console shorthand logging on stderr only.
 - **Command resolution**: Executes the command from CLI args, then the `OPENSHELL_SANDBOX_COMMAND` env var (set to `sleep infinity` by the server), then `/bin/bash` as a last resort.
 - **Startup seccomp prelude**: Before parsing CLI args or starting the async runtime, the supervisor sets `PR_SET_NO_NEW_PRIVS` and installs a narrow seccomp filter that blocks mount/remount, the new mount API syscalls, module loading, kexec, `bpf`, `perf_event_open`, and `userfaultfd`. This closes the privileged remount window while still leaving required child-setup syscalls such as `setns` available.
 - **Network namespace**: Requires successful namespace creation for proxy isolation; startup fails in proxy mode if required capabilities (`CAP_NET_ADMIN`, `CAP_SYS_ADMIN`) or `iproute2` are unavailable. If the `iptables` package is present, the supervisor installs OUTPUT chain rules (LOG + REJECT) inside the namespace to provide fast-fail behavior (immediate `ECONNREFUSED` instead of a 30-second timeout) and diagnostic logging when processes attempt direct connections that bypass the HTTP CONNECT proxy. If `iptables` is absent, the supervisor logs a warning and continues — core network isolation still works via routing.

crates/openshell-sandbox/src/main.rs

@@ -101,7 +101,7 @@ struct Args {
 fn main() -> Result<()> {
     let args = Args::parse();
 
-    // Try to open a rolling log file; fall back to stdout-only logging if it fails
+    // Try to open a rolling log file; fall back to stderr-only logging if it fails
     // (e.g., /var/log is not writable in custom workload images).
     // Rotates daily, keeps the 3 most recent files to bound disk usage.
     let file_logging = tracing_appender::rolling::RollingFileAppender::builder()
@@ -116,7 +116,7 @@ fn main() -> Result<()> {
             (writer, guard)
         });
 
-    let stdout_filter =
+    let console_filter =
         EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new(&args.log_level));
 
     let runtime = tokio::runtime::Builder::new_multi_thread()
@@ -176,9 +176,9 @@ fn main() -> Result<()> {
 
             tracing_subscriber::registry()
                 .with(
-                    OcsfShorthandLayer::new(std::io::stdout())
+                    OcsfShorthandLayer::new(std::io::stderr())
                         .with_non_ocsf(true)
-                        .with_filter(stdout_filter),
+                        .with_filter(console_filter),
                 )
                 .with(
                     OcsfShorthandLayer::new(file_writer)
@@ -192,14 +192,14 @@ fn main() -> Result<()> {
         } else {
             tracing_subscriber::registry()
                 .with(
-                    OcsfShorthandLayer::new(std::io::stdout())
+                    OcsfShorthandLayer::new(std::io::stderr())
                         .with_non_ocsf(true)
-                        .with_filter(stdout_filter),
+                        .with_filter(console_filter),
                 )
                 .with(push_layer)
                 .init();
             // Log the warning after the subscriber is initialized
-            warn!("Could not open /var/log for log rotation; using stdout-only logging");
+            warn!("Could not open /var/log for log rotation; using stderr-only logging");
             (None, None)
         };
 

crates/openshell-sandbox/tests/stdout_logging.rs

@@ -0,0 +1,41 @@
+// SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+use std::process::Command;
+
+#[test]
+fn startup_logs_go_to_stderr_not_stdout() {
+    let output = Command::new(env!("CARGO_BIN_EXE_openshell-sandbox"))
+        .arg("--")
+        .arg("/usr/bin/printf")
+        .arg("hello")
+        .env("OPENSHELL_LOG_LEVEL", "info")
+        .env_remove("RUST_LOG")
+        .env_remove("OPENSHELL_POLICY_RULES")
+        .env_remove("OPENSHELL_POLICY_DATA")
+        .env_remove("OPENSHELL_SANDBOX_ID")
+        .env_remove("OPENSHELL_ENDPOINT")
+        .output()
+        .expect("spawn openshell-sandbox");
+
+    assert!(
+        !output.status.success(),
+        "expected sandbox startup to fail without a policy source"
+    );
+
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    let stderr = String::from_utf8_lossy(&output.stderr);
+
+    assert!(
+        stdout.trim().is_empty(),
+        "expected startup logs on stderr only, got stdout: {stdout}"
+    );
+    assert!(
+        stderr.contains("Starting sandbox"),
+        "expected startup log on stderr, got: {stderr}"
+    );
+    assert!(
+        stderr.contains("Sandbox policy required"),
+        "expected missing-policy error on stderr, got: {stderr}"
+    );
+}