scratch: probe unshare semantics on runner

.github/workflows/probe-unshare.yml

@@ -0,0 +1,28 @@
+name: probe-unshare
+on:
+  workflow_dispatch:
+jobs:
+  probe:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Probe unshare semantics
+        run: |
+          echo "=== sysctls ==="
+          cat /proc/sys/kernel/unprivileged_userns_clone 2>/dev/null || echo "no unprivileged_userns_clone"
+          cat /proc/sys/kernel/apparmor_restrict_unprivileged_userns 2>/dev/null || echo "no apparmor_restrict_unprivileged_userns"
+          echo "=== identity ==="
+          id
+          echo "=== subuids ==="
+          getent passwd "$(id -un)" | head -1
+          grep -E "$(id -un)|^$(id -u):" /etc/subuid /etc/subgid 2>/dev/null || echo "no subuid/subgid entries"
+          echo "=== helpers ==="
+          which unshare newuidmap newgidmap
+          echo "=== plain ==="
+          unshare --user --map-root-user true; echo "plain rc=$?"
+          echo "=== map-auto ==="
+          unshare --user --map-root-user --map-auto true; echo "auto rc=$?"
+          echo "=== full launch shape (map-auto) ==="
+          unshare --user --map-root-user --map-auto --mount --ipc --pid --uts --fork sh -lc "echo alpha"; echo "full rc=$?"
+          echo "=== full launch shape (plain) ==="
+          unshare --user --map-root-user --mount --ipc --pid --uts --fork sh -lc "echo alpha"; echo "plain full rc=$?"

rust/crates/runtime/Cargo.toml

@@ -6,6 +6,7 @@ license.workspace = true
 publish.workspace = true
 
 [dependencies]
+libc = "0.2"
 sha2 = "0.10"
 glob = "0.3"
 plugins = { path = "../plugins" }

rust/crates/runtime/tests/probe_unshare.rs

@@ -0,0 +1,61 @@
+//! Scratch probe: dump GitHub runner unshare semantics (temporary, PRs will be closed).
+#![cfg(target_os = "linux")]
+
+use std::process::Command;
+
+fn run(args: &[&str]) -> (i32, String, String) {
+    let out = Command::new("unshare").args(args).output();
+    match out {
+        Ok(o) => (
+            o.status.code().unwrap_or(-1),
+            String::from_utf8_lossy(&o.stdout).trim().to_string(),
+            String::from_utf8_lossy(&o.stderr).trim().to_string(),
+        ),
+        Err(e) => (-1, String::new(), format!("spawn error: {e}")),
+    }
+}
+
+fn sh(cmd: &str) -> String {
+    Command::new("sh")
+        .args(["-lc", cmd])
+        .output()
+        .map(|o| String::from_utf8_lossy(&o.stdout).trim().to_string())
+        .unwrap_or_default()
+}
+
+#[test]
+fn dump_unshare_semantics() {
+    let mut report = String::new();
+    report.push_str(&format!("uid line: {}\n", sh("id")));
+    for f in ["/etc/subuid", "/etc/subgid"] {
+        report.push_str(&format!("--- {f} ---\n"));
+        if let Ok(s) = std::fs::read_to_string(f) {
+            report.push_str(&s);
+        } else {
+            report.push_str("(unreadable)\n");
+        }
+    }
+    for k in ["/proc/sys/kernel/unprivileged_userns_clone", "/proc/sys/kernel/apparmor_restrict_unprivileged_userns"] {
+        report.push_str(&format!("{k} = {}\n", std::fs::read_to_string(k).unwrap_or_else(|_| "(n/a)".into())));
+    }
+    for (name, args) in [
+        ("plain", &["--user", "--map-root-user", "true"][..]),
+        ("auto", &["--user", "--map-root-user", "--map-auto", "true"][..]),
+        (
+            "plain-full",
+            &["--user", "--map-root-user", "--mount", "--ipc", "--pid", "--uts", "--fork", "sh", "-lc", "echo alpha"][..],
+        ),
+        (
+            "auto-full",
+            &["--user", "--map-root-user", "--map-auto", "--mount", "--ipc", "--pid", "--uts", "--fork", "sh", "-lc", "echo alpha"][..],
+        ),
+        (
+            "auto-full-echo-multi",
+            &["--user", "--map-root-user", "--map-auto", "--mount", "--ipc", "--pid", "--uts", "--fork", "sh", "-lc", "echo alpha from bash"][..],
+        ),
+    ] {
+        let (rc, so, se) = run(args);
+        report.push_str(&format!("[{name}] rc={rc} stdout={so:?} stderr={se:?}\n"));
+    }
+    panic!("PROBE REPORT:\n{report}");
+}