Confine the Firecracker VMM with the jailer

.github/workflows/runner.yml

@@ -54,8 +54,20 @@ jobs:
         run: |
           echo '{"features": {"containerd-snapshotter": true}}' | sudo tee /etc/docker/daemon.json
           sudo systemctl restart docker
+      - name: Build scenario binaries
+        run: cargo test-runner scenarios --build-only
       - name: Run scenarios
-        run: cargo test-runner scenarios
+        # The scenarios run as root. The sandbox is built by dropping
+        # privilege rather than by starting without it: the jailer creates the
+        # chroot's device nodes with mknod, chowns the tree to the jail user,
+        # pivot_roots, and joins a network namespace. The udev rule above makes
+        # /dev/kvm usable by an unprivileged process, which is necessary but
+        # not sufficient.
+        #
+        # Only this step is elevated. The build above stays as the CI user, so
+        # cargo's cache and target directory are not left root-owned, and
+        # BENCHER_RUNNER_BIN keeps cargo out of the elevated step entirely.
+        run: sudo BENCHER_RUNNER_BIN=./target/debug/runner ./target/debug/test_runner scenarios
 
   build_runner:
     name: Build Runner (${{ matrix.build }})

CLAUDE.md

@@ -168,7 +168,10 @@ Defined in `.cargo/config.toml`:
   - `cargo test-api seed` needs the API server already running (`cargo run` in `services/api`) with a fresh database (`services/api/data` holds only a tracked `.gitignore`; delete `services/api/data/bencher.db`, not the whole directory)
   - `cargo test-api seed` also needs the `bencher` CLI binary already built (`cargo build --bin bencher`); it shells out via `assert_cmd`, which panics with `` `CARGO_BIN_EXE_bencher` is unset `` if the binary is missing
   - Pass `--no-git` when running the seed test in this repo: there is no colocated `.git`, so `bencher run` cannot derive a git context and the on-the-fly project naming assertions (`bencher` vs `Project`) will fail without it
-- `cargo test-runner` - Runner integration tests (requires Linux + KVM)
+- `cargo test-runner` - Runner integration tests (requires Linux + KVM + root)
+  - `cargo test-runner scenarios` always fails unelevated: the sandbox is built by dropping privilege, so the scenarios refuse to start without it. Build unprivileged, then run elevated, which also keeps `cargo` from leaving root-owned artifacts in `target`:
+    - `cargo test-runner scenarios --build-only`
+    - `sudo BENCHER_RUNNER_BIN=./target/debug/runner ./target/debug/test_runner scenarios`
 
 ## Git Flow
 

plus/bencher_runner/Cargo.toml

@@ -60,7 +60,13 @@ libc = { workspace = true }
 
 # Linux-only dependencies
 [target.'cfg(target_os = "linux")'.dependencies]
-nix = { workspace = true, features = ["feature", "poll", "sched", "signal"] }
+nix = { workspace = true, features = [
+    "feature",
+    "mount",
+    "poll",
+    "sched",
+    "signal",
+] }
 
 [build-dependencies]
 flate2 = { workspace = true }