slocker-lite/CLAUDE.md

# CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

## Project state

`slocker-lite` (C++20, built with Meson) mounts an OCI Image Layout tar (`oci-layout` +
`index.json` + `blobs/sha256/*`, as produced by `skopeo`/`podman save --format
oci-archive`/modern `docker save`) using `containers-storage` and `fuse-overlayfs`, then
(via `-r/--run`) runs a sandboxed command against it with `bwrap`. The real deployment
target is Android with a stock kernel, where `podman`/`docker` don't run (missing
namespace support) and there's no kernel overlayfs (hence `fuse-overlayfs`); `bwrap` is
invoked in "degraded mode" using only whichever `--unshare-xxx` namespaces the running
kernel actually supports. See `README.md` for the human-facing overview (build/usage/
status); this file stays the dense, file-by-file reference. Still early-stage.

Source layout (all under `src/`):
- `main.cpp` — the CLI entry point only, and deliberately tiny (~30 lines):
  loads the config file, applies its `global.log-level` (`apply_log_level()`,
  `cli_args.h`) before CLI parsing so an explicit `--log-level` can still
  override it afterward, calls `parse_args()` (`cli_args.h`) and returns its
  exit code immediately if it gives one (covers `-h`/`-V` and every parse
  error), otherwise calls `dispatch_command()` (`commands.h`) and returns its
  result. All of the actual option-parsing and command logic that used to live
  here moved out into `cli_args.{h,cpp}`/`commands.{h,cpp}`/`self_test.{h,cpp}`
  (see below) specifically to keep this file from re-growing into a dumping
  ground as more commands (docker-compose support, etc.) get added.
- `cli_args.{h,cpp}` — command-line parsing only, nothing else. `Mode` (the
  enum of every CLI action) and `ParsedArgs` (everything `parse_args()`
  extracts from `argv`) live in the header since `commands.h`'s
  `dispatch_command()` consumes them; `print_usage()`/`print_version()`, the
  `options` namespace of getopt long-option codes, and the `long_options`
  array itself are `.cpp`-local. `parse_args(argc, argv, out)` runs the
  `getopt_long` loop plus all of the post-loop validation that used to live at
  the top of `main()`: mode/`--volume` interaction (`-v` alone vs. combined
  with `-r`, see below), `--group` requires `--user`, no leftover positional
  args outside `-r`/`-e`, and (moved here from what used to be inline in the
  `Mode::exec` dispatch arm) `-x/--exec <pid>`'s own pid parsing/validation
  (`ParsedArgs::exec_pid`, a positive integer or a hard error) and its
  trailing-command requirement (`ParsedArgs::command`, required non-empty).
  `--kill <pid>` (`ParsedArgs::kill_pid`) shares that same positive-integer
  parsing/validation via a small extracted `parse_pid_arg()` helper (`.cpp`-local)
  rather than duplicating the `strtol` dance a second time — unlike `-x/--exec`,
  it takes no trailing command, so it's simply not added to the leftover-args
  exemption list (`Mode::run`/`Mode::exec` only). Returns an exit code `main()`
  should return immediately (`0` for `-h`/`-V`,
  `1` for any parse error) when set; `nullopt` means `out` is ready for
  `dispatch_command()`. `-D/--daemonize` (has a short form; `'D'` was free) is
  a plain boolean flag (`ParsedArgs::daemonize_flag`, set in its own `case
  'D':`, same pattern as `-n/--no-nsenter`). `--hostname <name>`/`--env
  VAR=VALUE`/`--env-file <file>` (all long-option only, `--env`/`--env-file`
  both repeatable) are collected here into `ParsedArgs::hostname_flag`/
  `env_specs` — `--env` pushes `{false, optarg}`, `--env-file` pushes `{true,
  optarg}` into the *same* ordered `std::vector<EnvSpec>` (not two separate
  lists), preserving their exact relative command-line order across both
  flags, since `resolve_env_specs()` (`env_spec.{h,cpp}`, see below) needs
  that order to let a later one override an earlier one for the same variable
  name — actually resolving them happens later, in `commands.cpp`'s
  `run_container()`. `-v/--volume` is dual-purpose: used alone it's a
  standalone `Mode::volume` request; combined with `-r/--run` it's repeatable
  and requests a volume mount instead (resolved later by
  `resolve_volume_mount()`, `volume_mount.{h,cpp}`, see below). Since `-v`
  must be repeatable with `-r` but each occurrence still takes two
  space-separated tokens, the getopt loop doesn't let `'v'` set `Mode`
  directly: it accumulates `(spec, path)` pairs into `ParsedArgs::volume_specs`
  (consuming the second token manually, with a guard against swallowing the
  next flag if there isn't one), and only *after* the loop decides whether
  that means one standalone `Mode::volume` call or, together with `-r`,
  passes `volume_specs` through unresolved for `dispatch_command()`/
  `run_container()` to handle.
- `commands.{h,cpp}` — every command's implementation, plus the dispatcher.
  `dispatch_command(args, config_path, config)` (the only externally-linked
  function; everything else in this file is `.cpp`-local) is a `switch
  (args.mode)` with **one explicit `case` per `Mode` enumerator and no
  `default:`**, so `-Wswitch` (this project builds at `warning_level=3`)
  forces a compile warning/error if a future `Mode` value is ever added
  without a matching dispatch case, instead of silently falling through to
  the wrong command — confirmed by testing (temporarily adding an unhandled
  enumerator triggered exactly the expected `-Wswitch` warning). `Mode::mount`
  has its own explicit case (`mount_command()`) for the same reason: it used
  to be handled only by *falling off the end* of a long `if`/`else` chain in
  `main()` with no explicit check at all — the very kind of implicit,
  easy-to-silently-break behavior this dispatcher redesign exists to close
  off, especially with more `Mode` values (docker-compose support) expected
  soon. `list_processes_command()` implements `--list-processes` (long-option
  only): calls `list_sessions()` (`pid_file.{h,cpp}`, see below) and prints
  one tab-aligned `pid`, `container name`, `running`/`exited` row per entry
  (same two-column tab-alignment scheme as `list_images_command()`/
  `list_volumes_command()`, extended to a third column), no header row, silent
  success on an empty list. `clean_processes_command()` implements
  `--clean-processes` (also long-option only): calls `clean_stale_sessions()`
  (`pid_file.{h,cpp}`) and prints one `removed stale pid file for '<name>' (pid
  <pid>)` line per file actually removed — nothing is printed for sessions still
  running, and an empty result (nothing stale) is silent success, same
  convention as the rest of this file's list/delete commands. `Mode::exec`'s
  dispatch case is a one-line call to `exec_in_session(*args.exec_pid,
  args.command)` (`exec_session.{h,cpp}`, see below) — the pid/command
  parsing and validation now happens in `cli_args.cpp`'s `parse_args()`
  instead (see above).
  `inspect_image_command()` implements `-i/--inspect
  <image.tar>`: prints every `OciImageConfig` field (user/group, exposed ports, env,
  volumes, default command) without mounting or running the image — extend it
  whenever `OciImageConfig` gains a new field (see `oci_image.{h,cpp}` below).
  `run_container()` unconditionally calls `read_oci_image_config()` and reuses the
  result for two independent defaults: the command to run (`Entrypoint ++ Cmd`) when
  none is given on the command line, and, when `--user` wasn't given, the sandboxed
  process's user/group (`config.User`, split into `OciImageConfig::user`/`group`) —
  an explicit `--user`/`--group` on the command line always takes precedence.
  `create_volume_command()` implements `-v/--volume <name> <directory>`;
  `list_volumes_command()` implements `--list-volumes` (same tab-alignment scheme as
  `list_images_command()`, reused as-is); `delete_volume_command()` implements both
  `--delete-volume <name>` (config entry only) and `--delete-volume-full <name>`
  (also `std::filesystem::remove_all()`s the host directory — errors out before
  touching the config if that fails, warns instead of failing if the directory was
  already gone) — see `config_file.{h,cpp}` below for what a "volume" means here (a
  distinct concept from `OciImageConfig::volumes`). `dispatch_command()`'s
  `Mode::volume`/`Mode::delete_volume`/`Mode::delete_volume_full` cases call these.
  `run_container()` (the `Mode::run` dispatch case) resolves each `-v` spec
  (erroring out, `ok = false`, same as a failed `--user` resolution — `bwrap`
  is skipped but unmount/cleanup still runs) into a `ResolvedVolumeMount`,
  rejecting a duplicate or non-absolute container path first, and passes the
  resolved list to `run_bwrap()`. `--hostname <name>` is likewise threaded
  straight through `run_container()` into `run_bwrap()`/`build_bwrap_args()`
  (`bwrap.{h,cpp}`) — see there for how/when it actually takes effect.
  `run_container()` also derives a `container_name` for the session-tracking
  pid file (see `pid_file.{h,cpp}` below): `read_image_ref()`
  (`oci_image.{h,cpp}`) applied to the single image tar being run, formatted
  as `name:tag`, falling back to the tar's own filename stem if
  `read_image_ref()` can't determine one — passed through to `run_bwrap()`
  alongside everything else. `--env`/`--env-file`'s already-ordered
  `env_specs` (see `cli_args.{h,cpp}` above) are resolved here, once, via
  `resolve_env_specs()` (`env_spec.{h,cpp}`, see below) — same `ok =
  false`-on-failure pattern as volume/user resolution — and the resolved list
  is passed to `run_bwrap()` as `extra_env`. `-D/--daemonize`'s
  `daemonize_flag` is also consumed here: `run_container()` computes
  `container_name` *before* `mount_image()` (needed so `daemonize()` below can
  use the real container name for the log file from its very first line, not
  just after a later rename) and, if daemonizing, calls
  `daemonize(container_name)` (`daemonize.{h,cpp}`, see below) immediately
  after: a returned value means this is the original (parent) process (or a
  hard daemonize failure) — print it and `return` right away; `nullopt` means
  this is the now-detached child, which falls through into the rest of
  `run_container()`'s existing body completely unchanged, including the
  unmount/cleanup that already runs after `run_bwrap()` returns (no separate
  watcher/reaper — the daemonized child *is* what runs the whole session,
  start to finish).
- `self_test.{h,cpp}` — `run_self_tests()` implements `-t/--test`, this
  project's own built-in self-test mode (distinct from the Meson-driven
  fixture smoke test under `tests/`, described in "Build & test commands"
  below). Currently just reports `detect_bwrap_unshare_args()`'s output
  (`bwrap.{h,cpp}`); deliberately its own small file since real tests are
  expected here soon.
- `env_spec.{h,cpp}` — `resolve_env_specs()` turns an ordered list of
  `EnvSpec {is_file, value}` (see `cli_args.{h,cpp}` above) into a flat, ordered list of
  `(key, value)` pairs. A literal (`--env`) is split at its *first* `=` (the
  value may itself contain `=`; the key must be non-empty). A file (`--env-file`)
  is read line by line: blank/whitespace-only lines and lines whose first
  non-whitespace character is `#` are skipped (comments), with a trailing `\r`
  stripped first for CRLF files; every other line is parsed the same way as a
  literal. Logs a specific error and returns `nullopt` on the first hard failure
  (malformed line, empty key, or an unreadable file) — deliberately stops at the
  first line, not "skip and warn", since an env file with a typo should fail
  loudly rather than silently omit a variable a container might depend on.
  `build_sandbox_env()` (`bwrap.cpp`, see below) appends the resolved list after
  its own built-in `PATH`/`HOME`/`PWD`/`TERM` — no deduplication needed there,
  since `run_process_foreground()`'s own `setenv(..., 1)` loop already lets the
  later occurrence in iteration order win for a repeated key, so an explicit
  `--env PATH=...` still overrides the default.
- `oci_image.{h,cpp}` — validates/parses the OCI Image Layout tar (libarchive +
  nlohmann_json) and extracts layer blobs. `list_oci_images()` scans a directory
  (non-recursively) for `*.tar`/`*.tar.*` files and, for each valid OCI archive,
  derives an image name/tag via `read_image_ref()` from its `index.json` manifest
  annotations (`io.containerd.image.name` preferred, else
  `org.opencontainers.image.ref.name`), falling back to the archive's filename and
  `"latest"` respectively. `read_image_ref()` is public (not just an internal
  helper of `list_oci_images()`) precisely so `run_container()` (`commands.cpp`) can
  reuse the exact same logic to name a *single* image tar's session pid file (see
  `pid_file.{h,cpp}` below) instead of duplicating it.
  `read_oci_image_config()` reads the image config blob referenced by the manifest
  and extracts `User` (split on `:` into `OciImageConfig::user`/`group`),
  `ExposedPorts`, `Env`, `Volumes`, and the effective default command
  (`Entrypoint ++ Cmd`). `user`/`group` and the default command are consumed by
  `-r/--run`, and every field is displayed by `-i/--inspect` (see `commands.{h,cpp}` above)
  — `ExposedPorts`/`Env`/`Volumes` are otherwise still just captured for when
  networking/volumes are implemented.
- `containers_storage.{h,cpp}` — wraps the `containers-storage` CLI (`import-layer`,
  `mount`, `unmount`, `layer --json`, `delete-layer`), forcing `fuse-overlayfs` as the
  overlay `mount_program`. `cleanup_layer_chain()` walks a layer's parent chain
  (children before parents) deleting each one.
- `bwrap.{h,cpp}` — `detect_bwrap_unshare_args()` probes the kernel (via a forked
  `unshare(2)` per namespace type) for which `--unshare-xxx` flags `bwrap` can actually
  use; `build_bwrap_args()`/`run_bwrap()` assemble and run the sandboxed command.
  `build_bwrap_args()` also takes a `std::vector<ResolvedVolumeMount>` (see
  `volume_mount.h` below) and appends one writable `--bind <host_directory>
  <container_path>` per entry. `wrap_for_root_namespace()` is `run_bwrap()`'s own
  nsenter-wrapping logic pulled out into a reusable, exported function — it's also
  what `volume_mount.cpp`'s copy-into-an-empty-volume step uses to reach the
  image's content when running rootless (see below); `run_bwrap()` itself now just
  calls it once on the assembled `bwrap` argv.
  `build_bwrap_args()`/`run_bwrap()` also take a `NamespaceConfig` (`bwrap.h`) — one
  plain `bool` field per `namespace_probes` entry (`user`/`ipc`/`pid`/`net`/`uts`/
  `cgroup`, default `true`), resolved by `run_container()` (`commands.cpp`) from
  `AppConfig`'s six `global.unshare-*` keys (`config_file.h`, see above) once, up
  front — `bwrap.{h,cpp}` itself never touches `AppConfig`/YAML, only this
  already-resolved struct. For each flag `detect_bwrap_unshare_args()` finds the
  kernel supports, `build_bwrap_args()` additionally requires the matching
  `NamespaceConfig` field to be `true` (looked up via a `.cpp`-local
  `namespace_policy_enabled()` if-chain over `namespace_probes`' `name`s) before
  actually passing it to `bwrap` — kernel support and policy are separate gates,
  both must allow a type. This replaced an earlier hardcoded special case that
  always dropped `--unshare-net` regardless of policy or kernel support (without
  any network setup, e.g. `slirp4netns`, unsharing it just left the sandbox with no
  network at all) — `net` now goes through the exact same policy path as every
  other type, defaulting to enabled like the rest. **This is a deliberate,
  user-acknowledged transitional behavior change**: as of this, a plain `-r/--run`
  with no config file override gets a real network namespace and thus no network
  access at all, until `slirp4netns` integration (the next task on this same
  branch) actually sets one up; `global.unshare-net: off` restores the prior
  no-isolation behavior in the meantime. `detect_bwrap_unshare_args()` itself is
  untouched by any of this — still an unfiltered kernel-capability probe, so
  `-t/--test`'s diagnostic report continues to reflect raw kernel support, not policy.
  `build_bwrap_args()`/`run_bwrap()` also take an optional `hostname` (from
  `--hostname`, long-option only): passed through as bwrap's own `--hostname` only
  when `--unshare-uts` is actually among the flags `bwrap` is being given (bwrap
  itself refuses `--hostname` without it) — otherwise logs a warning and leaves the
  sandbox's hostname alone, since a stock Android kernel in degraded mode may not
  support a UTS namespace at all.
  Never requests `--unshare-user` when running as root: root doesn't need a fresh
  user namespace for privilege, and bwrap's own single-mapping uid/gid setup for one
  triggers the kernel's unprivileged-userns setgroups() restriction, which showed up
  as every other supplementary group collapsing to the overflow gid ("nobody") in
  `id`, and `su` inside the sandbox failing with "can't set groups: Operation not
  permitted". Because of that, bwrap's own `--uid`/`--gid` (which require
  `--unshare-user`) aren't usable when running as root either — `--user`/`--group`
  work around this: when set, `build_bwrap_args()`/`run_bwrap()` bind-mount the
  separate `slocker-lite-priv-drop` helper (see below) into the sandbox at a fixed
  hidden path and route the real command through it as
  `<uid>:<gid> -- <command...>`. This only actually works without a user namespace
  (i.e. running as root) — under `--unshare-user`, the sandbox's uid map has only
  one valid entry, so the helper's own `setuid()` fails cleanly there instead of
  silently doing nothing. `run_bwrap()` fails fast (returns -1) if the helper can't
  be found next to this binary when `--user` was requested, rather than silently
  running the command as root. `run_bwrap()` also takes a `container_name` and
  tracks the running session with it: it passes a lambda as
  `run_process_foreground()`'s new `on_start` callback (see `process.{h,cpp}`
  below) that calls `create_session_lock(container_name, pid)` (`pid_file.{h,cpp}`,
  see below) the instant the real `bwrap` pid is known, then calls
  `release_session_lock()` once `run_process_foreground()` returns (covering
  every exit path — normal, nonzero, or a forwarded-signal exit — since that call
  always blocks until the child has actually exited).
  `build_bwrap_args()` no longer passes `--clearenv`/`--setenv` to `bwrap` itself;
  instead, `build_sandbox_env()` builds the sandboxed command's exact environment
  (`PATH`, `HOME`, `PWD` — hardcoded to `"/"`, matching `--chdir`'s own value; note
  per bwrap's own man page `--clearenv` never actually unset `PWD` in the first
  place, so this isn't a straight port of a prior `--setenv` — and `TERM`, only
  if the host process has one, followed by `extra_env` — the resolved
  `--env`/`--env-file` list from `resolve_env_specs()` (`env_spec.h`), appended
  last so it can override the built-in defaults for the same key) and
  `run_bwrap()` passes it straight to `run_process_foreground()`'s own `env`
  override (see `process.{h,cpp}` below). This works because `bwrap` (and
  `nsenter`, when interposed via `wrap_for_root_namespace()`) doesn't alter its
  own inherited environment unless told to, and neither does
  `slocker-lite-priv-drop` (just `setgroups()`/`setgid()`/`setuid()`/`execvp()`,
  no env manipulation) — so controlling it once, at the outermost exec, is
  sufficient for it to reach the final sandboxed command unchanged. **Because
  that outermost exec now uses this same explicitly-built environment**,
  `build_bwrap_args()`/`wrap_for_root_namespace()` resolve `bwrap`'s and
  `nsenter`'s own argv[0] to an absolute path via `find_in_path()` (called from
  this process's own, unmodified environment, before `fork()`) instead of
  leaving them as bare names — confirmed by direct testing: `--env PATH=...`
  used to break `execvp()`'s ability to even *locate* `bwrap`/`nsenter`
  (bare-name lookup happens in the child, using the already-overridden PATH),
  not just what the sandboxed command itself sees. With the fix, only the
  *sandboxed command's own* lookup is affected by a `--env PATH=...` override
  (as expected — same as overriding `PATH` in any real shell before running a
  bare command name), and `bwrap`/`nsenter` are always found regardless.
  `run_bwrap()` also takes an optional `on_bwrap_pid_known` callback, invoked
  alongside (not instead of) the session-lock-creation lambda, at the exact
  same `on_start` timing — `-D/--daemonize` (`daemonize.{h,cpp}`, see below)
  hooks in here via `report_daemon_started()` to learn the real pid at the
  same instant everything else that needs it does, rather than needing its own
  separate pid-discovery mechanism. That same `on_start` lambda also calls
  `create_session_cgroup()` (`session_cgroup.h`, see below), right alongside
  `create_session_lock()`, so `--kill` can later find every process the
  session ever starts via its dedicated cgroup; `remove_session_cgroup()` is
  called from the same post-`run_process_foreground()` spot
  `release_session_lock()` already is.
- `priv_drop_helper.cpp` → the separate `slocker-lite-priv-drop` binary (its own
  `executable()` target in `meson.build`, **built with `-static`**). Deliberately
  has zero dependencies on the rest of this project (no fmt/spdlog/etc.) and is
  fully statically linked: it gets bind-mounted *into the container image's own
  filesystem*, which won't have `slocker-lite`'s own shared library dependencies —
  a dynamically linked binary bind-mounted that way fails outright ("error while
  loading shared libraries"), which is exactly what happened before this was split
  out (the original approach bind-mounted `slocker-lite`'s own — dynamically linked
  — binary via `/proc/self/exe` and reexeced it; kept only as a lesson, not as
  working code). Usage: `slocker-lite-priv-drop <uid>:<gid> -- <command> [args...]`;
  does `setgroups(0,…)` → `setgid()` → `setuid()` → `execvp()`, in that order
  (dropping the group needs `CAP_SETGID`, which is lost once `setuid()` drops root).
  `find_priv_drop_helper()` and the `priv_drop::path`/`priv_drop::helper_name`
  constants live in `bwrap.h` (not just internal to `bwrap.cpp`) specifically so
  `exec_in_session()` (`exec_session.cpp`, see below) can reuse the exact same
  already-bind-mounted helper for `-x/--exec`'s own `--user`/`--group` support,
  instead of a second copy needing to be bind-mounted for it (which wouldn't even
  be possible — `-x/--exec` joins an *already-running* session's mount namespace,
  it doesn't get to add bind mounts to it). `find_priv_drop_helper()` itself only
  checks this binary's own host-side existence; it says nothing about whether a
  given session actually has it bind-mounted (only true when that session's
  `-r/--run` resolved a user in the first place).
- `user_spec.{h,cpp}` — `resolve_user_and_group()` resolves a user/group spec (each
  a name or numeric id) against the *container's own* `/etc/passwd`/`/etc/group`
  **content** (not the host's, and not a path — callers own reading it, since the
  two current callers get that content two different ways: `run_container()`
  reads it directly off the merged mount path, while `exec_in_session()` fetches
  it over `nsenter`, since a running session's mount namespace isn't otherwise
  reachable from this process — see below). `nullopt` content for either file
  means "unreadable/absent"; a numeric user with no group still resolves fine
  without it (defaults gid to the same numeric value as the uid) but a named one
  doesn't. `ResolvedUser` (`bwrap.h`) also carries `home`, looked up by the final
  resolved uid's `/etc/passwd` entry (field 5) regardless of whether `user` was
  given as a name or a number; falls back to `"/root"` for uid 0 or `"/"`
  otherwise when there's no matching row. `build_sandbox_env()` (`bwrap.cpp`)
  sets the sandboxed process's `HOME` from this — `"/root"` only when no user
  override applies at all (no `--user`, no image-declared `config.User`).
  `run_container()` (`commands.cpp`) calls `resolve_user_and_group()` with either
  the explicit `--user`/`--group` flags, or, when `--user` wasn't given, the
  image's own declared `config.User` (`OciImageConfig::user`/`group`) — so a
  container defaults to running as whatever user the image itself declares, not
  root, unless the image declares none.
- `process.{h,cpp}` — argv-based subprocess helpers (fork/execvp, no shell):
  `run_process()` captures stdout (used for `containers-storage` calls),
  `run_process_foreground()` inherits all of stdio (used for the interactive `bwrap`
  run). Also `find_in_path()`, a shared `$PATH` lookup. `run_process_foreground()`
  installs a SIGINT/SIGTERM handler around its `waitpid()` that forwards the signal
  to the running child and keeps waiting instead of letting the default disposition
  kill `slocker-lite` itself — without this, Ctrl-C (or `kill`) during `-r`'s `bwrap`
  run would skip `run_container()`'s unmount/cleanup entirely, leaving the layer
  imported and/or mounted. `run_process_foreground()` also takes an optional
  `on_start` callback, invoked with the child's real pid right after `fork()`
  succeeds (before the signal handlers go up and it blocks in `waitpid()`) — the
  only point where that pid is knowable, and still accurate even when `argv`
  itself execs into something else first (e.g. `nsenter` handing off to the final
  command via its own in-place `execvp()` — a pid never changes across `exec()`).
  `run_bwrap()` (`bwrap.cpp`) is the one caller that uses it, for session pid-file
  tracking (see `pid_file.{h,cpp}` below). `run_process_foreground()` also takes
  an optional `env` (list of key/value pairs): when set, the forked child
  replaces its entire environment via `clearenv()`/`setenv()` (plain POSIX, not
  the GNU-only `execvpe()` — the target platform includes musl) before `execvp()`,
  instead of inheriting this process's own. `nullopt` (the default) leaves the
  child's environment untouched. `run_bwrap()` is again the one caller that uses
  this, via `build_sandbox_env()` (`bwrap.cpp`) — see there.
- `pid_file.{h,cpp}` — tracks one running `-r/--run` session (a live `bwrap`
  process) as a locked pid file, so an outside process (or a later
  `slocker-lite` invocation) can tell whether it's still running.
  `sanitize_for_filename()` (anything outside `[A-Za-z0-9._-]` → `_`, falling
  back to `"container"` if that leaves nothing) is exported here (not just
  `.cpp`-local) specifically so `session_cgroup.{h,cpp}` (see below) can reuse
  the exact same `<name>-<pid>` naming rule for its own per-session cgroup
  directory without drifting from this file's own. `session_pid_file_path()` resolves
  `$XDG_STATE_HOME/slocker-lite/run/<container_name>-<pid>` (falling back to
  `$HOME/.local/state/...` when `XDG_STATE_HOME` is unset/empty — same
  resolution pattern as `config_file_path()` below, for state instead of
  config), sanitizing `container_name` first (anything outside `[A-Za-z0-9._-]`
  → `_`, since an image name/tag can contain `/` or `:`). `session_log_file_path()`
  is a sibling resolving to `$XDG_STATE_HOME/slocker-lite/logs/<container_name>-<pid>.log`
  instead — same sanitization, same `$XDG_STATE_HOME`/`$HOME` fallback, just a
  different subdirectory and a `.log` extension — used by `daemonize.{h,cpp}`
  (see below) for `-D/--daemonize`'s log file. `create_session_lock()`
  creates the file (`O_CREAT|O_WRONLY|O_TRUNC|O_CLOEXEC`, mode 0644 — `O_CLOEXEC`
  matters: this fd must never leak into the sandboxed command's own fd table),
  writes the pid as text, and takes an exclusive, non-blocking `flock()` on it —
  held only by that fd, so its lifetime tracks `slocker-lite`'s own process
  lifetime (released automatically on any exit, including a crash), which lines
  up with `bwrap` itself being invoked with `--die-with-parent`. Any external
  tool can check liveness the same way: attempt the same exclusive non-blocking
  `flock()` on the file — success means nothing holds it anymore (stale, safe to
  remove), `EWOULDBLOCK` means a live process still does. `release_session_lock()`
  closes the fd (releasing the flock immediately) and removes the file. Every
  failure path here (can't create the directory/file, can't lock, can't remove)
  is a `spdlog::warn`, never fatal — session tracking is best-effort and must
  never block or fail `-r/--run` itself. `list_sessions()` implements
  `--list-processes` (`commands.cpp`'s `list_processes_command()`): scans the same
  `run/` directory and reports one `SessionInfo {pid, container_name, running}`
  per readable pid file. `pid` is read from the file's own contents, not parsed
  from the filename (ambiguous for names that themselves contain `-`);
  `container_name` is then recovered by stripping that exact `-<pid>` suffix
  back off the filename. `running` reuses the same liveness check any external
  tool would do — a non-blocking exclusive `flock()` that succeeds means the
  file is actually stale, so `running` is false in that case; the lock is always
  released again immediately either way, never left held by the check itself. A
  file that can't be opened or doesn't parse as a pid (e.g. removed mid-scan) is
  silently skipped, not reported as an error — scanning a live directory is
  inherently racy. Both `list_sessions()` and `clean_stale_sessions()` (the
  latter implements `--clean-processes`) share a private `open_session_file()`
  helper for the open/read-pid/recover-name step. `clean_stale_sessions()`
  doesn't just remove whatever a separate `list_sessions()` call reported as not
  running — it re-takes the same non-blocking `flock()` used to test liveness
  and holds it across the `remove()` call itself, per file, so the stale check
  and the removal stay atomic against a new session starting in the gap between
  a check and a later removal. Only files it actually removes are reported back
  (as `SessionInfo`s with `running=false`); still-locked (running) files are
  left untouched and not reported.
- `session_cgroup.{h,cpp}` — gives `--kill` (`kill_session.{h,cpp}`, see
  below) a reliable way to find every process a session ever started, however
  deeply forked/daemonized/reparented, by putting it in a dedicated cgroup v2
  group from the moment it starts. `cgroup_v2_available()` checks for
  `/sys/fs/cgroup/cgroup.controllers` (the same signal systemd's own
  unified-hierarchy detection uses) — only cgroup v2 is supported; v1 (which
  splits per-controller into separate hierarchies with no unified
  `cgroup.procs` at the top) is deliberately out of scope, since this
  project's real target (Android) has used the unified v2 hierarchy by
  default since Android 12. `session_cgroup_path()` is deterministic —
  `/sys/fs/cgroup/slocker-lite/<name>-<pid>/`, reusing `pid_file.h`'s own
  `sanitize_for_filename()` — so no separate lookup state is needed anywhere.
  `create_session_cgroup()` is called from `run_bwrap()`'s `on_start` callback
  (`bwrap.cpp`, see below), the same spot `create_session_lock()` already
  fires from: `create_directories()`'s the leaf directory (also creating the
  `slocker-lite/` parent the first time — a plain grouping cgroup, no resource
  controllers are ever enabled on it via `cgroup.subtree_control`, so the "no
  internal processes" restriction that comes with actually delegating
  controllers never applies here) and writes the bwrap pid into its
  `cgroup.procs`. From that point on, every process bwrap (or anything it
  execs into) forks inherits this cgroup automatically, permanently —
  including anything that later daemonizes/double-forks and gets reparented,
  unlike pid-namespace child membership (only the processes `clone()` itself
  creates) or process-group membership (many daemonizing services explicitly
  `setpgid()`/`setsid()` away from it on purpose). Best-effort, mirroring
  `create_session_lock()`: returns `nullopt` (logging a warning, never fatal)
  if cgroup v2 isn't available, or the directory can't be created/written
  (no delegated subtree when running rootless, or an SELinux policy blocking
  cgroupfs writes even for a root-euid process, are both real, confirmed-by-
  testing causes on the two environments this project actually runs on).
  `remove_session_cgroup()` (called from the same post-`run_process_foreground()`
  spot `release_session_lock()` already is) only succeeds once the cgroup is
  empty — a straggler process still alive at normal exit (e.g. a daemonized
  process that outlived the session's own main command, a pre-existing
  exposure independent of this feature) leaves it in place with a warning, not
  a fatal error. `session_cgroup_pids()` reads `cgroup.procs` — this is the
  actual answer to "gather every process running inside the container":
  unlike anything derived from `/proc` parent-pid chains or pid namespaces,
  cgroup membership reliably includes every process the session ever started.
  `session_cgroup_supports_kill()`/`kill_session_cgroup()` wrap the
  `cgroup.kill` knob (Linux 5.14+): writing `"1"` to it atomically `SIGKILL`s
  every process currently in the cgroup in one step.
- `sandbox_process.{h,cpp}` — process-tree/namespace-resolution utilities
  shared by `exec_session.{h,cpp}` and `kill_session.{h,cpp}` (see both
  below); pulled into their own file (rather than staying private to
  `exec_session.cpp`, where `resolve_namespace_pid()` originally lived) once
  `--kill` needed the exact same "find the real sandboxed child" logic, to
  avoid a second, drifting copy. `resolve_namespace_pid()` is unchanged from
  its original `exec_session.cpp` form (see that entry for the full
  reasoning: bwrap's own outer/tracked pid never actually enters the
  pid/uts/ipc/cgroup namespaces it creates for its clone()'d child, only that
  child does). Two new utilities added alongside it for `kill_session()`:
  `pid_namespace_isolated(outer_pid, ns_pid)` compares
  `/proc/<outer_pid>/ns/pid` and `/proc/<ns_pid>/ns/pid`'s own `readlink()`
  targets directly — true only when bwrap's `clone()` actually created a
  separate pid namespace for its child (`--unshare-pid` was requested *and*
  the kernel supported it), the precondition for the kernel's own guarantee
  that killing a pid namespace's pid 1 forcibly tears down every remaining
  process in it. `collect_descendant_pids(root)` generalizes
  `resolve_namespace_pid()`'s own `/proc/<n>/stat` ppid-scanning fallback to
  collect a whole transitive tree (root included) instead of just one child,
  sharing the actual stat-parsing loop between both via a private
  `build_ppid_map()` (one `/proc` pass, used by both the single-child lookup
  and the full-tree collection). Reliable specifically when `root` is a
  genuinely isolated pid namespace's own pid 1: anything that reparents
  within it (e.g. a daemonizing service) is guaranteed by the kernel to land
  back on `root` itself, unlike on a kernel without pid namespace support,
  where it escapes to the *host's* real pid 1 instead (see
  `kill_session.{h,cpp}` below for exactly this scenario, confirmed on a real
  target device).
- `daemonize.{h,cpp}` — implements `-D/--daemonize`'s fork/detach mechanics.
  `daemonize(container_name)` sets up a `pipe2(..., O_CLOEXEC)` pair (so it
  never leaks into `bwrap`/the sandboxed command, same reasoning as the pid
  file's own `O_CLOEXEC`) and `fork()`s. The **child** calls `setsid()` —
  deliberately here, not via re-adding bwrap's own `--new-session` (removed
  earlier, see the `build_bwrap_args()` comment): `--new-session` only calls
  `setsid()` for the deeply-nested sandboxed command *inside* bwrap's own
  namespace setup, leaving the outer `bwrap`/`nsenter`/`slocker-lite` processes
  still attached to the *original* session and still receiving its signals
  (e.g. a `SIGHUP` when the controlling terminal closes) — not real
  daemonization. Calling `setsid()` in our own forked child, *before* it execs
  into `nsenter`/`bwrap`, detaches the entire chain at once, since `exec()`
  never changes session membership — confirmed by direct testing
  (`ps -o pid,sid,pgid,tty`): the daemon child becomes its own session leader
  with no controlling tty, and `bwrap` (a later descendant) shares that same
  session, also with no tty. The child also `sigaction()`s `SIGHUP` to
  `SIG_IGN` (survives the later `exec()` into `nsenter`/`bwrap`, unlike a real
  handler, which `exec()` resets to default — confirmed by sending `SIGHUP`
  directly to a running daemonized `bwrap` pid and it staying alive), then
  redirects stdin to `/dev/null` and stdout/stderr to a log file at
  `session_log_file_path(container_name, getpid())` (`pid_file.h`) — named
  after its *own* pid since the real session pid (`bwrap`'s) isn't known yet.
  If the log directory/file can't be set up at all, that's a **hard** failure
  here (`_exit(1)`), not best-effort — silently losing the very output
  `--daemonize` was asked to capture would defeat the point of the flag. The
  child reports `"LOG <path>\n"` over the pipe immediately (so the parent can
  show a useful location even on failure) and returns `nullopt` to its caller
  (`run_container()`, `commands.cpp`), which then falls through into the rest of
  that function's existing body completely unchanged — **the daemonized child
  is what runs the whole rest of `run_container()`, including the unmount/
  cleanup that already existed after `run_bwrap()` returns; no separate
  watcher/reaper process exists**. The **parent** blocks reading the pipe until
  EOF, returning the accumulated `"LOG "`/`"PID "` lines as a `DaemonizeResult`
  — the caller then prints it and exits immediately without running any
  session logic itself. `report_daemon_started(container_name, pid)` (called
  from `run_bwrap()`'s new `on_bwrap_pid_known` callback — see `bwrap.{h,cpp}`
  below — the instant the real `bwrap` pid is known) renames the pid-named log
  file to `<container_name>-<pid>.log`, re-reports the *updated* `"LOG "` line
  (a real bug caught by testing: the parent's first `"LOG "` line names the
  pre-rename, daemon-pid-named path — without a second one, the parent would
  print a stale filename that doesn't match where the file actually ends up),
  then `"PID <pid>\n"` and closes its own end of the pipe — must happen here,
  explicitly, rather than waiting for the pipe to close naturally at the end of
  the (potentially very long) daemon's lifetime, or the parent would block for
  as long as the session runs instead of returning promptly. The pipe's write
  fd and the current log path are tracked as private file-scope state in
  `daemonize.cpp` (matching `process.cpp`'s own `g_foreground_child_pid`
  pattern for "there's only ever one of these per process" runtime state),
  since `report_daemon_started()` is called later, from a different function,
  not threaded explicitly through every call in between.
- `exec_session.{h,cpp}` — implements `-x/--exec <pid>`: joins an already-running
  `-r/--run` session's namespaces via `nsenter` and runs a command inside it in
  the foreground. `exec_in_session()` first confirms `pid` is a tracked, running
  session via `list_sessions()` (`pid_file.h`) — same liveness check
  `--list-processes`/`--clean-processes` already use, no new logic needed there.
  **Key discovery, confirmed by direct testing, not assumed**: `pid` (the one
  `run_process_foreground()` captured and pid-file-tracked when `-r` launched
  `bwrap`) is bwrap's own *outer* process — it sets up the mount and user
  namespaces itself, then `clone()`s the actual sandboxed command into fresh
  pid/uts/ipc/cgroup namespaces, and `clone()`'s namespace-creation flags only
  ever affect the newly created child, never the caller. So the outer process
  itself never actually enters those namespaces — comparing
  `/proc/<outer_pid>/ns/{pid,uts,ipc,cgroup}` against this process's own showed
  them identical, while only `mnt`/`user` differed. `resolve_namespace_pid()`
  (`sandbox_process.{h,cpp}` — moved out of this file once `--kill`
  needed the exact same logic, see that entry) finds that real inner process
  so this can join *its* namespaces instead. For each of `{mnt→--mount, uts→--uts, ipc→--ipc, pid→--pid,
  cgroup→--cgroup, user→--user}` (`net` deliberately excluded — this project
  never isolates networking, see `bwrap.cpp` below), `readlink()`s both
  `/proc/<ns_pid>/ns/<type>` and `/proc/self/ns/<type>` and only passes
  nsenter's corresponding `--type=/proc/<ns_pid>/ns/<type>` flag when they
  differ — an identical-namespace re-entry attempt can fail outright
  (`setns()`'s own `EINVAL` restriction on re-entering a namespace you're
  already in), so skipping is deliberate, not just an optimization. `mnt` is the
  one type where a *read* failure (permission denied, or the process vanished)
  is treated as fatal, since without it "joining the container" is meaningless;
  every other type just degrades to a skip. Always appends
  `--preserve-credentials`: without it, `nsenter --user` also tries to
  `setuid()`/`setgid()`/`setgroups()` to the target's identity within the new
  user namespace, which fails outright (`setgroups failed: Operation not
  permitted`) against the `setgroups`-denied unprivileged user namespace bwrap
  creates whenever `-r/--run` isn't root — confirmed by hitting this exact
  failure during manual testing before adding the flag. Runs the final
  `nsenter ... -- <command>` via the existing `run_process_foreground()`
  (`process.h`) — same inherited stdio and SIGINT/SIGTERM forwarding as every
  other foreground external command, no new process-running logic needed.
  `exec_in_session()` also takes optional `--user`/`--group` (mirroring `-r/--run`'s
  own): given, they resolve against the session's own `/etc/passwd`/`/etc/group`
  (fetched via `cat` run through the same `nsenter` join, since this process can't
  otherwise see into that namespace, then handed to `resolve_user_and_group()` —
  `user_spec.h`, see below); if unset, defaults to whatever uid/gid the session's
  own sandboxed command is *already* running as (read from `/proc/<ns_pid>/status`),
  rather than root/the caller — fixing a real bug (reported after this project's
  own `-x/--exec` and priv-drop features had both shipped separately): without this,
  `-x/--exec` always ran as whatever the *host* invocation was, ignoring any
  `--user`/`--group` the session itself was started with. Either way, the resolved
  identity is applied by running `command` through the session's already
  bind-mounted `slocker-lite-priv-drop` helper (`priv_drop::path`, `bwrap.h`) —
  reused as-is, not bind-mounted again (`-x/--exec` can't add bind mounts to an
  already-running session's namespace anyway). Skipped entirely when the resolved
  uid *and* gid are both 0: a session that was never given a resolvable user at
  `-r/--run` time never got the helper bind-mounted at all, and dropping to 0:0
  would be a no-op regardless; a missing helper for a genuinely non-root
  resolution instead surfaces as `nsenter`'s own "No such file or directory" once
  it tries to exec `priv_drop::path`, diagnostic enough on its own. **Second real
  bug, caught by direct testing on a rootless dev machine before this shipped**:
  when the session's own `-r/--run` used `--unshare-user` (i.e. ran rootless —
  see the root-vs-rootless paragraph below), "root inside the container" is
  achieved purely through the kernel's own uid mapping for that namespace, not a
  real privilege drop — so `/proc/<ns_pid>/status`'s uid/gid, read from *outside*
  that namespace, shows the host-mapped id (e.g. `1000`), not the
  container-relative one (`0`). Treating that as "needs a priv-drop to 1000" is
  wrong two ways: the helper is typically never bind-mounted for a session with
  no resolved `--user`, and even when it is, `setuid()` fails outright under the
  single-entry uid map an unprivileged user namespace gets (confirmed directly:
  `failed to drop privileges to 0:0: Operation not permitted`). Fixed by tracking
  whether the `user` namespace type was actually one of the ones joined (it only
  is when it differs from this process's own, i.e. exactly when `-r/--run` used
  `--unshare-user`) and, when so, leaving the default identity unresolved (no
  priv-drop) for that case — joining that same user namespace with
  `--preserve-credentials` (already done regardless) already reproduces the
  container's own view correctly via that same kernel mapping, with nothing
  further needed. Verified end-to-end on this same rootless dev machine: a
  daemonized `-r --run` busybox session with no declared user, `--exec`'d with no
  `--user`, now correctly shows `uid=0(root)` (previously would have attempted,
  and failed, a priv-drop to the host-mapped uid); an explicit `--exec --user 0`
  against the same session correctly resolves to `0:0` and skips the priv-drop
  step; `--exec --user portage` against it correctly resolves the name to its
  real `250:250` via the fetched `/etc/passwd` and then fails clearly (helper not
  bind-mounted, since the session itself had no declared user) rather than
  silently running as the wrong identity.
- `kill_session.{h,cpp}` — implements `--kill <pid>`, stopping a tracked,
  running `-r/--run` session and everything it started. `kill_session()`
  validates `pid` the same way `exec_in_session()` does (via `list_sessions()`,
  `pid_file.h`). **Real bug reported by the user against their own actual
  target device, confirmed via a captured session log**: a plain `kill
  <tracked_bwrap_pid>` doesn't kill everything a container started — their
  `/init` script `php-fpm --daemonize`s (double-forks, detaches) then `exec
  caddy ...`s (replaces itself); after killing the tracked pid, both `caddy`
  and the `php-fpm` master+workers kept running as orphans. **Root cause**: on
  that device, `bwrap`'s `--unshare-pid` isn't actually in effect at all —
  `detect_bwrap_unshare_args()` (`bwrap.cpp`) only requests `--unshare-xxx`
  flags the kernel actually supports, and that kernel doesn't support pid
  namespaces (independently confirmed elsewhere this session, see
  `exec_session.{h,cpp}`'s own `CONFIG_CHECKPOINT_RESTORE` bug above) — so
  `php-fpm --daemonize` reparents to the *host's own* pid 1, completely
  disconnected from the sandboxed session; the classic "kill a pid namespace's
  pid 1, the kernel guarantees the whole namespace collapses" trick simply
  doesn't apply there. Per the user's own explicit request (they want to
  choose the mechanism per host capability, and may need an even more basic
  one later for some hypothetical older device), `kill_session()` picks
  between three independently-named strategies, selected dynamically per
  session (not a single cached host-wide capability flag, since e.g. cgroup
  creation can fail for session-specific reasons like permissions even on a
  host that generally supports cgroups) — each runs its own complete
  `SIGTERM` → wait-up-to-`grace_period_seconds` (10s default, no CLI flag) →
  forced-`SIGKILL` escalation internally, with no cross-strategy
  fallback-after-failure chaining:
  1. `kill_via_cgroup()` — preferred whenever the session has a non-empty
     dedicated cgroup (`session_cgroup_pids()`, `session_cgroup.h`): `SIGTERM`
     to every pid currently in it, and, if forcing is needed, either the
     atomic `cgroup.kill` knob or a fresh re-read-and-`SIGKILL` sweep (fresh,
     not the original snapshot, since a process could have forked a new child
     after the graceful sweep but before dying). The **only** mechanism that
     reliably reaches every process regardless of pid namespace support.
     **Critical correctness point, caught during design review before this
     shipped**: the "is it stopped yet" poll must gate on the *cgroup being
     empty*, not `list_sessions()`'s running flag — that flag only reflects
     the pid file's flock, released the moment the tracked outer `bwrap` pid
     exits, and the `SIGTERM` sweep necessarily hits `bwrap` itself too (it's
     a cgroup member) — `bwrap` dies and gets reaped in well under a second,
     long before slower descendants (`caddy` shutting down gracefully,
     `php-fpm` finishing in-flight requests) actually exit. Gating on the pid
     file instead would make the poll resolve "done" almost immediately, the
     forced-kill step would never run, and the original bug would reproduce
     with unused machinery around it.
  2. `kill_via_pid_namespace()` — used when no cgroup exists for the session,
     but `resolve_namespace_pid()`/`pid_namespace_isolated()`
     (`sandbox_process.h`) confirm `--unshare-pid` was genuinely in effect for
     it. `SIGTERM`s `collect_descendant_pids(ns_pid)` (reliable here
     specifically because reparenting within a genuinely isolated pid
     namespace always lands back on that namespace's own pid 1); if forcing
     is needed, a single `SIGKILL` to `ns_pid` alone is *guaranteed* complete
     by the kernel itself, independent of whatever the graceful sweep missed.
     "Stopped" is simply `kill(ns_pid, 0)` failing with `ESRCH`. Verified
     end-to-end on this project's rootless dev machine (which does support
     pid namespaces, unlike the user's real target device): a daemonized
     busybox session running `sh -c 'sleep 300 & exec sleep 300'` (mirroring
     the daemonize-then-exec shape of the original bug) was fully cleaned up
     by `--kill`, including the backgrounded child, with no leftover
     processes, mounts, or layers; a second run using `sh -c 'trap "" TERM;
     sleep 300'` (ignoring `SIGTERM` entirely) confirmed the forced-`SIGKILL`
     escalation path too, taking the full 10s grace period before the pid
     namespace's own collapse-on-kill guarantee cleaned it up regardless.
  3. `kill_via_tracked_pid()` — fallback when neither of the above applies:
     signals the tracked `bwrap` pid directly, `SIGTERM` then `SIGKILL`,
     polling `list_sessions()` for "stopped" since that's the only signal
     available without a cgroup or an isolated pid namespace to check
     directly. Exactly today's manual-`kill` behavior — least complete, but
     always available, and strictly no worse than before this feature
     existed. This is the path the user's own real target device actually
     takes today (no cgroup delegation confirmed working there yet; no pid
     namespace support at all) — a future, even more basic strategy (for some
     hypothetical still-more-limited device) would slot in here the same way,
     per the user's own explicit request to keep this extensible.

  `poll_until()`/`sleep_ms()` (`.cpp`-local) use `nanosleep()` in an
  `EINTR`-retry loop — matching this project's existing direct-POSIX style
  (`process.cpp` already retries `waitpid()` the same way) — rather than
  `<thread>`/`<chrono>` (unused anywhere else in this project).
- `config_file.{h,cpp}` — `load_config_file()` reads and parses (via libyaml's
  document API, `<yaml.h>`) the `global` and `volumes` sections of the local YAML
  config file located by `config_file_path()` (`$XDG_CONFIG_HOME/slocker-lite/config.yaml`,
  falling back to `$HOME/.config/slocker-lite/config.yaml`). Supported `global` keys:
  `log-level`, and six `unshare-<type>` keys (`unshare-user`/`unshare-ipc`/
  `unshare-pid`/`unshare-net`/`unshare-uts`/`unshare-cgroup`, one per
  `bwrap.cpp`'s own `namespace_probes` entry) controlling whether `-r/--run`
  requests each of bwrap's `--unshare-xxx` flags — every other long option is a
  one-shot flag, not a setting, so it doesn't belong in a persistent config file.
  Each `unshare-*` key accepts `"1"`/`"on"`/`"yes"`/`"true"` (enabled) or
  `"0"`/`"off"`/`"no"`/`"false"` (disabled), case-insensitively (`.cpp`-local
  `parse_bool_flag()`); an unset key defaults to enabled, and an unrecognized
  value logs a `spdlog::warn` and is treated as unset (default enabled) rather
  than failing the whole config load — consistent with this file's existing
  forward-compatible/ignore-malformed-entries policy (only malformed *YAML
  syntax* is a hard error). A missing file returns a default-constructed
  (empty) `AppConfig`, not an error; unknown sections/keys (and malformed
  individual volume entries) are likewise ignored for forward-compatibility.
  `main()` applies `config->log_level` (via the existing `apply_log_level()`)
  right after `spdlog::cfg::load_env_levels()` and before parsing CLI options,
  so an explicit `--log-level` on the command line always overwrites it
  afterward — same precedence pattern already used for `SPDLOG_LEVEL`.
  `write_config_file()` writes the whole file back out (via libyaml's
  document-building/emitter API, symmetric to the read side) — used by
  `-v/--volume` (`create_volume_command()`, `commands.cpp`) to persist a new
  `VolumeEntry {name, directory}` into the `volumes` section, preserving
  `global` (including any set `unshare-*` keys, re-serialized as canonical
  `"true"`/`"false"`) untouched. **`VolumeEntry`/the `volumes` section is a
  distinct concept from `OciImageConfig::volumes`**: this is a user-defined
  `name -> host directory` mapping created via `-v/--volume`, not an image's
  own declared mount points (still unconsumed, see `oci_image.{h,cpp}` above).
  `AppConfig`/`config.volumes` is looked up by name in `resolve_volume_mount()`
  (`volume_mount.{h,cpp}`, see below), which is how `-r/--run`'s own `-v` usage
  finds a named volume's host directory. `run_container()` (`commands.cpp`)
  resolves the six `unshare-*` fields (each `value_or(true)`) into a
  `NamespaceConfig` (`bwrap.h`, see below) once, up front, and passes it to
  `run_bwrap()` — `bwrap.{h,cpp}` itself has no dependency on this file or on
  YAML parsing at all, only on the already-resolved, defaults-applied struct.
- `volume_mount.{h,cpp}` — `is_valid_volume_name()` (no `/`, checked by both
  `create_volume_command()` and to tell a `-v` spec's name/path apart) and
  `resolve_volume_mount()`, called once per `-v` occurrence from `run_container()`
  when running with `-r`. A spec with no `/` is looked up in `config.volumes` by
  name (error if unknown); one with `/` is treated as a host directory path and
  `create_directories()`'d if missing. If the resulting host directory is empty,
  `initialize_volume_directory()` reconciles it against the image's own directory
  at the given container path: if that image directory is non-empty, its contents
  are copied in first; then, **whether or not there was content to copy**, the
  host directory's own mode/ownership/timestamps (and xattrs/ACLs where
  supported) are always set to match the image directory's own — **real bug
  fixed by the user, not assumed**: an earlier version only ever copied when the
  image directory was non-empty, so an image declaring an *empty* directory with
  specific ownership/permissions (e.g. a data directory owned by a non-root
  uid/gid) got a host directory with default `create_directories()` permissions
  instead, and even the non-empty case never reconciled the directory's *own*
  attributes (only each copied entry's). The existence check, content copy, and
  attribute reconciliation all run as a single `sh -c` invocation wrapped through
  `wrap_for_root_namespace()` (`bwrap.h`) — **not** a plain `std::filesystem`
  check — because a rootless `containers-storage mount`'s content isn't visible
  to this process at all without `nsenter`, the same constraint `run_bwrap()`
  itself works around (see the root-vs-rootless paragraph below). `cp -a
  --preserve=mode,ownership,timestamps,links[,xattr] --attributes-only -T` does
  the attribute-reconciliation step; whether `,xattr` is included is decided by
  a direct `setxattr()`/`removexattr()` probe on the host directory (no new
  library dependency — Linux POSIX ACLs are themselves stored as xattrs, so this
  one probe stands in for both, logging a single `spdlog::warn` if unsupported).
  `-T`/`--no-target-directory` is required on that second `cp` — confirmed by
  direct testing: without it, since the host directory already exists, plain
  `cp SRC DST` copies `SRC` *into* `DST` as a nested `DST/basename(SRC)`
  subdirectory instead of reconciling `DST`'s own attributes, which is exactly
  the bug this fix closes. A nonzero `cp` exit is only ever a warning, never
  fatal — often just an ownership-preservation shortfall when not running as
  root. Verified end-to-end against `images/gitea.tar`'s real declared
  `/etc/gitea`/`/var/lib/gitea` volumes under a real rootless mount: the
  resulting host directories' mode/ownership matched the image's own declared
  values in both cases, and no mounts/layers were left behind afterward.

Errors are logged via `spdlog::error`; every external command is also traced at debug
level in `run_process()`/`run_process_foreground()` (`src/process.cpp`) — visible via
`SPDLOG_LEVEL=debug`, since spdlog's default level is `info` — and a failed external
command additionally logs a `spdlog::warn`, which is visible by default (no env var
needed). The final "mounted image at: ..." success line is direct stdout program
output, not a log.

Because `containers-storage mount` runs rootless, it reexecs itself into a private
user+mount namespace to gain the privilege it needs for the overlay mount — which
leaves the result invisible to a plain shell or child process outside that namespace.
Confirmed `containers-storage unshare` does **not** rejoin an already-running mount's
namespace; only `nsenter` targeting the live `fuse-overlayfs` daemon's PID does.
`-r/--run` handles this automatically by locating that PID and running `bwrap` via
`nsenter` into its namespaces (`wrap_for_root_namespace()`, `src/bwrap.h`) — reused
as-is by `volume_mount.cpp`'s copy-into-an-empty-volume step, since that also needs
to read image content that's otherwise invisible outside the same namespace.
**Running as root sidesteps all of this**: no privilege
reexec is needed, so the mount is already directly visible in the current namespace,
and `nsenter --user=...` into it then fails ("reassociate to namespace 'ns/user'
failed: Invalid argument") since the caller is already in that same user namespace.
`-r/--run` detects `geteuid() == 0` and skips `nsenter` automatically in that case;
`-n/--no-nsenter` forces it off manually for any other situation where the mount turns
out to already be directly visible.

**Mutable global state and multi-container support:** an audit ahead of planned
docker-compose support (running multiple containers at once) found exactly three
pieces of mutable global/file-scope state in `src/`: `g_mount_program`
(`containers_storage.{h,cpp}`, the resolved `fuse-overlayfs` path — genuinely
process-wide, invariant across containers), `g_foreground_child_pid`
(`process.cpp`, plus `run_process_foreground()`'s process-wide `SIGINT`/`SIGTERM`
handler installation — tracks one foreground child at a time), and
`g_report_fd`/`g_log_path` (`daemonize.cpp`, one in-flight `-D/--daemonize`
handshake's report-pipe fd and log path). **Decision, confirmed by the user:**
multi-container/compose support will run each container's session in its own
forked OS process — the same model `-D/--daemonize` already uses — rather than
one process managing multiple containers concurrently without forking. Under
that model, "one OS process" and "one running container" stay the same thing
they already are today, so **none of these globals need to become per-container
state** — each forked child only ever tracks/signals one foreground child and
handles one daemonize handshake, exactly as today. This is a load-bearing
constraint for however the compose orchestrator ends up implemented: it must
fork (not thread, not run an in-process event loop over N containers) one child
per service, each child reusing `run_container()`'s existing single-container
code path unchanged.

## Build & test commands

Build directory is `buildDir/` (already configured).

- Configure (only needed if `buildDir/` is missing or deleted): `meson setup buildDir`
- Build: `meson compile -C buildDir` (or `ninja -C buildDir`) — also builds
  `buildDir/slocker-lite-priv-drop`, the statically-linked helper `-r --user` needs
  (see `priv_drop_helper.cpp` in "Project state")
- Run the executable: `./buildDir/slocker-lite -m <image.tar>` (see `--help` for the
  full flag list: `-m/--mount`, `-r/--run`, `-u/--umount`, `-c/--cleanup`,
  `-l/--list-images`, `-i/--inspect`, `-x/--exec`, `--kill`, `-n/--no-nsenter`, `-D/--daemonize`,
  `--user`, `--group`, `--hostname`, `--env`, `--env-file`, `-v/--volume`, `--list-volumes`, `--delete-volume`,
  `--delete-volume-full`, `--list-processes`, `--clean-processes`, `-t/--test`, `--log-level`,
  `-h/--help`, `-V/--version`)
- Run tests: `meson test -C buildDir`

## Code style

- Null-pointer checks: prefer `if (!ptr)` / `if (ptr)` over `if (ptr == nullptr)` / `if (ptr != nullptr)`.
- Constants: no `k` Hungarian-notation prefix. `enum class` values are already qualified by
  the enum's own name (e.g. `Mode::run`, `OciPortProtocol::tcp`), so plain snake_case
  enumerators are enough on their own. Free-standing constants also use plain snake_case;
  when several are conceptually related, group them under a named `namespace` instead of
  relying on a shared prefix to imply the grouping (e.g. `cli_args.cpp`'s `getopt_long` long-option
  codes live in `namespace options { constexpr int log_level = ...; }`, and `bwrap.cpp`'s
  priv-drop-helper path/binary-name pair live in `namespace priv_drop { ... }`) — nest the named
  namespace inside the file's existing anonymous namespace where one is already present, so
  internal linkage is unchanged. A `kXxx`-named identifier that turns out not to actually be
  `const` (mutable global/static state) instead follows this codebase's existing `g_` prefix
  convention (e.g. `containers_storage.cpp`'s `g_mount_program`, matching `process.cpp`'s
  `g_foreground_child_pid` and `daemonize.cpp`'s `g_report_fd`/`g_log_path`).

## Licensing

- Every `.c`/`.cpp`/`.h` file under `src/` must start with the GPLv2-or-later copyright
  header (see any existing file under `src/` for the exact text).
- After adding a new source file under `src/`, run `./add-license.sh` from the repo
  root to prepend the header (it reads `copyright-header` and inserts it via `sed`,
  skipping files that already have it, so it's safe to re-run at any time).

## Build configuration notes

- `meson.build` sets `warning_level=3` and `cpp_std=c++20` — keep new code warning-clean under `-Wall -Wextra -Wpedantic`-equivalent settings.
- The single Meson `test()` target runs `slocker-lite` against a fixture OCI image tar generated at build time by `tests/gen_fixture.py` (a `custom_target`) and checks its exit code (no test framework is wired in yet).