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` — CLI entry point, dependency checks, orchestration (`mount_image()`,
  `run_container()`, `cleanup_image()`, `unmount_image()`, `list_images_command()`,
  `inspect_image_command()`, `create_volume_command()`, `list_volumes_command()`,
  `delete_volume_command()`). `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`). `-v/--volume` is dual-purpose:
  used alone it's `create_volume_command()`; combined with `-r/--run` it instead
  requests a volume mount (repeatable) and is resolved by `resolve_volume_mount()`
  (see `volume_mount.{h,cpp}` below) instead. Since `-v` must be repeatable with
  `-r` but each occurrence still takes two space-separated tokens, `main()`'s
  getopt loop no longer lets `'v'` set `Mode` itself: it accumulates
  `(spec, path)` pairs into `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::kVolume` call
  or, together with `-r`, threads `volume_specs` through to `run_container()`.
  `run_container()` resolves each 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>`
  (long-option only, no short form) 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.
- `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 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_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 `main.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()` deliberately drops `--unshare-net` from what's actually passed
  to `bwrap` even when the kernel supports it — without any network setup (e.g.
  `slirp4netns`), unsharing it just leaves the sandbox with no network at all. Re-add
  once network isolation is implemented; `detect_bwrap_unshare_args()` itself still
  probes/reports it (e.g. via `-t/--test`), since that's kernel capability, 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.
- `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()` (`src/bwrap.cpp`) locates it next to `slocker-lite`'s own
  binary (via `/proc/self/exe`'s directory), which holds both when run straight from
  `buildDir/` and after a real `meson install`.
- `user_spec.{h,cpp}` — `resolve_user_and_group()` resolves a user/group spec (each
  a name or numeric id) against the *mounted image's own* `/etc/passwd`/`/etc/group`
  (not the host's), since names like `git` only mean anything inside that image's own
  user database. A numeric user with no group and no matching `/etc/passwd` entry
  defaults gid to the same numeric value as the uid. `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_bwrap_args()` (`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()` (`main.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 `$PATH` lookup. `run_process_foreground()`
  installs a SIGINT/SIGTERM handler around its `waitpid()` that forwards the signal
  to the running child and keeps waiting instead of letting the default disposition
  kill `slocker-lite` itself — without this, Ctrl-C (or `kill`) during `-r`'s `bwrap`
  run would skip `run_container()`'s unmount/cleanup entirely, leaving the layer
  imported and/or mounted.
- `config_file.{h,cpp}` — `load_config_file()` reads and parses (via libyaml's
  document API, `<yaml.h>`) the `global` and `volumes` sections of the local YAML
  config file located by `config_file_path()` (`$XDG_CONFIG_HOME/slocker-lite/config.yaml`,
  falling back to `$HOME/.config/slocker-lite/config.yaml`). `global.log-level` is the
  only supported `global` key — other long options are one-shot flags, not settings,
  so they don't belong in a persistent config file. A missing file returns a
  default-constructed (empty) `AppConfig`, not an error; unknown sections/keys (and
  malformed individual volume entries) are ignored for forward-compatibility;
  malformed YAML syntax is a hard error. `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()`, `main.cpp`) to persist a new
  `VolumeEntry {name, directory}` into the `volumes` section, preserving `global`
  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.
- `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 and
  the image already has non-empty content at the given container path, that content
  is copied in first. Both the existence check and the copy run as a single
  `sh -c '[ -d ... ] && cp -a ...'` 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]` does the copy; 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). A nonzero `cp` exit is only ever a warning, never
  fatal — often just an ownership-preservation shortfall when not running as root.

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.

## 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`, `-n/--no-nsenter`, `--user`, `--group`,
  `--hostname`, `-v/--volume`, `--list-volumes`, `--delete-volume`,
  `--delete-volume-full`, `-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)`.

## 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).