Document apt field trial and gap G7

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 
+## 0.1.3 - 2026-10-23
+- Docs: field trial of apt and common tools, new gap G7 (host-owned lower layer), README limits section. No code change.
+
 ## 0.1.2 - 2026-10-23
 - Docs: researched solutions for all known gaps (G1 to G6) in `docs/RESEARCH.md`, with measurements. Nothing implemented yet.
 

README.md

@@ -1,6 +1,6 @@
 # doos
 
-Version: see `VERSION` (currently 0.1.2). Release history: `CHANGELOG.md`.
+Version: see `VERSION` (currently 0.1.3). Release history: `CHANGELOG.md`.
 
 User-level containers in C++20: overlayfs over the host's `/usr`, `/etc`, `/opt`, user/mount/pid namespaces, networking through slirp4netns. No root at runtime.
 
@@ -38,6 +38,14 @@ Container writes to `/usr`, `/etc`, `/opt` stay in the container (copy-on-write)
 A container is `running` while its first `attach` command is alive; more `attach` calls
 join it. Data persists until `delete --yes`.
 
+## Current limits you will hit
+
+Tools run fine (curl, git, gcc, python3, pip, ping all verified). Package management does
+not: `apt-get update` needs a seeded `/var` and `-o APT::Sandbox::User=root`, and
+`apt-get install` cannot finish configuring packages because host-root-owned files are
+unwritable and unchownable inside. Details, measurements and the fix are in
+`docs/RESEARCH.md` (G7, "Field trial").
+
 ## Configuration
 
 | Variable | Meaning |

VERSION

@@ -1 +1 @@
-0.1.2
+0.1.3

docs/RESEARCH.md

@@ -54,6 +54,7 @@ removed afterwards. Rows marked "unmeasured" have no run behind them.
 | G4 | Host `/etc` readable | Allowlist-only private `/etc` instead of an overlay of host `/etc` | minor | `spec.hpp` lists, `setupRoot` |
 | G5 | `--net host` shares abstract sockets | Keep opt-in; evaluate pasta as an isolated replacement | patch or minor | `network.cpp` |
 | G6 | No seccomp, nested userns allowed | Hand-built seccomp-bpf denylist after `no_new_privs` | minor | new `Seccomp` class, `sandbox.cpp` |
+| G7 | Host-owned lower dirs are not writable; `chown --reference` of host files fails; `/var` is empty so apt has no databases | Image-based lower layer (extracted rootfs, owned by mapped ids) plus G3; short term: pre-seeded directory skeleton | major (changes layout) | `Container::prepareLayout`, `setupRoot`, new image import |
 
 ### G1: memory, cpu and pids limits (cgroup v2)
 
@@ -133,3 +134,30 @@ Unmeasured: compatibility impact on real workloads.
 3. G4 (private `/etc`): needs the per-tool allowlist runs first.
 4. G2, then G3 (G3 changes the delete path, do it deliberately).
 5. G5: needs a test matrix before any change.
+
+## Field trial: apt and common tools (v0.1.2, measured)
+
+Run in a fresh `--net slirp` container on the host above (Ubuntu with apt 3.2.0, dpkg 1.23.7).
+
+| Step | Result |
+|---|---|
+| curl https, git ls-remote, gcc compile+run, python3 (`ssl`, `sqlite3`), pip, ping 1.1.1.1 | all work |
+| `apt-get update` as shipped | fails: `/var/lib/apt/lists/partial is missing` (`/var` is an empty private dir) |
+| `apt-get install` as shipped | fails: `/var/lib/dpkg/lock-frontend` missing, no dpkg status database |
+| after seeding `/var` (dirs plus copy of host `dpkg/status`, `info`, `alternatives`, 40 MB) | apt starts, but the download method dies: `setegid 65534 failed`, `seteuid 42 failed` (sandbox user `_apt` cannot exist with a single id) |
+| with `-o APT::Sandbox::User=root` | `apt-get update` works: 58.9 MB fetched in 3 s |
+| `apt-get install cowsay` | download works; unpack fails: `unable to create '/usr/games/cowsay.dpkg-new': Permission denied` |
+| after pre-creating the host directory tree in the upper dirs (13164 dirs for `/usr`, done with a shell loop, 38 s; not yet implemented in C++) | unpack works, `/usr/games/cowsay hoi` runs; host `/usr/games/cowsay` still absent (isolation holds) |
+| configure step | fails: `chown: changing ownership of '/etc/shells.tmp': Invalid argument` (debianutils postinst copies owner of `/etc/shells`, which shows as `65534`), so dpkg reports 4 packages not fully installed |
+
+Root causes:
+1. Host files owned by host root appear as `nobody` (uid 65534). Container root has `CAP_DAC_OVERRIDE` only over inodes whose owner is mapped, so it cannot write into host-root-owned directories of the lower layer, and cannot `chown` to or from an unmapped owner.
+2. `/var` has no package database.
+3. apt wants a second uid (G3).
+
+Conclusion: the current design (host `/usr`, `/etc`, `/opt` as lower layer) is good for running tools and for isolated experiments, not for package management. Bare tools and compilers work. Honest status: **`apt-get install` does not complete today.**
+
+Researched ways forward, in order of effort:
+- Short term, writable skeleton: at create time mirror every directory of the lower dirs into the upper dir with the same mode (directories owned by the user are writable; merged view is unchanged). Directories unreadable to the user (for example `/etc/ssl/private`, mode 700, root-owned) are skipped and stay as they are. Measured effect: writes into `/usr/bin` work, attach time unchanged (41 ms). Unmeasured: C++ implementation speed, and it does not fix the `chown` failure.
+- Proper fix, image layer: use an extracted distro rootfs (for example from `podman export` or `skopeo copy` plus unpack, both installed here; `mmdebstrap` and `debootstrap` are not) as the lower dir, with ownership shifted into the subuid range (G3). Then the tree is owned by mapped ids, `/var/lib/dpkg` exists, `chown` works, `_apt` exists, and host `/etc` stops leaking (G4). This also removes the dependence on the host distro. Cost: image import step, disk per image (shared lower, so once), and a layout change, hence a major bump.
+- Alternative to evaluate: `fuse-overlayfs` (installed) supports uid mappings between layers. Unmeasured; adds a FUSE process per container.