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Phase 1: imagepull package — pull, flatten, ext4

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New internal/imagepull/ subpackage. Three concerns, each
independently testable:

Pull (imagepull.go):
 - github.com/google/go-containerregistry's remote.Image with the
   linux/amd64 platform pinned. Anonymous pulls only for v1.
 - Layer blobs cached on disk via cache.NewFilesystemCache under
   <cacheDir>/blobs/sha256/<hex> — OCI-standard layout so
   skopeo/crane could co-exist later.
 - Eagerly touches every layer once so network errors surface at
   Pull time, not deep in Flatten.

Flatten (flatten.go):
 - Replays layers oldest-first into destDir.
 - Whiteout-aware: .wh.<name> deletes the named entry,
   .wh..wh..opq wipes the parent directory's contents from prior
   layers.
 - Path-traversal hardening mirrored from kernelcat extractTar:
   reject .., absolute paths, and symlinks/hardlinks whose
   resolved target escapes destDir.
 - Handles tar.TypeReg, TypeDir, TypeSymlink, TypeLink. Skips
   device/fifo nodes silently (need privilege; udev/devtmpfs
   handles them in the guest).

BuildExt4 (ext4.go):
 - Truncates outFile to sizeBytes, then runs `mkfs.ext4 -F -d
   <srcDir> -E root_owner=0:0`. No mount, no sudo, no loopback.
 - 64 MiB floor; callers handle real sizing with content-aware
   headroom.
 - File ownership in the resulting ext4 reflects srcDir's on-disk
   ownership — runner's uid/gid since extraction was unprivileged.
   Documented in package doc as a Phase A v1 limitation; Phase B
   will add a debugfs- or tar2ext4-based ownership fixup.

paths.Layout gains OCICacheDir at $XDG_CACHE_HOME/banger/oci/,
ensured at startup alongside the other dirs.

Tests use go-containerregistry's in-process registry to push and
pull synthetic multi-layer images. Cover: layer caching round-trip,
whiteout + opaque-marker handling, path-traversal rejection, unsafe
symlink rejection, real mkfs.ext4 round-trip (skipped if mkfs.ext4
absent), and tiny-size rejection.

go-containerregistry v0.21.5 added as a direct dep, plus its
transitive closure (containerd/stargz, opencontainers/go-digest,
docker/cli config helpers, etc).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
This commit is contained in:
Thales Maciel 2026-04-16 17:22:13 -03:00
parent da4a6bf45b
commit 78376ba6ec
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GPG key ID: 33112E6833C34679
7 changed files with 733 additions and 33 deletions
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4
internal/paths/paths.go
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@ -23,6 +23,7 @@ type Layout struct {
VMsDir string
ImagesDir string
KernelsDir string
OCICacheDir string
}
func Resolve() (Layout, error) {
@ -54,11 +55,12 @@ func Resolve() (Layout, error) {
layout.VMsDir = filepath.Join(layout.StateDir, "vms")
layout.ImagesDir = filepath.Join(layout.StateDir, "images")
layout.KernelsDir = filepath.Join(layout.StateDir, "kernels")
layout.OCICacheDir = filepath.Join(layout.CacheDir, "oci")
return layout, nil
}
func Ensure(layout Layout) error {
for _, dir := range []string{layout.ConfigDir, layout.StateDir, layout.CacheDir, layout.RuntimeDir, layout.VMsDir, layout.ImagesDir, layout.KernelsDir} {
for _, dir := range []string{layout.ConfigDir, layout.StateDir, layout.CacheDir, layout.RuntimeDir, layout.VMsDir, layout.ImagesDir, layout.KernelsDir, layout.OCICacheDir} {
if err := os.MkdirAll(dir, 0o755); err != nil {
return err
}