diff --git a/.buildkite/docker-compose.yml b/.buildkite/docker-compose.yml
new file mode 100644
index 000000000..05baf3479
--- /dev/null
+++ b/.buildkite/docker-compose.yml
@@ -0,0 +1,17 @@
+version: '3'
+
+services:
+  fixperms-tests:
+    image: golang:latest
+    working_dir: /code
+    volumes:
+      - ..:/code:ro
+    command: go test -v ./...
+
+  fixperms-build:
+    image: golang:latest
+    working_dir: /code
+    volumes:
+      - ..:/code
+      - /var/lib/buildkite-agent/git-mirrors:/var/lib/buildkite-agent/git-mirrors
+    command: .buildkite/steps/build-fixperms.sh
diff --git a/.buildkite/pipeline.yaml b/.buildkite/pipeline.yaml
index e0ee9a893..27475aae8 100644
--- a/.buildkite/pipeline.yaml
+++ b/.buildkite/pipeline.yaml
@@ -7,14 +7,28 @@ steps:
     agents:
       queue: "${BUILDKITE_AGENT_META_DATA_QUEUE}"
 
-  - id: "bats-tests"
-    name: ":bash: Unit tests"
+  - id: "fixperms-tests"
+    name: ":go: fixperms tests"
     agents:
       queue: "${BUILDKITE_AGENT_META_DATA_QUEUE}"
     plugins:
-      docker-compose#v2.1.0:
-        run: unit-tests
-        config: docker-compose.unit-tests.yml
+      - docker-compose#v2.1.0:
+          run: fixperms-tests
+          config: .buildkite/docker-compose.yml
+
+  - id: "fixperms-build"
+    name: ":go: fixperms build"
+    agents:
+      queue: "${BUILDKITE_AGENT_META_DATA_QUEUE}"
+    depends_on:
+      - "fixperms-tests"
+    artifact_paths: "build/fix-perms-*"
+    plugins:
+      - docker-compose#v2.1.0:
+          run: fixperms-build
+          config: .buildkite/docker-compose.yml
+      - artifacts#v1.9.0:
+          upload: "builds/fix-perms-*"
 
   - id: "deploy-service-role-stack"
     name: ":aws-iam: :cloudformation:"
@@ -23,7 +37,8 @@ steps:
     command: .buildkite/steps/deploy-service-role-stack.sh
     depends_on:
       - "lint"
-      - "bats-tests"
+      - "fixperms-tests"
+      - "fixperms-build"
 
   - id: "packer-windows-amd64"
     name: ":packer: :windows:"
@@ -34,7 +49,8 @@ steps:
       queue: "${BUILDKITE_AGENT_META_DATA_QUEUE}"
     depends_on:
       - "lint"
-      - "bats-tests"
+      - "fixperms-tests"
+      - "fixperms-build"
 
   - id: "launch-windows-amd64"
     name: ":cloudformation: :windows: AMD64 Launch"
@@ -77,7 +93,8 @@ steps:
       queue: "${BUILDKITE_AGENT_META_DATA_QUEUE}"
     depends_on:
       - "lint"
-      - "bats-tests"
+      - "fixperms-tests"
+      - "fixperms-build"
 
   - id: "launch-linux-amd64"
     name: ":cloudformation: :linux: AMD64 Launch"
@@ -119,7 +136,8 @@ steps:
       queue: "${BUILDKITE_AGENT_META_DATA_QUEUE}"
     depends_on:
       - "lint"
-      - "bats-tests"
+      - "fixperms-tests"
+      - "fixperms-build"
 
   - id: "launch-linux-arm64"
     name: ":cloudformation: :linux: ARM64 Launch"
diff --git a/.buildkite/steps/build-fixperms.sh b/.buildkite/steps/build-fixperms.sh
new file mode 100755
index 000000000..a3b9cac34
--- /dev/null
+++ b/.buildkite/steps/build-fixperms.sh
@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+set -euo pipefail
+for arch in amd64 arm64; do
+  GOOS=linux GOARCH="${arch}" go build -v -o "build/fix-perms-linux-${arch}" ./internal/fixperms
+done
diff --git a/.buildkite/steps/packer.sh b/.buildkite/steps/packer.sh
index 47333ad75..b04ffbb93 100755
--- a/.buildkite/steps/packer.sh
+++ b/.buildkite/steps/packer.sh
@@ -16,6 +16,12 @@ fi
 
 mkdir -p "build/"
 
+if [[ "$os" == "linux" ]] ; then
+  buildkite-agent artifact download "build/fix-perms-linux-${arch}" ./build
+  mv "build/fix-perms-linux-${arch}" packer/linux/conf/buildkite-agent/scripts/fix-buildkite-agent-builds-permissions
+  chmod 755 packer/linux/conf/buildkite-agent/scripts/fix-buildkite-agent-builds-permissions
+fi
+
 # Build a hash of packer files and the agent versions
 packer_files_sha=$(find Makefile "packer/${os}" plugins/ -type f -print0 | xargs -0 sha1sum | awk '{print $1}' | sort | sha1sum | awk '{print $1}')
 stable_agent_sha=$(curl -Lfs "https://download.buildkite.com/agent/stable/latest/${agent_binary}.sha256")
diff --git a/docker-compose.unit-tests.yml b/docker-compose.unit-tests.yml
deleted file mode 100644
index a68c7a539..000000000
--- a/docker-compose.unit-tests.yml
+++ /dev/null
@@ -1,9 +0,0 @@
-version: '3'
-
-services:
-  unit-tests:
-    image: bats/bats
-    volumes:
-      - .:/code:ro
-      - ./unit-tests/fixtures:/var/lib/buildkite-agent/builds:ro
-    command: /code/unit-tests
\ No newline at end of file
diff --git a/go.mod b/go.mod
new file mode 100644
index 000000000..6d7d7bd23
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,8 @@
+module github.com/buildkite/elastic-ci-stack-for-aws/v6
+
+go 1.20
+
+require (
+	github.com/google/go-cmp v0.5.9
+	golang.org/x/sys v0.12.0
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 000000000..eb4ae603c
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,4 @@
+github.com/google/go-cmp v0.5.9 h1:O2Tfq5qg4qc4AmwVlvv0oLiVAGB7enBSJ2x2DqQFi38=
+github.com/google/go-cmp v0.5.9/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
+golang.org/x/sys v0.12.0 h1:CM0HF96J0hcLAwsHPJZjfdNzs0gftsLfgKt57wWHJ0o=
+golang.org/x/sys v0.12.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
diff --git a/internal/fixperms/fdfs/fdfs.go b/internal/fixperms/fdfs/fdfs.go
new file mode 100644
index 000000000..8d1641fa1
--- /dev/null
+++ b/internal/fixperms/fdfs/fdfs.go
@@ -0,0 +1,64 @@
+//go:build linux
+
+// Package fdfs is like os.DirFS, but with a file descriptor and openat(2),
+// fchownat(2), etc, to ensure symlinks do not escape.
+package fdfs
+
+import (
+	"io/fs"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+const resolveFlags = unix.RESOLVE_BENEATH | unix.RESOLVE_NO_SYMLINKS | unix.RESOLVE_NO_MAGICLINKS | unix.RESOLVE_NO_XDEV
+
+// FS uses a file descriptor for a directory as the base of a fs.FS.
+type FS uintptr
+
+// DirFS opens the directory dir, and returns an FS rooted at that directory.
+// It uses open(2) with O_PATH+O_DIRECTORY+O_CLOEXEC.
+func DirFS(dir string) (FS, error) {
+	bd, err := os.OpenFile(dir, unix.O_PATH|unix.O_DIRECTORY|unix.O_CLOEXEC, 0)
+	if err != nil {
+		return 0, err
+	}
+	return FS(bd.Fd()), nil
+}
+
+// Close closes the file descriptor.
+func (s FS) Close() error {
+	return unix.Close(int(s))
+}
+
+// Open wraps openat2(2) with O_RDONLY+O_NOFOLLOW+O_CLOEXEC.
+func (s FS) Open(path string) (fs.File, error) {
+	fd, err := unix.Openat2(int(s), path, &unix.OpenHow{
+		Flags:   unix.O_RDONLY | unix.O_NOFOLLOW | unix.O_CLOEXEC,
+		Mode:    0,
+		Resolve: resolveFlags,
+	})
+	if err != nil {
+		return nil, err
+	}
+	f := os.NewFile(uintptr(fd), path)
+	return f, nil
+}
+
+// Lchown wraps fchownat(2) (with AT_SYMLINK_NOFOLLOW).
+func (s FS) Lchown(path string, uid, gid int) error {
+	return unix.Fchownat(int(s), path, uid, gid, unix.AT_SYMLINK_NOFOLLOW)
+}
+
+// Sub wraps openat2(2) (with O_PATH+O_DIRECTORY+O_NOFOLLOW+O_CLOEXEC), and returns an FS.
+func (s FS) Sub(dir string) (FS, error) {
+	subFD, err := unix.Openat2(int(s), dir, &unix.OpenHow{
+		Flags:   unix.O_PATH | unix.O_DIRECTORY | unix.O_NOFOLLOW | unix.O_CLOEXEC,
+		Mode:    0,
+		Resolve: resolveFlags,
+	})
+	if err != nil {
+		return 0, err
+	}
+	return FS(subFD), nil
+}
diff --git a/internal/fixperms/fdfs/fdfs_test.go b/internal/fixperms/fdfs/fdfs_test.go
new file mode 100644
index 000000000..ccb61cc34
--- /dev/null
+++ b/internal/fixperms/fdfs/fdfs_test.go
@@ -0,0 +1,61 @@
+//go:build linux
+
+package fdfs
+
+import (
+	"io/fs"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestTOCTOUShenanigans(t *testing.T) {
+	path := "/tmp/TestTOCTOUShenanigans/foo"
+	if err := os.MkdirAll(path, 0o777); err != nil {
+		t.Fatalf("os.MkdirAll(%s, %o) = %v", path, 0o777, err)
+	}
+	fp := filepath.Join(path, "data")
+	if err := os.WriteFile(fp, []byte("innocent"), 0o666); err != nil {
+		t.Fatalf("os.WriteFile(%s, nil, 0o666) = %v", fp, err)
+	}
+
+	path2 := "/tmp/TestTOCTOUShenanigans/crimes"
+	if err := os.MkdirAll(path2, 0o777); err != nil {
+		t.Fatalf("os.MkdirAll(%s, %o) = %v", path2, 0o777, err)
+	}
+	fp2 := filepath.Join(path2, "data")
+	if err := os.WriteFile(fp2, []byte("guilty"), 0o666); err != nil {
+		t.Fatalf("os.WriteFile(%s, nil, 0o666) = %v", fp2, err)
+	}
+
+	// Do it in two steps, to simulate a trusted directory and an untrusted
+	// subpath.
+	fsys, err := DirFS("/tmp/TestTOCTOUShenanigans")
+	if err != nil {
+		t.Fatalf("DirFS(/tmp/TestTOCTOUShenanigans) error = %v", err)
+	}
+	defer fsys.Close()
+	fooFS, err := fsys.Sub("foo")
+	if err != nil {
+		t.Fatalf("DirFS(/tmp/TestTOCTOUShenanigans).Sub(foo) error = %v", err)
+	}
+	defer fooFS.Close()
+
+	// Replace foo with a symlink to crimes...
+	path3 := "/tmp/TestTOCTOUShenanigans/foo.bak"
+	if err := os.Rename(path, path3); err != nil {
+		t.Fatalf("os.Rename(%s, %s) = %v", path, path3, err)
+	}
+	if err := os.Symlink(path2, path); err != nil {
+		t.Fatalf("os.Symlink(%s, %s) = %v", path2, path, err)
+	}
+
+	// What do we get?
+	df, err := fs.ReadFile(fooFS, "data")
+	if err != nil {
+		t.Fatalf("fs.ReadFile(DirFS(%s), data) error = %v", path, err)
+	}
+	if got, want := string(df), "innocent"; got != want {
+		t.Fatalf("fs.ReadFile(DirFS(%s), data) contents = %q, want %q", path, got, want)
+	}
+}
diff --git a/internal/fixperms/fixer/fixer.go b/internal/fixperms/fixer/fixer.go
new file mode 100644
index 000000000..49a688ec2
--- /dev/null
+++ b/internal/fixperms/fixer/fixer.go
@@ -0,0 +1,85 @@
+//go:buid linux
+
+package fixer
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"os/user"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	"github.com/buildkite/elastic-ci-stack-for-aws/v6/internal/fixperms/fdfs"
+)
+
+// Main contains the higher-level operations of the permissions fixer.
+func Main(argv []string, baseDir, uname string) (string, int) {
+	if len(argv) != 4 {
+		return exitf(1, "Usage: %s AGENT_DIR ORG_DIR PIPELINE_DIR", argv[0])
+	}
+	for _, seg := range argv[1:] {
+		if seg != filepath.Clean(seg) {
+			return exitf(2, "Invalid argument %q", seg)
+		}
+		if seg == "." || seg == ".." || strings.ContainsRune(seg, '/') {
+			return exitf(2, "Invalid argument %q", seg)
+		}
+	}
+	subpath := filepath.Join(argv[1:]...)
+
+	// Get a file descriptor for the base builds directory.
+	bd, err := fdfs.DirFS(baseDir)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return exit0()
+		}
+		return exitf(3, "Couldn't open %s: %v", baseDir, err)
+	}
+	defer bd.Close()
+
+	// Get a file descriptor for the agentdir/orgdir/pipelinedir within the
+	// builds directory.
+	// openat2(2) flags ensures this is within the builds directory, and does
+	// not involve a symlink.
+	pd, err := bd.Sub(subpath)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			return exit0()
+		}
+		return exitf(3, "Couldn't open %s: %v", subpath, err)
+	}
+	defer pd.Close()
+
+	// Get the uid and gid of buildkite-agent
+	agentUser, err := user.Lookup(uname)
+	if err != nil {
+		return exitf(4, "Couldn't look up buildkite-agent user: %v", err)
+	}
+	uid, err := strconv.Atoi(agentUser.Uid)
+	if err != nil {
+		return exitf(4, "buildkite-agent uid %q not an integer: %v", agentUser.Uid, err)
+	}
+	gid, err := strconv.Atoi(agentUser.Gid)
+	if err != nil {
+		return exitf(4, "buildkite-agent gid %q not an integer: %v", agentUser.Gid, err)
+	}
+
+	// fs.WalkDir to find everything within the directory.
+	// fchownat(2) to change the owner of the item.
+	// We allow symlinks here, but operate on the symlinks themselves.
+	if err := fs.WalkDir(pd, ".", func(path string, d fs.DirEntry, err error) error {
+		return pd.Lchown(path, uid, gid)
+	}); err != nil {
+		return exitf(5, "Couldn't recursively chown %s: %v", subpath, err)
+	}
+
+	return exit0()
+}
+
+func exit0() (string, int) { return "", 0 }
+
+func exitf(code int, f string, v ...any) (string, int) {
+	return fmt.Sprintf(f, v...), code
+}
diff --git a/internal/fixperms/fixer/fixer_test.go b/internal/fixperms/fixer/fixer_test.go
new file mode 100644
index 000000000..838c5e7c9
--- /dev/null
+++ b/internal/fixperms/fixer/fixer_test.go
@@ -0,0 +1,138 @@
+//go:build linux
+
+package fixer
+
+import (
+	"fmt"
+	"io/fs"
+	"os"
+	"os/exec"
+	"syscall"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestFixer_SlashesErrors(t *testing.T) {
+	tests := [][]string{
+		{"os.Args[0]", "/", "abc", "abc"},
+		{"os.Args[0]", "abc/", "abc", "abc"},
+		{"os.Args[0]", "/abc", "abc", "abc"},
+		{"os.Args[0]", "abc/def", "abc", "abc"},
+		{"os.Args[0]", "abc/def/ghi", "abc", "abc"},
+		{"os.Args[0]", "/abc/", "abc", "abc"},
+		{"os.Args[0]", "abc", "/", "abc"},
+		{"os.Args[0]", "abc", "abc/", "abc"},
+		{"os.Args[0]", "abc", "/abc", "abc"},
+		{"os.Args[0]", "abc", "abc/def", "abc"},
+		{"os.Args[0]", "abc", "abc/def/ghi", "abc"},
+		{"os.Args[0]", "abc", "/abc/", "abc"},
+		{"os.Args[0]", "abc", "abc", "/"},
+		{"os.Args[0]", "abc", "abc", "abc/"},
+		{"os.Args[0]", "abc", "abc", "/abc"},
+		{"os.Args[0]", "abc", "abc", "abc/def"},
+		{"os.Args[0]", "abc", "abc", "abc/def/ghi"},
+		{"os.Args[0]", "abc", "abc", "/abc/"},
+	}
+	for _, test := range tests {
+		_, code := Main(test, "/code/internal/fixperms/fixtures", "root")
+		if got, want := code, 2; got != want {
+			t.Errorf("Main(%v) code = %d, want %d", test, got, want)
+		}
+	}
+}
+
+func TestFixer_DotsErrors(t *testing.T) {
+	tests := [][]string{
+		{"os.Args[0]", ".", "abc", "abc"},
+		{"os.Args[0]", "..", "abc", "abc"},
+		{"os.Args[0]", "abc", ".", "abc"},
+		{"os.Args[0]", "abc", "..", "abc"},
+		{"os.Args[0]", "abc", "abc", "."},
+		{"os.Args[0]", "abc", "abc", ".."},
+	}
+	for _, test := range tests {
+		_, code := Main(test, "/code/internal/fixperms/fixtures", "root")
+		if got, want := code, 2; got != want {
+			t.Errorf("Main(%v) code = %d, want %d", test, got, want)
+		}
+	}
+}
+
+func TestFixer_SymlinksErrors(t *testing.T) {
+	tests := [][]string{
+		{"os.Args[0]", "link", "b", "c"},
+		{"os.Args[0]", "a", "link", "c"},
+		{"os.Args[0]", "a", "b", "link"},
+	}
+	for _, test := range tests {
+		_, code := Main(test, "/code/internal/fixperms/fixtures", "root")
+		if got, want := code, 3; got != want {
+			t.Errorf("Main(%v) code = %d, want %d", test, got, want)
+		}
+	}
+}
+
+func TestFixer_NonDirectoryErrors(t *testing.T) {
+	argv := []string{"os.Args[0]", "d", "e", "f"}
+	_, code := Main(argv, "/code/internal/fixperms/fixtures", "root")
+	if got, want := code, 3; got != want {
+		t.Errorf("Main(%v) code = %d, want %d", argv, got, want)
+	}
+}
+
+func TestFixer_NonExistSkips(t *testing.T) {
+	argv := []string{"os.Args[0]", "g", "h", "i"}
+	_, code := Main(argv, "/code/internal/fixperms/fixtures", "root")
+	if got, want := code, 0; got != want {
+		t.Errorf("Main(%v) code = %d, want %d", argv, got, want)
+	}
+}
+
+func TestFixer_Fixes(t *testing.T) {
+	if err := exec.Command("/usr/bin/cp", "-r", "/code/internal/fixperms/fixtures/a", "/tmp").Run(); err != nil {
+		t.Fatalf("cp -r fixtures/a /tmp: %v", err)
+	}
+
+	argv := []string{"os.Args[0]", "a", "b", "c"}
+	_, code := Main(argv, "/tmp", "nobody")
+	if got, want := code, 0; got != want {
+		t.Errorf("Main(%v) code = %d, want %d", argv, got, want)
+	}
+
+	var gotFiles []string
+	wantFiles := []string{
+		".",
+		"d",
+		"d/e",
+		"d/link",
+		"link",
+	}
+
+	if err := fs.WalkDir(os.DirFS("/tmp/a/b/c"), ".", func(path string, d fs.DirEntry, err error) error {
+		gotFiles = append(gotFiles, path)
+
+		fi, err := d.Info()
+		if err != nil {
+			return err
+		}
+		st, ok := fi.Sys().(*syscall.Stat_t)
+		if !ok {
+			return fmt.Errorf("file info for %s not a *syscall.Stat_t: %T", path, fi.Sys())
+		}
+		if st.Uid != 65534 {
+			t.Errorf("uid of %s = %d, want 65534", path, st.Uid)
+		}
+		if st.Gid != 65534 {
+			t.Errorf("gid of %s = %d, want 65534", path, st.Gid)
+		}
+		return nil
+
+	}); err != nil {
+		t.Errorf("fs.WalkDir(/tmp/a/b/c, .) = %v", err)
+	}
+
+	if diff := cmp.Diff(gotFiles, wantFiles); diff != "" {
+		t.Errorf("walked files diff (-got +want):\n%s", diff)
+	}
+}
diff --git a/internal/fixperms/fixperms.go b/internal/fixperms/fixperms.go
new file mode 100644
index 000000000..495adc17e
--- /dev/null
+++ b/internal/fixperms/fixperms.go
@@ -0,0 +1,63 @@
+//go:build linux
+
+// The fixperms tool changes the ownership of certain files to buildkite-agent.
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/buildkite/elastic-ci-stack-for-aws/v6/internal/fixperms/fixer"
+)
+
+// Files that are created by Docker containers end up with strange user and
+// group ids, usually 0 (root). Docker namespacing will one day save us, but it
+// can only map a single docker user id to a given user id (not any docker user
+// id to a single system user id).
+//
+// Until we can map any old user id back to buildkite-agent automatically with
+// Docker, then we just need to fix the permissions manually before each build
+// runs so git clean can work as expected.
+//
+// In order to fix ownership of files owned by root, we need to be root. Thus,
+// buildkite-agent has rights to run this program with sudo (see sudoers.conf).
+// That means we have to take extra care to not chown things we shouldn't.
+//
+// Q1: Why not `chown -Rh /var/lib/buildkite-agents/builds`?
+// A1: That gets slower as more and more builds are run on this agent, hence the
+//     args that specify a particular pipeline dir. See #340.
+//
+// Q2: Why not a small script that checks the args for shenanigans, then runs
+//     `chown -Rh ...`?
+// A2: Because of TOCTOU. There's a race between checking that there are no
+//     symlink shenanigans, and calling `chown`, which provides time for an
+//     attacker to put some shenanigans back in before `chown` is called.
+//
+// Q3: What about running `chown -Rh` in a chroot that also contains the dir?
+// A3: You have to copy the tools you need to run into the chroot. A job could
+//     overwrite the tool with its own binary, which is then run as root.
+//     And if you think you can carefully lay out the chroot and set permissions
+//     to prevent that, you still need to stop the script receiving a symlink to
+//     the directory containing chown, changing its own perms. If you add a
+//     check for that first, then there's still TOCTOU.
+//
+// Q4: Containers!
+// A4: *sigh*
+//
+// File paths are not a good security interface for files. But! We can use file
+// descriptors. openat(2), fchownat(2), etc provide a way to resolve file paths
+// relative to a given parent directory, and prevent symlink resolution at the
+// same time.
+
+const (
+	buildsDir = "/var/lib/buildkite-agent/builds"
+	username  = "buildkite-agent"
+)
+
+func main() {
+	msg, code := fixer.Main(os.Args, buildsDir, username)
+	if code != 0 {
+		fmt.Fprintln(os.Stderr, msg)
+		os.Exit(code)
+	}
+}
diff --git a/unit-tests/fixtures/a/b/c/.gitkeep b/internal/fixperms/fixtures/a/b/c/d/e
similarity index 100%
rename from unit-tests/fixtures/a/b/c/.gitkeep
rename to internal/fixperms/fixtures/a/b/c/d/e
diff --git a/internal/fixperms/fixtures/a/b/c/d/link b/internal/fixperms/fixtures/a/b/c/d/link
new file mode 120000
index 000000000..9cbe6ea56
--- /dev/null
+++ b/internal/fixperms/fixtures/a/b/c/d/link
@@ -0,0 +1 @@
+e
\ No newline at end of file
diff --git a/internal/fixperms/fixtures/a/b/c/link b/internal/fixperms/fixtures/a/b/c/link
new file mode 120000
index 000000000..c59d9b634
--- /dev/null
+++ b/internal/fixperms/fixtures/a/b/c/link
@@ -0,0 +1 @@
+d
\ No newline at end of file
diff --git a/unit-tests/fixtures/a/b/link b/internal/fixperms/fixtures/a/b/link
similarity index 100%
rename from unit-tests/fixtures/a/b/link
rename to internal/fixperms/fixtures/a/b/link
diff --git a/unit-tests/fixtures/a/link b/internal/fixperms/fixtures/a/link
similarity index 100%
rename from unit-tests/fixtures/a/link
rename to internal/fixperms/fixtures/a/link
diff --git a/unit-tests/fixtures/d/e/f b/internal/fixperms/fixtures/d/e/f
similarity index 100%
rename from unit-tests/fixtures/d/e/f
rename to internal/fixperms/fixtures/d/e/f
diff --git a/unit-tests/fixtures/g/.gitkeep b/internal/fixperms/fixtures/g/.gitkeep
similarity index 100%
rename from unit-tests/fixtures/g/.gitkeep
rename to internal/fixperms/fixtures/g/.gitkeep
diff --git a/unit-tests/fixtures/link b/internal/fixperms/fixtures/link
similarity index 100%
rename from unit-tests/fixtures/link
rename to internal/fixperms/fixtures/link
diff --git a/packer/linux/conf/buildkite-agent/scripts/fix-buildkite-agent-builds-permissions b/packer/linux/conf/buildkite-agent/scripts/fix-buildkite-agent-builds-permissions
deleted file mode 100755
index ae6ab8885..000000000
--- a/packer/linux/conf/buildkite-agent/scripts/fix-buildkite-agent-builds-permissions
+++ /dev/null
@@ -1,100 +0,0 @@
-#!/usr/bin/env bash
-
-# To run the unit tests for this file, run the following command in the root of
-# the project:
-# $ docker-compose -f docker-compose.unit-tests.yml run unit-tests
-
-# Files that are created by Docker containers end up with strange user and
-# group ids, usually 0 (root). Docker namespacing will one day save us, but it
-# can only map a single docker user id to a given user id (not any docker user
-# id to a single system user id).
-#
-# Until we can map any old user id back to
-# buildkite-agent automatically with Docker, then we just need to fix the
-# permissions manually before each build runs so git clean can work as
-# expected.
-
-set -eu -o pipefail
-
-# We need to scope the next bit to only the currently running agent dir and
-# pipeline, but we also need to control security and make sure arbitrary folders
-# can't be chmoded.
-#
-# We prepare the agent build directory basename in the environment hook and pass
-# it as the first argument, org name as second argument, and the pipeline dir as
-# the third.
-#
-# In here we need to check that they both don't contain slashes or contain a
-# traversal component.
-
-AGENT_DIR="$1"
-# => "my-agent-1"
-
-ORG_DIR="$2"
-# => "my-org"
-
-PIPELINE_DIR="$3"
-# => "my-pipeline"
-
-# Make sure it doesn't contain any slashes by substituting slashes with nothing
-# and making sure it doesn't change
-function exit_if_contains_slashes() {
-	if [[ "${1//\//}" != "${1}" ]]; then
-		exit 1
-	fi
-}
-
-function exit_if_contains_traversal() {
-	if [[ "${1}" == "." || "${1}" == ".." ]]; then
-		exit 2
-	fi
-}
-
-function exit_if_blank() {
-	if [[ -z "${1}" ]]; then
-		exit 3
-	fi
-}
-
-# Check them for slashes
-exit_if_contains_slashes "${AGENT_DIR}"
-exit_if_contains_slashes "${ORG_DIR}"
-exit_if_contains_slashes "${PIPELINE_DIR}"
-
-# Check them for traversals
-exit_if_contains_traversal "${AGENT_DIR}"
-exit_if_contains_traversal "${ORG_DIR}"
-exit_if_contains_traversal "${PIPELINE_DIR}"
-
-# Check them for blank values
-exit_if_blank "${AGENT_DIR}"
-exit_if_blank "${ORG_DIR}"
-exit_if_blank "${PIPELINE_DIR}"
-
-# We know the builds path:
-BUILDS_PATH="/var/lib/buildkite-agent/builds"
-
-# And now we can reconstruct the full agent builds path:
-PIPELINE_PATH="${BUILDS_PATH}/${AGENT_DIR}/${ORG_DIR}/${PIPELINE_DIR}"
-# => "/var/lib/buildkite-agent/builds/my-agent-1/my-org/my-pipeline"
-
-# If it doesn't exist, then we won't do anything.
-if [[ ! -e "${PIPELINE_PATH}" ]]; then
-	exit 0
-fi
-
-
-# Check for symlink shenanigans
-if [[ "$(realpath "${PIPELINE_PATH}")" != "${PIPELINE_PATH}" ]]; then
-	exit 4
-fi
-
-# It should be a directory.
-if [[ ! -d "${PIPELINE_PATH}" ]]; then
-	exit 5
-fi
-
-# If we make it here, we're safe to go!
-
-/bin/chown -R buildkite-agent:buildkite-agent "${PIPELINE_PATH}"
-
diff --git a/unit-tests/fix-buildkite-agent-builds-permissions.bats b/unit-tests/fix-buildkite-agent-builds-permissions.bats
deleted file mode 100644
index e7bde924f..000000000
--- a/unit-tests/fix-buildkite-agent-builds-permissions.bats
+++ /dev/null
@@ -1,162 +0,0 @@
-#!/usr/bin/env bats
-
-FIX_PERMISSIONS_SCRIPT="/code/packer/linux/conf/buildkite-agent/scripts/fix-buildkite-agent-builds-permissions"
-
-@test "Slashes in the agent arg cause an exit 1 (A)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "/" "abc" "abc"
-	[ "$status" -eq 1 ]
-}
-
-@test "Slashes in the agent arg cause an exit 1 (B)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc/" "abc" "abc"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the agent arg cause an exit 1 (C)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "/abc" "abc" "abc"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the agent arg cause an exit 1 (D)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc/def" "abc" "abc"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the agent arg cause an exit 1 (E)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc/def/ghi" "abc" "abc"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the agent arg cause an exit 1 (F)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "/abc/" "abc" "abc"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the org arg cause an exit 1 (A)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "/" "abc"
-	[ "$status" -eq 1 ]
-}
-
-@test "Slashes in the org arg cause an exit 1 (B)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc/" "abc" "abc"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the org arg cause an exit 1 (C)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "/abc" "abc"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the org arg cause an exit 1 (D)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "abc/def" "abc"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the org arg cause an exit 1 (E)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "abc/def/ghi" "abc"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the org arg cause an exit 1 (F)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "/abc/" "abc"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the pipeline arg cause an exit 1 (A)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "abc" "/"
-	[ "$status" -eq 1 ]
-}
-
-@test "Slashes in the pipeline arg cause an exit 1 (B)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "abc" "abc/"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the pipeline arg cause an exit 1 (C)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "abc" "/abc"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the pipeline arg cause an exit 1 (D)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "abc" "abc/def"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the pipeline arg cause an exit 1 (E)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "abc" "abc/def/ghi"
-  [ "$status" -eq 1 ]
-}
-
-@test "Slashes in the pipeline arg cause an exit 1 (F)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "abc" "/abc/"
-  [ "$status" -eq 1 ]
-}
-
-@test "Single dot traversal in the agent arg cause an exit 2" {
-  run "$FIX_PERMISSIONS_SCRIPT" "." "abc" "abc"
-  [ "$status" -eq 2 ]
-}
-
-@test "Double dot traversal in the agent arg cause an exit 2" {
-  run "$FIX_PERMISSIONS_SCRIPT" ".." "abc" "abc"
-  [ "$status" -eq 2 ]
-}
-
-@test "Single dot traversal in the org arg cause an exit 2" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "." "abc"
-  [ "$status" -eq 2 ]
-}
-
-@test "Double dot traversal in the org arg cause an exit 2" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" ".." "abc"
-  [ "$status" -eq 2 ]
-}
-
-@test "Single dot traversal in the pipeline arg cause an exit 2" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "abc" "."
-  [ "$status" -eq 2 ]
-}
-
-@test "Double dot traversal in the pipeline arg cause an exit 2" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "abc" ".."
-  [ "$status" -eq 2 ]
-}
-
-@test "Blank agent arg cause an exit 3" {
-  run "$FIX_PERMISSIONS_SCRIPT" "" "abc" "abc"
-  [ "$status" -eq 3 ]
-}
-
-@test "Blank org arg cause an exit 3" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "" "abc"
-  [ "$status" -eq 3 ]
-}
-
-@test "Blank pipeline arg cause an exit 3" {
-  run "$FIX_PERMISSIONS_SCRIPT" "abc" "abc" ""
-  [ "$status" -eq 3 ]
-}
-
-@test "Non-existing path is skipped" {
-  "$FIX_PERMISSIONS_SCRIPT" "g" "h" "i"
-}
-
-@test "Symlinks in the args cause an exit 4 (A)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "link" "b" "c"
-  [ "$status" -eq 4 ]
-}
-
-@test "Symlinks in the args cause an exit 4 (B)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "a" "link" "c"
-  [ "$status" -eq 4 ]
-}
-
-@test "Symlinks in the args cause an exit 4 (C)" {
-  run "$FIX_PERMISSIONS_SCRIPT" "a" "b" "link"
-  [ "$status" -eq 4 ]
-}
-
-@test "Path not a directory causes an exit 5" {
-  run "$FIX_PERMISSIONS_SCRIPT" "d" "e" "f"
-  [ "$status" -eq 5 ]
-}