Add a test for #41723

Capabilities are serialised in VFS_CAP_REVISION_3 when an image is
built in a user-namespaced daemon, instead of VFS_CAP_REVISION_2.

This adds a test for this, though it's currently wired to fail if
the capabilities are serialised in VFS_CAP_REVISION_2 instead in this
situation, since this is unexpected.

Signed-off-by: Eric Mountain <eric.mountain@datadoghq.com>

integration/build/build_userns_linux_test.go

@@ -0,0 +1,140 @@
+package build // import "github.com/docker/docker/integration/build"
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"io"
+	"io/ioutil"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/docker/docker/api/types"
+	"github.com/docker/docker/integration/internal/container"
+	"github.com/docker/docker/pkg/stdcopy"
+	"github.com/docker/docker/testutil/daemon"
+	"github.com/docker/docker/testutil/fakecontext"
+	"gotest.tools/v3/assert"
+	"gotest.tools/v3/skip"
+)
+
+// Implements a test for https://github.com/moby/moby/issues/41723
+// Images built in a user-namespaced daemon should have capabilities serialised in
+// VFS_CAP_REVISION_2 (no user-namespace root uid) format rather than V3 (that includes
+// the root uid).
+func TestBuildUserNamespaceValidateCapabilitiesAreV2(t *testing.T) {
+	skip.If(t, testEnv.DaemonInfo.OSType != "linux")
+	skip.If(t, testEnv.IsRemoteDaemon())
+	skip.If(t, !testEnv.IsUserNamespaceInKernel())
+	skip.If(t, testEnv.IsRootless())
+
+	const imageTag = "capabilities:1.0"
+
+	tmp, err := ioutil.TempDir("", "integration-")
+	assert.NilError(t, err)
+	defer os.RemoveAll(tmp)
+
+	dUserRemap := daemon.New(t)
+	dUserRemap.StartWithBusybox(t, "--userns-remap", "default")
+	dUserRemapRunning := true
+	defer func() {
+		if dUserRemapRunning {
+			dUserRemap.Stop(t)
+		}
+	}()
+
+	dockerfile := `
+		FROM debian:bullseye
+		RUN setcap CAP_NET_BIND_SERVICE=+eip /bin/sleep
+	`
+
+	ctx := context.Background()
+	source := fakecontext.New(t, "", fakecontext.WithDockerfile(dockerfile))
+	defer source.Close()
+
+	clientUserRemap := dUserRemap.NewClientT(t)
+	resp, err := clientUserRemap.ImageBuild(ctx,
+		source.AsTarReader(t),
+		types.ImageBuildOptions{
+			Tags: []string{imageTag},
+		})
+	assert.NilError(t, err)
+	defer resp.Body.Close()
+	buf := make([]byte, 1024)
+	for {
+		n, err := resp.Body.Read(buf)
+		if err != nil && err != io.EOF {
+			t.Fatalf("Error reading ImageBuild response: %v", err)
+			break
+		}
+		if n == 0 {
+			break
+		}
+	}
+
+	reader, err := clientUserRemap.ImageSave(ctx, []string{imageTag})
+	assert.NilError(t, err, "failed to download capabilities image")
+	defer reader.Close()
+
+	tar, err := os.Create(tmp + "/image.tar")
+	assert.NilError(t, err, "failed to create image tar file")
+	defer tar.Close()
+
+	_, err = io.Copy(tar, reader)
+	assert.NilError(t, err, "failed to write image tar file")
+
+	dUserRemap.Stop(t)
+	dUserRemap.Cleanup(t)
+	dUserRemapRunning = false
+
+	dNoUserRemap := daemon.New(t)
+	dNoUserRemap.StartWithBusybox(t)
+	defer dNoUserRemap.Stop(t)
+
+	clientNoUserRemap := dNoUserRemap.NewClientT(t)
+
+	tarFile, err := os.Open(tmp + "/image.tar")
+	assert.NilError(t, err, "failed to open image tar file")
+
+	tarReader := bufio.NewReader(tarFile)
+	loadResp, err := clientNoUserRemap.ImageLoad(ctx, tarReader, false)
+	assert.NilError(t, err, "failed to load image tar file")
+	defer loadResp.Body.Close()
+	for {
+		n, err := loadResp.Body.Read(buf)
+		if err != nil && err != io.EOF {
+			t.Fatalf("Error reading ImageLoad response: %v", err)
+			break
+		}
+		if n == 0 {
+			break
+		}
+	}
+
+	cid := container.Run(ctx, t, clientNoUserRemap,
+		container.WithImage(imageTag),
+		container.WithCmd("/sbin/getcap", "-n", "/bin/sleep"),
+	)
+	logReader, err := clientNoUserRemap.ContainerLogs(ctx, cid, types.ContainerLogsOptions{
+		ShowStdout: true,
+	})
+	assert.NilError(t, err)
+
+	actualStdout := new(bytes.Buffer)
+	actualStderr := ioutil.Discard
+	_, err = stdcopy.StdCopy(actualStdout, actualStderr, logReader)
+	assert.NilError(t, err)
+	if strings.TrimSpace(actualStdout.String()) != "/bin/sleep cap_net_bind_service=eip" {
+		// Activate when fix is merged: https://github.com/moby/moby/pull/41724
+		//t.Fatalf("run produced invalid output: %q, expected %q", actualStdout.String(), "/bin/sleep cap_net_bind_service=eip")
+		// t.Logf("run produced invalid output (expected until #41724 merges): %q, expected %q",
+		// 	actualStdout.String(),
+		// 	"/bin/sleep cap_net_bind_service=eip")
+	} else {
+		// Shouldn't happen until fix is merged: https://github.com/moby/moby/pull/41724
+		t.Fatalf("run produced valid output (unexpected until #41724 merges): %q, expected %q",
+			actualStdout.String(),
+			"/bin/sleep cap_net_bind_service=eip")
+	}
+}