daemon/command: disable c8d snapshotter when userns remapping enabled

Buildkit fails when userns remapping is enabled and c8d snapshotter is
used. As a temporary workaround, disable c8d snapshotter when userns
remapping is enabled. This will need a proper fix in the future.

Signed-off-by: Albin Kerouanton <albinker@gmail.com>

daemon/command/daemon_linux.go

@@ -1,7 +1,10 @@
 package command
 
 import (
+	"context"
+
 	cdcgroups "github.com/containerd/cgroups/v3"
+	"github.com/containerd/log"
 	systemdDaemon "github.com/coreos/go-systemd/v22/daemon"
 	"github.com/moby/moby/v2/daemon"
 	"github.com/moby/moby/v2/daemon/config"
@@ -22,6 +25,20 @@ func setPlatformOptions(conf *config.Config) error {
 	conf.ContainerdNamespace = containerdNamespace
 	conf.ContainerdPluginNamespace = containerdPluginNamespace
 
+	// Buildkit breaks when userns remapping is enabled and containerd snapshotter is used. As a temporary workaround,
+	// if containerd snapshotter is explicitly enabled, and userns remapping is enabled too, return an error. If userns
+	// remapping is enabled, but containerd-snapshotter is enabled by default, disable it. See https://github.com/moby/moby/issues/47377.
+	enabled := conf.Features["containerd-snapshotter"]
+	if enabled {
+		return errors.New("containerd-snapshotter is explicitly enabled, but is not compatible with userns remapping. Please disable userns remapping or containerd-snapshotter")
+	}
+
+	log.G(context.TODO()).Warn("userns remapping enabled, disabling containerd snapshotter")
+	if conf.Features == nil {
+		conf.Features = make(map[string]bool)
+	}
+	conf.Features["containerd-snapshotter"] = false
+
 	return nil
 }
 

daemon/command/daemon_linux_test.go

@@ -8,6 +8,7 @@ import (
 	"strconv"
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
 	"github.com/moby/moby/v2/daemon/config"
 	"github.com/moby/sys/reexec"
 	"golang.org/x/sys/unix"
@@ -86,3 +87,67 @@ func TestLoadListenerNoAddr(t *testing.T) {
 	assert.NilError(t, json.NewDecoder(stdout).Decode(&resp))
 	assert.Equal(t, resp.Err, "")
 }
+
+func TestC8dSnapshotterWithUsernsRemap(t *testing.T) {
+	testcases := []struct {
+		name   string
+		cfg    *config.Config
+		expCfg *config.Config
+		expErr string
+	}{
+		{
+			name:   "no remap, no snapshotter",
+			cfg:    &config.Config{},
+			expCfg: &config.Config{},
+		},
+		{
+			name: "userns remap, no explicit containerd-snapshotter feature",
+			cfg:  &config.Config{RemappedRoot: "default"},
+			expCfg: &config.Config{
+				RemappedRoot: "dockremap:dockremap",
+				CommonConfig: config.CommonConfig{
+					ContainerdNamespace:       "-100000.100000",
+					ContainerdPluginNamespace: "-100000.100000",
+					Features:                  map[string]bool{"containerd-snapshotter": false},
+				},
+			},
+		},
+		{
+			name: "userns remap, explicit containerd-snapshotter feature",
+			cfg: &config.Config{
+				RemappedRoot: "default",
+				CommonConfig: config.CommonConfig{Features: map[string]bool{"containerd-snapshotter": true}},
+			},
+			expCfg: &config.Config{
+				RemappedRoot: "dockremap:dockremap",
+				CommonConfig: config.CommonConfig{
+					ContainerdNamespace:       "-100000.100000",
+					ContainerdPluginNamespace: "-100000.100000",
+					Features:                  map[string]bool{"containerd-snapshotter": true},
+				},
+			},
+			expErr: "containerd-snapshotter is explicitly enabled, but is not compatible with userns remapping. Please disable userns remapping or containerd-snapshotter",
+		},
+		{
+			name: "no remap, explicit containerd-snapshotter feature",
+			cfg: &config.Config{
+				CommonConfig: config.CommonConfig{Features: map[string]bool{"containerd-snapshotter": true}},
+			},
+			expCfg: &config.Config{
+				CommonConfig: config.CommonConfig{Features: map[string]bool{"containerd-snapshotter": true}},
+			},
+		},
+	}
+
+	for _, tc := range testcases {
+		t.Run(tc.name, func(t *testing.T) {
+			err := setPlatformOptions(tc.cfg)
+			assert.DeepEqual(t, tc.expCfg, tc.cfg, cmp.AllowUnexported(config.DefaultBridgeConfig{}))
+			if tc.expErr != "" {
+				assert.Equal(t, tc.expErr, err.Error())
+			} else {
+				assert.NilError(t, err)
+			}
+		})
+	}
+}

integration-cli/docker_cli_userns_test.go

@@ -26,7 +26,7 @@ func (s *DockerDaemonSuite) TestDaemonUserNamespaceRootSetting(c *testing.T) {
 	testRequires(c, UserNamespaceInKernel)
 
 	ctx := testutil.GetContext(c)
-	s.d.StartWithBusybox(ctx, c, "--userns-remap", "default")
+	s.d.StartWithBusybox(ctx, c, "--userns-remap", "default", "--storage-driver", "vfs")
 
 	out, err := s.d.Cmd("run", "busybox", "stat", "-c", "%u:%g", "/bin/cat")
 	assert.Check(c, err)

integration/build/build_userns_linux_test.go

@@ -38,8 +38,8 @@ func TestBuildUserNamespaceValidateCapabilitiesAreV2(t *testing.T) {
 
 	tmpDir := t.TempDir()
 
-	dUserRemap := daemon.New(t)
-	dUserRemap.Start(t, "--userns-remap", "default")
+	dUserRemap := daemon.New(t, daemon.WithUserNsRemap("default"))
+	dUserRemap.Start(t)
 	clientUserRemap := dUserRemap.NewClientT(t)
 	defer clientUserRemap.Close()
 

internal/testutil/daemon/ops.go

@@ -2,6 +2,7 @@ package daemon
 
 import (
 	"net/netip"
+	"os"
 	"os/user"
 
 	"github.com/moby/moby/v2/internal/testutil/environment"
@@ -23,6 +24,21 @@ func WithContainerdSocket(socket string) Option {
 func WithUserNsRemap(remap string) Option {
 	return func(d *Daemon) {
 		d.usernsRemap = remap
+		// The dind container used by the CI has the env var DOCKER_GRAPHDRIVER
+		// set to 'overlayfs' which is a valid driver when the containerd image
+		// store is used, but not a valid graphdriver. OTOH the test daemon
+		// started by this package uses DOCKER_GRAPHDRIVER to set the storage
+		// backend. However, the daemon doesn't enable the containerd image
+		// store automatically when userns remapping is enabled, so using the
+		// storage driver set through DOCKER_GRAPHDRIVER will cause the daemon
+		// to fail to start. This should be removed once a proper fix for [1]
+		// is implemented.
+		//
+		// [1]: https://github.com/moby/moby/issues/47377
+		d.storageDriver = ""
+		if storage := os.Getenv("DOCKER_GRAPHDRIVER"); storage == "overlayfs" {
+			d.storageDriver = "overlay2"
+		}
 	}
 }