fuse: automatically use squashfuse for images, deprecate --sif-fuse

Deprecate the explicit `--sif-fuse` flag and `sif fuse` directive for
`singularity.conf`. These were previously used to enable experimental
FUSE mount of SIF/SquashFS containers.

Modify image handling so that we now try squashfuse mounts
automatically, with fall back to temporary sandbox extraction, when:

* squashfs kernel mounts have been disabled in `singularity.conf`
* we are running in a non-setuid / user namespace flow.

Fixes sylabs#2216

CHANGELOG.md

@@ -2,6 +2,14 @@
 
 ## Changes Since Last Release
 
+### Changed defaults / behaviours
+
+- In native mode, SIF/SquashFS container images will now be mounted with
+  squashfuse when kernel mounts are disabled in `singularity.conf`, or cannot be
+  used (non-setuid / user namespace workflow). If the FUSE mount fails,
+  Singularity will fall back to extracting the container to a temporary sandbox
+  in order to run it.
+
 ### New Features & Functionality
 
 - The `registry login` and `registry logout` commands now support a `--authfile
@@ -27,10 +35,15 @@
   executable, then the `run` / `exec` / `shell` commands in `--oci` mode can be
   given the `--app <appname>` flag, and will automatically invoke the relevant
   SCIF command.
-- SIF/SquashFS container images can now be mounted using FUSE in all native mode
-  flows, including setuid mode. To enable, use the `--sif-fuse` flag, or set
-  `sif fuse = yes` in `singularity.conf`. Overlay partitions and extfs images
-  are not yet supported.
+
+### Deprecated Functionality
+
+- The experimental `--sif-fuse` flag, and `sif fuse` directive in
+  `singularity.conf` are deprecated. The flag and directive were used to enable
+  experimental mounting of SIF/SquashFS container images with FUSE in prior
+  versions of Singularity. From 4.1, FUSE mounts are used automatically when
+  kernel mounts are disabled / not available.
+
 
 ### Bug Fixes
 

cmd/internal/cli/action_flags.go

@@ -766,6 +766,7 @@ var actionSIFFUSEFlag = cmdline.Flag{
 	Name:         "sif-fuse",
 	Usage:        "attempt FUSE mount of SIF",
 	EnvKeys:      []string{"SIF_FUSE"},
+	Deprecated:   "FUSE mounts are now used automatically when kernel mounts are disabled / unavailable.",
 }
 
 // --proot (hidden)

e2e/actions/actions.go

@@ -2507,7 +2507,9 @@ func (c actionTests) actionCompat(t *testing.T) {
 	}
 }
 
-// actionSquashfuse tests that squashfuse SIF mount works.
+// actionSquashfuse tests that squashfuse SIF mount works. Currently forced here
+// via deprecated `--sif-fuse` flag as this is convenient to include non-userns
+// profiles without changing global config.
 func (c actionTests) actionSIFFUSE(t *testing.T) {
 	require.Command(t, "squashfuse")
 	require.Command(t, "fusermount")
@@ -2536,12 +2538,12 @@ func (c actionTests) actionSIFFUSE(t *testing.T) {
 	}
 }
 
-// Verify that the FUSE mounts, and the CleanupHost() process are not seen when
+// Verify that the FUSE mounts, and the PostStartHost/CleanupHost() processes are not seen when
 // --sif-fuse should not be in effect.
 func (c actionTests) actionNoSIFFUSE(t *testing.T) {
 	e2e.EnsureImage(t, c.env)
 
-	for _, p := range e2e.NativeProfiles {
+	for _, p := range []e2e.Profile{e2e.RootProfile, e2e.UserProfile} {
 		c.env.RunSingularity(
 			t,
 			e2e.AsSubtest(p.String()),
@@ -2552,6 +2554,7 @@ func (c actionTests) actionNoSIFFUSE(t *testing.T) {
 			e2e.ExpectExit(
 				0,
 				e2e.ExpectError(e2e.UnwantedContainMatch, "squashfuse"),
+				e2e.ExpectError(e2e.UnwantedContainMatch, "PostStartHost()"),
 				e2e.ExpectError(e2e.UnwantedContainMatch, "CleanupHost()"),
 			),
 		)

e2e/config/config.go

@@ -533,7 +533,8 @@ func (c configTests) configGlobal(t *testing.T) {
 			profile:        e2e.UserProfile,
 			directive:      "allow kernel squashfs",
 			directiveValue: "no",
-			exit:           255,
+			exit:           0,
+			resultOp:       e2e.ExpectError(e2e.ContainMatch, "Mounting image with FUSE"),
 		},
 		{
 			name:           "AllowKernelSquashfsYes_Container",
@@ -542,6 +543,7 @@ func (c configTests) configGlobal(t *testing.T) {
 			directive:      "allow kernel squashfs",
 			directiveValue: "yes",
 			exit:           0,
+			resultOp:       e2e.ExpectError(e2e.UnwantedContainMatch, "Mounting image with FUSE"),
 		},
 		// Standalone ext3 rootfs
 		{
@@ -635,7 +637,8 @@ func (c configTests) configGlobal(t *testing.T) {
 			profile:        e2e.UserProfile,
 			directive:      "allow kernel squashfs",
 			directiveValue: "no",
-			exit:           255,
+			exit:           0,
+			resultOp:       e2e.ExpectError(e2e.ContainMatch, "Mounting image with FUSE"),
 		},
 		{
 			name:           "AllowKernelSquashfsYes_SIF",
@@ -644,6 +647,7 @@ func (c configTests) configGlobal(t *testing.T) {
 			directive:      "allow kernel squashfs",
 			directiveValue: "yes",
 			exit:           0,
+			resultOp:       e2e.ExpectError(e2e.UnwantedContainMatch, "Mounting image with FUSE"),
 		},
 		// Encrypted squashFS rootfs in SIF
 		{

internal/pkg/runtime/launcher/native/launcher_linux.go

@@ -977,9 +977,8 @@ func (l *Launcher) setCgroups(instanceName string) error {
 	return nil
 }
 
-// PrepareImage perfoms any image preparation required before execution.
-// This is currently limited to extraction or FUSE mount when using the user namespace,
-// and activating any image driver plugins that might handle the image mount.
+// PrepareImage performs any image preparation required before execution.
+// This is currently limited to extraction or FUSE mount.
 func (l *Launcher) prepareImage(c context.Context, image string) error {
 	if strings.HasPrefix(image, "instance://") {
 		return nil
@@ -1002,8 +1001,8 @@ func (l *Launcher) prepareImage(c context.Context, image string) error {
 	case imgutil.SANDBOX:
 		return nil
 	case imgutil.SQUASHFS:
-		if isUserNs || l.cfg.SIFFUSE {
-			return l.prepareSquashfs(c, img, l.cfg.SIFFUSE)
+		if !l.engineConfig.File.AllowContainerSquashfs || !isUserNs || l.cfg.SIFFUSE {
+			return l.prepareSquashfs(c, img)
 		}
 		// setuid, kernel squashfs permitted, fuse not requested - no action needed
 		return nil
@@ -1015,7 +1014,7 @@ func (l *Launcher) prepareImage(c context.Context, image string) error {
 		return nil
 	case imgutil.EXT3:
 		if isUserNs || l.cfg.SIFFUSE {
-			return l.prepareExtfs(c, img, l.cfg.SIFFUSE)
+			return l.prepareExtfs(c, img)
 		}
 		// setuid, kernel extfs permitted, fuse not requested - no action needed
 		return nil
@@ -1024,33 +1023,31 @@ func (l *Launcher) prepareImage(c context.Context, image string) error {
 	return fmt.Errorf("unsupported image rootfs type: %d", part.Type)
 }
 
-func (l *Launcher) prepareSquashfs(ctx context.Context, img *imgutil.Image, tryFuse bool) error {
+func (l *Launcher) prepareSquashfs(ctx context.Context, img *imgutil.Image) error {
 	tempDir, imageDir, err := mkContainerDirs()
 	if err != nil {
 		return err
 	}
 
-	if tryFuse {
-		allowOther := false
-		// In fakeroot mode, the users is able to assume a subuid/subgid, so allow
-		// others to access the FUSE mount.
-		if l.cfg.Fakeroot {
-			allowOther = true
-		}
-
-		sylog.Infof("Mounting image with FUSE.")
-		err = squashfuseMount(ctx, img, imageDir, allowOther)
-		if err == nil {
-			l.engineConfig.SetImage(imageDir)
-			l.engineConfig.SetImageFuse(true)
-			l.engineConfig.SetDeleteTempDir(tempDir)
-			l.generator.AddProcessEnv("SINGULARITY_CONTAINER", imageDir)
-			return nil
-		}
+	allowOther := false
+	// In fakeroot mode, the users is able to assume a subuid/subgid, so allow
+	// others to access the FUSE mount.
+	if l.cfg.Fakeroot {
+		allowOther = true
+	}
 
-		sylog.Warningf("squashfuse mount failed, falling back to extraction: %v", err)
+	sylog.Infof("Mounting image with FUSE.")
+	err = squashfuseMount(ctx, img, imageDir, allowOther)
+	if err == nil {
+		l.engineConfig.SetImage(imageDir)
+		l.engineConfig.SetImageFuse(true)
+		l.engineConfig.SetDeleteTempDir(tempDir)
+		l.generator.AddProcessEnv("SINGULARITY_CONTAINER", imageDir)
+		return nil
 	}
 
+	sylog.Warningf("squashfuse mount failed, falling back to extraction: %v", err)
+
 	if l.cfg.NoTmpSandbox || !l.engineConfig.File.TmpSandboxAllowed {
 		return fmt.Errorf("unpacking image to temporary sandbox dir required, but is prohibited by 'tmp sandbox = no' in singularity.conf or --no-tmp-sandbox command-line flag")
 	}
@@ -1069,7 +1066,7 @@ func (l *Launcher) prepareSquashfs(ctx context.Context, img *imgutil.Image, tryF
 	return fmt.Errorf("extraction failed: %v", err)
 }
 
-func (l *Launcher) prepareExtfs(_ context.Context, _ *imgutil.Image, _ bool) error {
+func (l *Launcher) prepareExtfs(_ context.Context, _ *imgutil.Image) error {
 	// TODO - Enable fuse2fs handling
 	return fmt.Errorf("extfs images can only be run in setuid mode with kernel extfs mounts enabled")
 }

pkg/util/singularityconf/config.go

@@ -329,7 +329,8 @@ allow container dir = {{ if eq .AllowContainerDir true }}yes{{ else }}no{{ end }
 # ALLOW KERNEL SQUASHFS: [BOOL]
 # DEFAULT: yes
 # If set to no, Singularity will not perform any kernel mounts of squashfs filesystems.
-# This affects both stand-alone image files and filesystems embedded in a SIF file.
+# Instead, for SIF / SquashFS containers, a squashfuse mount will be attempted, with
+# extraction to a temporary sandbox directory if this fails.
 # Applicable to setuid mode only.
 allow kernel squashfs = {{ if eq .AllowKernelSquashfs true }}yes{{ else }}no{{ end }}
 
@@ -523,7 +524,7 @@ systemd cgroups = {{ if eq .SystemdCgroups true }}yes{{ else }}no{{ end }}
 
 # SIF FUSE: [BOOL]
 # DEFAULT: no
-# EXPERIMENTAL
+# DEPRECATED - FUSE mounts are now used automatically when kernel mounts are disabled / unavailable.
 # Whether to try mounting SIF images with Squashfuse by default.
 # Applies only to unprivileged / user namespace flows. Requires squashfuse and
 # fusermount on PATH. Will fall back to extracting the SIF on failure.