runtime: improve EROFS snapshotter support

To better support containerd 2.1 and later versions, remove the
hardcoded `layer.erofs` and instead parse `/proc/mounts` to obtain the
real mount source (and `/sys/block/loopX/loop/backing_file` if needed).

If the mount source doesn't end with `layer.erofs`, it should be marked
as unsupported, as it may be a filesystem meta file generated by later
containerd versions for the EROFS flattened filesystem feature.

Also check whether the filesystem type is `overlay` or not, since the
containerd mount manager [1] may change it after being introduced.

[1] https://github.com/containerd/containerd/issues/11303

Fixes: f63ec50ba3 ("runtime: Add EROFS snapshotter with block device support")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

src/runtime/virtcontainers/container.go

@@ -821,12 +821,48 @@ func (c *Container) createMounts(ctx context.Context) error {
 	return c.createBlockDevices(ctx)
 }
 
+func findMountSource(mnt string) (string, error) {
+	output, err := os.ReadFile("/proc/mounts")
+	if err != nil {
+		return "", err
+	}
+
+	// /proc/mounts has 6 fields per line, one mount per line, e.g.
+	// /dev/loop0 /var/lib/containerd/io.containerd.snapshotter.v1.erofs/snapshots/1/fs erofs ro,relatime,user_xattr,acl,cache_strategy=readaround 0 0
+	for _, line := range strings.Split(string(output), "\n") {
+		parts := strings.Split(line, " ")
+		if len(parts) == 6 {
+			switch parts[2] {
+			case "erofs":
+				if parts[1] == mnt {
+					return parts[0], nil
+				}
+			}
+		}
+	}
+	return "", fmt.Errorf("erofs mount not found for %s", mnt)
+}
+
 func (c *Container) createErofsDevices() ([]config.DeviceInfo, error) {
 	var deviceInfos []config.DeviceInfo
-	if HasErofsOptions(c.rootFs.Options) {
-		parsedOptions := parseRootFsOptions(c.rootFs.Options)
-		for _, path := range parsedOptions {
-			di, err := c.createDeviceInfo(path+"/layer.erofs", path+"/layer.erofs", true, true)
+	if IsErofsRootFS(c.rootFs) {
+		lowerdirs := parseErofsRootFsOptions(c.rootFs.Options)
+		for _, path := range lowerdirs {
+			s, err := findMountSource(path)
+			if err != nil {
+				return nil, err
+			}
+			if strings.HasPrefix(s, "/dev/loop") {
+				b, err := os.ReadFile(fmt.Sprintf("/sys/block/loop%s/loop/backing_file", strings.TrimPrefix(s, "/dev/loop")))
+				if err != nil {
+					return nil, err
+				}
+				s = strings.TrimSuffix(string(b), "\n")
+			}
+			if filepath.Base(s) != "layer.erofs" {
+				return nil, fmt.Errorf("unsupported mount source %s for %s", s, path)
+			}
+			di, err := c.createDeviceInfo(s, s, true, true)
 			if err != nil {
 				return nil, err
 			}
@@ -1076,7 +1112,7 @@ func (c *Container) create(ctx context.Context) (err error) {
 		}
 	}()
 
-	if c.checkBlockDeviceSupport(ctx) && !IsNydusRootFSType(c.rootFs.Type) && !HasErofsOptions(c.rootFs.Options) {
+	if c.checkBlockDeviceSupport(ctx) && !IsNydusRootFSType(c.rootFs.Type) && !IsErofsRootFS(c.rootFs) {
 		// If the rootfs is backed by a block device, go ahead and hotplug it to the guest
 		if err = c.hotplugDrive(ctx); err != nil {
 			return

src/runtime/virtcontainers/fs_share_linux.go

@@ -626,7 +626,7 @@ func (f *FilesystemShare) ShareRootFilesystem(ctx context.Context, c *Container)
 		return f.shareRootFilesystemWithNydus(ctx, c)
 	}
 
-	if HasErofsOptions(c.rootFs.Options) {
+	if IsErofsRootFS(c.rootFs) {
 		return f.shareRootFilesystemWithErofs(ctx, c)
 	}
 

src/runtime/virtcontainers/kata_agent.go

@@ -2703,9 +2703,13 @@ func IsNydusRootFSType(s string) bool {
 	return strings.HasPrefix(path.Base(s), "nydus-overlayfs")
 }
 
-// HasErofsOptions checks if any of the options contain io.containerd.snapshotter.v1.erofs path
-func HasErofsOptions(options []string) bool {
-	for _, opt := range options {
+// IsErofsRootFS checks if any of the options contain io.containerd.snapshotter.v1.erofs path
+func IsErofsRootFS(root RootFs) bool {
+	// TODO: support containerd mount manager: https://github.com/containerd/containerd/issues/11303
+	if root.Type != "overlay" {
+		return false
+	}
+	for _, opt := range root.Options {
 		if strings.Contains(opt, "io.containerd.snapshotter.v1.erofs") {
 			return true
 		}
@@ -2713,21 +2717,15 @@ func HasErofsOptions(options []string) bool {
 	return false
 }
 
-func parseRootFsOptions(options []string) []string {
+func parseErofsRootFsOptions(options []string) []string {
 	lowerdirs := []string{}
 
 	for _, opt := range options {
 		if strings.HasPrefix(opt, "lowerdir=") {
 			lowerdirValue := strings.TrimPrefix(opt, "lowerdir=")
 
-			paths := strings.Split(lowerdirValue, ":")
-
-			for _, path := range paths {
-				path = strings.TrimSuffix(path, "/fs")
-				lowerdirs = append(lowerdirs, path)
-			}
+			lowerdirs = append(lowerdirs, strings.Split(lowerdirValue, ":")...)
 		}
 	}
-
 	return lowerdirs
 }