From 09ed95db86c58da975eefd31212725e67a320be1 Mon Sep 17 00:00:00 2001
From: Sebastiaan van Stijn <github@gone.nl>
Date: Thu, 1 Oct 2020 17:23:58 +0200
Subject: [PATCH] mount: support FUSE helper

When m.Type starts with either fuse. or fuse3, the mount helper binary mount.fuse
or mount.fuse3 is executed.

Similar to the changes made in containerd:

> This is expected to be used by fuse-overlayfs plugin: https://github.com/AkihiroSuda/containerd-fuse-overlayfs
>
> Motivation
>
> The purpose of the containerd fuse-overlayfs snapshotter plugin is to provide
> OverlayFS functionality for rootless mode without depending on the Ubuntu/Debian
> kernel patch.
> Although fuse-overlayfs provides shiftfs functionality and supports CRFS plugin,
> these functionalities are not planned to be supported by the containerd fuse-overlayfs

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
---
 mount/unmount_unix.go | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/mount/unmount_unix.go b/mount/unmount_unix.go
index 924d059a..c077cbef 100644
--- a/mount/unmount_unix.go
+++ b/mount/unmount_unix.go
@@ -2,13 +2,49 @@
 
 package mount
 
-import "golang.org/x/sys/unix"
+import (
+	"os/exec"
+
+	"golang.org/x/sys/unix"
+)
 
 func unmountBare(target string, flags int) error {
 	return unix.Unmount(target, flags)
 }
 
+// fuseSuperMagic is defined in statfs(2)
+const fuseSuperMagic = 0x65735546
+
+func isFUSE(dir string) bool {
+	var st unix.Statfs_t
+	if err := unix.Statfs(dir, &st); err != nil {
+		return false
+	}
+	return st.Type == fuseSuperMagic
+}
+
+// unmountFUSE attempts to unmount using fusermount/fusermount3 helper binary.
+//
+// For FUSE mounts, using these helper binaries is preferred, see:
+// https://github.com/containerd/containerd/pull/3765#discussion_r342083514
+func unmountFUSE(target string) error {
+	var err error
+	for _, helperBinary := range []string{"fusermount3", "fusermount"} {
+		cmd := exec.Command(helperBinary, "-u", target)
+		err = cmd.Run()
+		if err == nil {
+			return nil
+		}
+	}
+	return err
+}
+
 func unmount(target string, flags int) error {
+	if isFUSE(target) {
+		if err := unmountFUSE(target); err == nil {
+			return nil
+		}
+	}
 	err := unmountBare(target, flags)
 	if err == nil || err == unix.EINVAL {
 		// Ignore "not mounted" error here. Note the same error