Add support for passing FUSE file descriptors as mount point

doc/CONFIGURATION.md

@@ -298,6 +298,25 @@ ExecStop=/usr/bin/fusermount -u /home/ec2-user/s3-bucket-mount
 WantedBy=remote-fs.target
 ```
 
+### Providing a FUSE file descriptor for mounting
+
+Mountpoint supports mounting S3 buckets at a given path, or using a provided FUSE file descriptor (only on Linux).
+
+For directory mount points, the passed path must be an existing directory.
+
+For FUSE file descriptors on Linux, you can specify an open FUSE file descriptor as a mount point with `/dev/fd/N` syntax.
+This is useful in container environments to achieve unprivileged mounts.
+In this case, the caller is responsible for the following:
+1. Opening the FUSE device (`/dev/fuse`) in read-write mode to obtain a file descriptor.
+2. Performing the `mount` syscall with the desired mount point, the file descriptor, and any mount options.
+   Mountpoint by default uses and recommends enabling `nodev`, `nosuid`, `default_permissions`, and `noatime` mount options.
+   See the [Linux kernel documentation](https://man7.org/linux/man-pages/man8/mount.fuse3.8.html#OPTIONS) for more details on mount options.
+3. Spawning Mountpoint with the file descriptor using `/dev/fd/N` syntax as the mount point argument.
+4. Closing the file descriptor in the parent process.
+5. Performing the `unmount` syscall on the mount point when unmounting is desired or when the Mountpoint process terminates.
+
+See [mounthelper.go](https://github.com/awslabs/mountpoint-s3/tree/main/examples/fuse-fd-mount-point/mounthelper.go) as an example usage of this feature.
+
 ## Caching configuration
 
 Mountpoint can optionally cache object metadata and content to reduce cost and improve performance for repeated reads to the same file.

examples/fuse-fd-mount-point/mounthelper.go

@@ -0,0 +1,116 @@
+// An example script showing usage of FUSE file descriptor as a mount point.
+//
+// Example usage:
+// $ go build mounthelper.go
+// $ sudo /sbin/setcap 'cap_sys_admin=ep' ./mounthelper # `mount` syscall requires `CAP_SYS_ADMIN`, alternatively, `mounthelper` can be run as root
+// $ ./mounthelper -mountpoint /tmp/mountpoint -bucket bucketname
+// $ # Mountpoint mounted at /tmp/mountpoint until `mounthelper` is terminated with ctrl+c.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"os/signal"
+	"strings"
+	"syscall"
+)
+
+var mountPoint = flag.String("mountpoint", "", "Path to mount the filesystem at")
+var bucket = flag.String("bucket", "", "S3 Bucket to mount")
+
+func main() {
+	flag.Parse()
+
+	// `os.MkdirAll` will return `nil` if `mountPoint` is an already existing directory.
+	if err := os.MkdirAll(*mountPoint, 0644); err != nil {
+		log.Panicf("Failed to create target mount point %s: %v\n", *mountPoint, err)
+	}
+
+	// 1. Open FUSE device
+	const USE_DEFAULT_PERM = 0
+	fd, err := syscall.Open("/dev/fuse", os.O_RDWR, USE_DEFAULT_PERM)
+	if err != nil {
+		log.Panicf("Failed to open /dev/fuse: %v\n", err)
+	}
+
+	// Ensure to close the file descriptor in case of an error, if there are no errors,
+	// the file descriptor will be closed and `closeFd` will set to `false`.
+	closeFd := true
+	defer func() {
+		if closeFd {
+			err := syscall.Close(fd)
+			if err != nil {
+				log.Panicf("Failed to close fd on parent: %v\n", err)
+			}
+		}
+	}()
+
+	var stat syscall.Stat_t
+	err = syscall.Stat(*mountPoint, &stat)
+	if err != nil {
+		log.Panicf("Failed to stat mount point %s: %v\n", *mountPoint, err)
+	}
+
+	// 2. Perform `mount` syscall
+    // These mount options and flags match those typically set when using Mountpoint.
+    // Some are set by the underlying FUSE library.
+    // Mountpoint sets (correct at the time of authoring this comment):
+    // * `noatime` to avoid unsupported access time updates.
+    // * `default_permissions` to tell the Kernel to evaluate permissions itself, since Mountpoint does not currently provide any handler for FUSE `access`.
+	options := []string{
+		fmt.Sprintf("fd=%d", fd),
+		fmt.Sprintf("rootmode=%o", stat.Mode),
+		fmt.Sprintf("user_id=%d", os.Geteuid()),
+		fmt.Sprintf("group_id=%d", os.Getegid()),
+		"default_permissions",
+	}
+	var flags uintptr = syscall.MS_NOSUID | syscall.MS_NODEV | syscall.MS_NOATIME
+	err = syscall.Mount("mountpoint-s3", *mountPoint, "fuse", flags, strings.Join(options, ","))
+	if err != nil {
+		log.Panicf("Failed to call mount syscall: %v\n", err)
+	}
+
+	// 3. Define and defer call to `unmount` syscall, to be invoked once script terminates
+	defer func() {
+		err := syscall.Unmount(*mountPoint, 0)
+		if err != nil {
+			log.Printf("Failed to unmount %s: %v\n", *mountPoint, err)
+		} else {
+			log.Printf("Succesfully unmounted %s\n", *mountPoint)
+		}
+	}()
+
+	// 4. Spawn Mountpoint with the fd
+	mountpointCmd := exec.Command("./target/release/mount-s3",
+		*bucket,
+		fmt.Sprintf("/dev/fd/%d", fd),
+		// Other mount options can be added here
+		"--prefix=some_s3_prefix/",
+		"--allow-delete",
+	)
+	mountpointCmd.Stdout = os.Stdout
+	mountpointCmd.Stderr = os.Stderr
+	err = mountpointCmd.Run()
+	if err != nil {
+		log.Panicf("Failed to start Mountpoint: %v\n", err)
+	}
+
+	// 4. Close fd on parent
+	err = syscall.Close(fd)
+	if err != nil {
+		log.Panicf("Failed to close fd on parent: %v\n", err)
+	}
+	// As we expliclity closed it, no need for `defer`red close to happen.
+	closeFd = false
+
+	done := make(chan os.Signal, 1)
+	signal.Notify(done, syscall.SIGINT, syscall.SIGTERM)
+
+	log.Print("Filesystem mounted, waiting for ctrl+c signal to terminate")
+	<-done
+
+	// 5. Unmounting will happen here due to `defer` in step 3.
+}

mountpoint-s3/CHANGELOG.md

@@ -1,5 +1,12 @@
 ## Unreleased
 
+### New features
+
+* Mountpoint now supports specifying an open FUSE file descriptor in place of the mount path by using the syntax `/dev/fd/N`.
+  See [mounthelper.go](https://github.com/awslabs/mountpoint-s3/tree/main/examples/fuse-fd-mount-point/mounthelper.go) as an example usage and see
+  [Configuring mount point](https://github.com/awslabs/mountpoint-s3/blob/main/doc/CONFIGURATION.md#configuring-mount-point) about more details on configuring this feature.
+  ([#1103](https://github.com/awslabs/mountpoint-s3/pull/1103))
+
 ### Other changes
 
 * Fix an issue where an interrupt during `readdir` syscall leads to an error. ([#965](https://github.com/awslabs/mountpoint-s3/pull/965))