Merge pull request #28 from juicedata/revert-26-feat/fd

Revert "fuse: support special /dev/fd/N mountpoint"
juicedata · Sep 20, 2024 · 0752e53 · 0752e53
2 parents 5dbad4f + 20d1190
commit 0752e53
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Showing 4 changed files with 14 additions and 175 deletions.
diff --git a/fuse/api.go b/fuse/api.go
@@ -88,45 +88,6 @@
 // filesystems in terms of path names. Working with path names is somewhat
 // easier compared to inodes, however renames can be racy. Do not use pathfs if
 // you care about correctness.
-//
-// # Mount styles
-//
-// The NewServer() handles mounting the filesystem, which
-// involves opening `/dev/fuse` and calling the
-// `mount(2)` syscall. The latter needs root permissions.
-// This is handled in one of three ways:
-//
-// 1) go-fuse opens `/dev/fuse` and executes the `fusermount`
-// setuid-root helper to call `mount(2)` for us. This is the default.
-// Does not need root permissions but needs `fusermount` installed.
-//
-// 2) If `MountOptions.DirectMount` is set, go-fuse calls `mount(2)` itself.
-// Needs root permissions, but works without `fusermount`.
-//
-// 3) If `mountPoint` has the magic `/dev/fd/N` syntax, it means that that a
-// privileged parent process:
-//
-// * Opened /dev/fuse
-//
-// * Called mount(2) on a real mountpoint directory that we don't know about
-//
-// * Inherited the fd to /dev/fuse to us
-//
-// * Informs us about the fd number via /dev/fd/N
-//
-// This magic syntax originates from libfuse [1] and allows the FUSE server to
-// run without any privileges and without needing `fusermount`, as the parent
-// process performs all privileged operations.
-//
-// The "privileged parent" is usually a container manager like Singularity [2],
-// but for testing, it can also be  the `mount.fuse3` helper with the
-// `drop_privileges,setuid=$USER` flags. Example below for gocryptfs:
-//
-//	$ sudo mount.fuse3 "/usr/local/bin/gocryptfs#/tmp/cipher" /tmp/mnt -o drop_privileges,setuid=$USER
-//
-// [1] https://github.com/libfuse/libfuse/commit/64e11073b9347fcf9c6d1eea143763ba9e946f70
-//
-// [2] https://sylabs.io/guides/3.7/user-guide/bind_paths_and_mounts.html#fuse-mounts
 package fuse
 
 // Types for users to implement.

diff --git a/fuse/mount_linux.go b/fuse/mount_linux.go
@@ -12,7 +12,6 @@ import (
 	"os/exec"
 	"path"
 	"path/filepath"
-	"strconv"
 	"strings"
 	"syscall"
 	"unsafe"
@@ -117,12 +116,18 @@ func mountDirect(mountPoint string, opts *MountOptions, ready chan<- error) (fd
 	return
 }
 
-// callFusermount calls the `fusermount` suid helper with the right options so
-// that it:
-// * opens `/dev/fuse`
-// * mount()s this file descriptor to `mountPoint`
-// * passes this file descriptor back to use via a unix domain socket
-func callFusermount(mountPoint string, opts *MountOptions) (fd int, err error) {
+// Create a FUSE FS on the specified mount point.  The returned
+// mount point is always absolute.
+func mount(mountPoint string, opts *MountOptions, ready chan<- error) (fd int, err error) {
+	if opts.DirectMount {
+		fd, err := mountDirect(mountPoint, opts, ready)
+		if err == nil {
+			return fd, nil
+		} else if opts.Debug {
+			log.Printf("mount: failed to do direct mount: %s", err)
+		}
+	}
+
 	local, remote, err := unixgramSocketpair()
 	if err != nil {
 		return
@@ -164,53 +169,11 @@ func callFusermount(mountPoint string, opts *MountOptions) (fd int, err error) {
 		return -1, err
 	}
 
-	return
-}
-
-// parseFuseFd checks if `mountPoint` is the special form /dev/fd/N (with N >= 0),
-// and returns N in this case. Returns -1 otherwise.
-func parseFuseFd(mountPoint string) (fd int) {
-	dir, file := path.Split(mountPoint)
-	if dir != "/dev/fd/" {
-		return -1
-	}
-	fd, err := strconv.Atoi(file)
-	if err != nil || fd <= 0 {
-		return -1
-	}
-	return fd
-}
-
-// Create a FUSE FS on the specified mount point.  The returned
-// mount point is always absolute.
-func mount(mountPoint string, opts *MountOptions, ready chan<- error) (fd int, err error) {
-	if opts.DirectMount {
-		fd, err := mountDirect(mountPoint, opts, ready)
-		if err == nil {
-			return fd, nil
-		} else if opts.Debug {
-			log.Printf("mount: failed to do direct mount: %s", err)
-		}
-	}
-
-	// Magic `/dev/fd/N` mountpoint. See the docs for NewServer() for how this
-	// works.
-	fd = parseFuseFd(mountPoint)
-	if fd >= 0 {
-		if opts.Debug {
-			log.Printf("mount: magic mountpoint %q, using fd %d", mountPoint, fd)
-		}
-	} else {
-		// Usual case: mount via the `fusermount` suid helper
-		fd, err = callFusermount(mountPoint, opts)
-		if err != nil {
-			return
-		}
-	}
 	// golang sets CLOEXEC on file descriptors when they are
 	// acquired through normal operations (e.g. open).
 	// Buf for fd, we have to set CLOEXEC manually
 	syscall.CloseOnExec(fd)
+
 	close(ready)
 	return fd, err
 }

diff --git a/fuse/mount_linux_test.go b/fuse/mount_linux_test.go
diff --git a/fuse/server.go b/fuse/server.go
@@ -118,20 +118,10 @@ func (ms *Server) RecordLatencies(l LatencyMap) {
 // Unmount calls fusermount -u on the mount. This has the effect of
 // shutting down the filesystem. After the Server is unmounted, it
 // should be discarded.
-//
-// Does not work when we were mounted with the magic /dev/fd/N mountpoint syntax,
-// as we do not know the real mountpoint. Unmount using
-//
-//   fusermount -u /path/to/real/mountpoint
-//
-/// in this case.
 func (ms *Server) Unmount() (err error) {
 	if ms.mountPoint == "" {
 		return nil
 	}
-	if parseFuseFd(ms.mountPoint) >= 0 {
-		return fmt.Errorf("Cannot unmount magic mountpoint %q. Please use `fusermount -u REALMOUNTPOINT` instead.", ms.mountPoint)
-	}
 	delay := time.Duration(0)
 	for try := 0; try < 5; try++ {
 		err = unmount(ms.mountPoint, ms.opts)
@@ -154,11 +144,7 @@ func (ms *Server) Unmount() (err error) {
 	return err
 }
 
-// NewServer creates a FUSE server and attaches ("mounts") it to the
-// `mountPoint` directory.
-//
-// See the "Mount styles" section in the package documentation if you want to
-// know about the inner workings of the mount process. Usually you do not.
+// NewServer creates a server and attaches it to the given directory.
 func NewServer(fs RawFileSystem, mountPoint string, opts *MountOptions) (*Server, error) {
 	if opts == nil {
 		opts = &MountOptions{
@@ -1131,10 +1117,5 @@ func (ms *Server) WaitMount() error {
 	if err != nil {
 		return err
 	}
-	if parseFuseFd(ms.mountPoint) >= 0 {
-		// Magic `/dev/fd/N` mountpoint. We don't know the real mountpoint, so
-		// we cannot run the poll hack.
-		return nil
-	}
 	return pollHack(ms.mountPoint)
 }