From b2724070d95234a8cd281f275211e0f827a8bbe1 Mon Sep 17 00:00:00 2001
From: Jakob Unterwurzacher
Date: Mon, 16 Aug 2021 18:40:48 +0200
Subject: reverse mode: implement -one-file-system

Fixes https://github.com/rfjakob/gocryptfs/issues/475
---
 Documentation/MANPAGE.md                      |  8 +++
 cli_args.go                                   |  3 +-
 crossbuild.bash                               |  6 +--
 internal/fusefrontend/args.go                 |  4 ++
 internal/fusefrontend_reverse/node.go         | 13 ++++-
 internal/fusefrontend_reverse/node_dir_ops.go | 23 +++++---
 internal/fusefrontend_reverse/node_helpers.go |  5 +-
 internal/fusefrontend_reverse/root_node.go    | 14 +++++
 mount.go                                      |  1 +
 tests/reverse/one_file_system_test.go         | 77 +++++++++++++++++++++++++++
 10 files changed, 140 insertions(+), 14 deletions(-)
 create mode 100644 tests/reverse/one_file_system_test.go

diff --git a/Documentation/MANPAGE.md b/Documentation/MANPAGE.md
index 648a6c7..3a2f6c5 100644
--- a/Documentation/MANPAGE.md
+++ b/Documentation/MANPAGE.md
@@ -331,6 +331,14 @@ See `-suid, -nosuid`.
 Send USR1 to the specified process after successful mount. This is
 used internally for daemonization.
 
+#### -one-file-system
+Don't cross filesystem boundaries (like rsync's `--one-file-system`).
+Mountpoints will appear as empty directories.
+
+Only applicable to reverse mode.
+
+Limitation: Mounted single files (yes this is possible) are NOT hidden.
+
 #### -rw, -ro
 Mount the filesystem read-write (`-rw`, default) or read-only (`-ro`).
 If both are specified, `-ro` takes precedence.
diff --git a/cli_args.go b/cli_args.go
index fa9e35c..1ece378 100644
--- a/cli_args.go
+++ b/cli_args.go
@@ -30,7 +30,7 @@ type argContainer struct {
 	plaintextnames, quiet, nosyslog, wpanic,
 	longnames, allow_other, reverse, aessiv, nonempty, raw64,
 	noprealloc, speed, hkdf, serialize_reads, forcedecode, hh, info,
-	sharedstorage, devrandom, fsck bool
+	sharedstorage, devrandom, fsck, one_file_system bool
 	// Mount options with opposites
 	dev, nodev, suid, nosuid, exec, noexec, rw, ro, kernel_cache, acl bool
 	masterkey, mountpoint, cipherdir, cpuprofile,
@@ -178,6 +178,7 @@ func parseCliOpts(osArgs []string) (args argContainer) {
 	flagSet.BoolVar(&args.sharedstorage, "sharedstorage", false, "Make concurrent access to a shared CIPHERDIR safer")
 	flagSet.BoolVar(&args.devrandom, "devrandom", false, "Use /dev/random for generating master key")
 	flagSet.BoolVar(&args.fsck, "fsck", false, "Run a filesystem check on CIPHERDIR")
+	flagSet.BoolVar(&args.one_file_system, "one-file-system", false, "Don't cross filesystem boundaries")
 
 	// Mount options with opposites
 	flagSet.BoolVar(&args.dev, "dev", false, "Allow device files")
diff --git a/crossbuild.bash b/crossbuild.bash
index 0904d54..43bfd32 100755
--- a/crossbuild.bash
+++ b/crossbuild.bash
@@ -5,7 +5,8 @@
 cd "$(dirname "$0")"
 
 export GO111MODULE=on
-B="go build -tags without_openssl"
+# Discard resulting binary by writing to /dev/null
+B="go build -tags without_openssl -o /dev/null"
 
 set -x
 
@@ -26,6 +27,3 @@ GOOS=darwin GOARCH=amd64 $B
 if go tool dist list | grep ios/arm64 ; then
 	GOOS=darwin GOARCH=arm64 $B
 fi
-
-# The cross-built binary is not useful on the compile host.
-rm gocryptfs
diff --git a/internal/fusefrontend/args.go b/internal/fusefrontend/args.go
index ae1c30c..d92c3ff 100644
--- a/internal/fusefrontend/args.go
+++ b/internal/fusefrontend/args.go
@@ -49,4 +49,8 @@ type Args struct {
 	// SharedStorage disables caching & hard link tracking,
 	// enabled via cli flag "-sharedstorage"
 	SharedStorage bool
+	// OneFileSystem disables crossing filesystem boundaries,
+	// like rsync's `--one-file-system` does.
+	// Only applicable to reverse mode.
+	OneFileSystem bool
 }
diff --git a/internal/fusefrontend_reverse/node.go b/internal/fusefrontend_reverse/node.go
index 787b99b..1b2fd67 100644
--- a/internal/fusefrontend_reverse/node.go
+++ b/internal/fusefrontend_reverse/node.go
@@ -22,6 +22,10 @@ import (
 // in a `gocryptfs -reverse` mount.
 type Node struct {
 	fs.Inode
+	// isOtherFilesystem is used for --one-filesystem.
+	// It is set when the device number of this file or directory
+	// is different from n.rootNode().rootDev.
+	isOtherFilesystem bool
 }
 
 // Lookup - FUSE call for discovering a file.
@@ -31,7 +35,14 @@ func (n *Node) Lookup(ctx context.Context, cName string, out *fuse.EntryOut) (ch
 	if t == typeDiriv {
 		// gocryptfs.diriv
 		return n.lookupDiriv(ctx, out)
-	} else if t == typeName {
+	}
+	rn := n.rootNode()
+	if rn.args.OneFileSystem && n.isOtherFilesystem {
+		// With --one-file-system, we present mountpoints as empty. That is,
+		// it contains only a gocryptfs.diriv file (allowed above).
+		return nil, syscall.ENOENT
+	}
+	if t == typeName {
 		// gocryptfs.longname.*.name
 		return n.lookupLongnameName(ctx, cName, out)
 	} else if t == typeConfig {
diff --git a/internal/fusefrontend_reverse/node_dir_ops.go b/internal/fusefrontend_reverse/node_dir_ops.go
index c287284..21b9775 100644
--- a/internal/fusefrontend_reverse/node_dir_ops.go
+++ b/internal/fusefrontend_reverse/node_dir_ops.go
@@ -23,6 +23,22 @@ import (
 // This function is symlink-safe through use of openBackingDir() and
 // ReadDirIVAt().
 func (n *Node) Readdir(ctx context.Context) (stream fs.DirStream, errno syscall.Errno) {
+	// Virtual files: at least one gocryptfs.diriv file
+	virtualFiles := []fuse.DirEntry{
+		{Mode: virtualFileMode, Name: nametransform.DirIVFilename},
+	}
+	rn := n.rootNode()
+
+	// This directory is a mountpoint. Present it as empty.
+	if rn.args.OneFileSystem && n.isOtherFilesystem {
+		if rn.args.PlaintextNames {
+			return fs.NewListDirStream(nil), 0
+		} else {
+			// An "empty" directory still has a gocryptfs.diriv file!
+			return fs.NewListDirStream(virtualFiles), 0
+		}
+	}
+
 	d, errno := n.prepareAtSyscall("")
 	if errno != 0 {
 		return
@@ -41,8 +57,6 @@ func (n *Node) Readdir(ctx context.Context) (stream fs.DirStream, errno syscall.
 		return nil, fs.ToErrno(err)
 	}
 
-	rn := n.rootNode()
-
 	// Filter out excluded entries
 	entries = rn.excludeDirEntries(d, entries)
 
@@ -50,11 +64,6 @@ func (n *Node) Readdir(ctx context.Context) (stream fs.DirStream, errno syscall.
 		return n.readdirPlaintextnames(entries)
 	}
 
-	// Virtual files: at least one gocryptfs.diriv file
-	virtualFiles := []fuse.DirEntry{
-		{Mode: virtualFileMode, Name: nametransform.DirIVFilename},
-	}
-
 	dirIV := pathiv.Derive(d.cPath, pathiv.PurposeDirIV)
 	// Encrypt names
 	for i := range entries {
diff --git a/internal/fusefrontend_reverse/node_helpers.go b/internal/fusefrontend_reverse/node_helpers.go
index 92f6a87..7b286a0 100644
--- a/internal/fusefrontend_reverse/node_helpers.go
+++ b/internal/fusefrontend_reverse/node_helpers.go
@@ -91,6 +91,7 @@ func (n *Node) prepareAtSyscall(child string) (d *dirfdPlus, errno syscall.Errno
 // newChild attaches a new child inode to n.
 // The passed-in `st` will be modified to get a unique inode number.
 func (n *Node) newChild(ctx context.Context, st *syscall.Stat_t, out *fuse.EntryOut) *fs.Inode {
+	isOtherFilesystem := (uint64(st.Dev) != n.rootNode().rootDev)
 	// Get unique inode number
 	rn := n.rootNode()
 	rn.inoMap.TranslateStat(st)
@@ -101,7 +102,9 @@ func (n *Node) newChild(ctx context.Context, st *syscall.Stat_t, out *fuse.Entry
 		Gen:  1,
 		Ino:  st.Ino,
 	}
-	node := &Node{}
+	node := &Node{
+		isOtherFilesystem: isOtherFilesystem,
+	}
 	return n.NewInode(ctx, node, id)
 }
 
diff --git a/internal/fusefrontend_reverse/root_node.go b/internal/fusefrontend_reverse/root_node.go
index b072f85..d57e1e6 100644
--- a/internal/fusefrontend_reverse/root_node.go
+++ b/internal/fusefrontend_reverse/root_node.go
@@ -36,17 +36,31 @@ type RootNode struct {
 	// inoMap translates inode numbers from different devices to unique inode
 	// numbers.
 	inoMap *inomap.InoMap
+	// rootDev stores the device number of the backing directory. Used for
+	// --one-file-system.
+	rootDev uint64
 }
 
 // NewRootNode returns an encrypted FUSE overlay filesystem.
 // In this case (reverse mode) the backing directory is plain-text and
 // ReverseFS provides an encrypted view.
 func NewRootNode(args fusefrontend.Args, c *contentenc.ContentEnc, n *nametransform.NameTransform) *RootNode {
+	var rootDev uint64
+	if args.OneFileSystem {
+		var st syscall.Stat_t
+		err := syscall.Stat(args.Cipherdir, &st)
+		if err != nil {
+			log.Panicf("Could not stat backing directory %q: %v", args.Cipherdir, err)
+		}
+		rootDev = uint64(st.Dev)
+	}
+
 	rn := &RootNode{
 		args:          args,
 		nameTransform: n,
 		contentEnc:    c,
 		inoMap:        inomap.New(),
+		rootDev:       rootDev,
 	}
 	if len(args.Exclude) > 0 || len(args.ExcludeWildcard) > 0 || len(args.ExcludeFrom) > 0 {
 		rn.excluder = prepareExcluder(args)
diff --git a/mount.go b/mount.go
index 3f190d4..ababa81 100644
--- a/mount.go
+++ b/mount.go
@@ -275,6 +275,7 @@ func initFuseFrontend(args *argContainer) (rootNode fs.InodeEmbedder, wipeKeys f
 		Suid:            args.suid,
 		KernelCache:     args.kernel_cache,
 		SharedStorage:   args.sharedstorage,
+		OneFileSystem:   args.one_file_system,
 	}
 	// confFile is nil when "-zerokey" or "-masterkey" was used
 	if confFile != nil {
diff --git a/tests/reverse/one_file_system_test.go b/tests/reverse/one_file_system_test.go
new file mode 100644
index 0000000..5d8b76b
--- /dev/null
+++ b/tests/reverse/one_file_system_test.go
@@ -0,0 +1,77 @@
+package reverse
+
+import (
+	"io/ioutil"
+	"net/url"
+	"os"
+	"runtime"
+	"syscall"
+	"testing"
+
+	"github.com/rfjakob/gocryptfs/tests/test_helpers"
+)
+
+func doTestOneFileSystem(t *testing.T, plaintextnames bool) {
+	// Let's not explode with "TempDir: pattern contains path separator"
+	myEscapedName := url.PathEscape(t.Name())
+	mnt, err := ioutil.TempDir(test_helpers.TmpDir, myEscapedName)
+	if err != nil {
+		t.Fatal(err)
+	}
+	cliArgs := []string{"-reverse", "-zerokey", "-one-file-system"}
+	if plaintextnames {
+		cliArgs = append(cliArgs, "-plaintextnames")
+	}
+	test_helpers.MountOrFatal(t, "/", mnt, cliArgs...)
+	defer test_helpers.UnmountErr(mnt)
+
+	// Copied from inomap
+	const maxPassthruIno = 1<<48 - 1
+
+	entries, err := os.ReadDir(mnt)
+	if err != nil {
+		t.Fatal(err)
+	}
+	mountpoints := []string{}
+	for _, e := range entries {
+		i, err := e.Info()
+		if err != nil {
+			continue
+		}
+		if !e.IsDir() {
+			// We are only interested in directories
+			continue
+		}
+		st := i.Sys().(*syscall.Stat_t)
+		// The inode numbers of files with a different device number are remapped
+		// to something above maxPassthruIno
+		if st.Ino > maxPassthruIno {
+			mountpoints = append(mountpoints, e.Name())
+		}
+	}
+	if len(mountpoints) == 0 {
+		t.Skip("no mountpoints found, nothing to test")
+	}
+	for _, m := range mountpoints {
+		e, err := os.ReadDir(mnt + "/" + m)
+		if err != nil {
+			t.Error(err)
+		}
+		expected := 1
+		if plaintextnames {
+			expected = 0
+		}
+		if len(e) != expected {
+			t.Errorf("mountpoint %q does not look empty: %v", m, e)
+		}
+	}
+	t.Logf("tested %d mountpoints: %v", len(mountpoints), mountpoints)
+}
+
+func TestOneFileSystem(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skip("only works on linux")
+	}
+	t.Run("normal", func(t *testing.T) { doTestOneFileSystem(t, false) })
+	t.Run("plaintextnames", func(t *testing.T) { doTestOneFileSystem(t, true) })
+}
-- 
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