fusefrontend: add quirks for MacOS ExFAT

This also moves the quirks logic into fusefrontend.

Fixes https://github.com/rfjakob/gocryptfs/issues/585

internal/fusefrontend/file.go

@@ -118,7 +118,7 @@ func (f *File) createHeader() (fileID []byte, err error) {
 	h := contentenc.RandomHeader()
 	buf := h.Pack()
 	// Prevent partially written (=corrupt) header by preallocating the space beforehand
-	if !f.rootNode.args.NoPrealloc {
+	if !f.rootNode.args.NoPrealloc && f.rootNode.quirks&quirkBrokenFalloc == 0 {
 		err = syscallcompat.EnospcPrealloc(f.intFd(), 0, contentenc.HeaderLen)
 		if err != nil {
 			if !syscallcompat.IsENOSPC(err) {

internal/fusefrontend/node_helpers.go

@@ -87,7 +87,7 @@ func (n *Node) newChild(ctx context.Context, st *syscall.Stat_t, out *fuse.Entry
 	out.Attr.FromStat(st)
 
 	var gen uint64 = 1
-	if rn.args.SharedStorage {
+	if rn.args.SharedStorage || rn.quirks&quirkDuplicateIno1 != 0 {
 		// Make each directory entry a unique node by using a unique generation
 		// value - see the comment at RootNode.gen for details.
 		gen = atomic.AddUint64(&rn.gen, 1)

internal/fusefrontend/quirks.go

@@ -0,0 +1,52 @@
+package fusefrontend
+
+import (
+	"runtime"
+
+	"golang.org/x/sys/unix"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+const (
+	quirkBrokenFalloc = uint64(1 << iota)
+	quirkDuplicateIno1
+)
+
+func detectQuirks(cipherdir string) (q uint64) {
+	const (
+		// From Linux' man statfs
+		BTRFS_SUPER_MAGIC = 0x9123683e
+
+		// From https://github.com/rfjakob/gocryptfs/issues/585#issuecomment-887370065
+		DARWIN_EXFAT_MAGIC = 35
+	)
+
+	var st unix.Statfs_t
+	err := unix.Statfs(cipherdir, &st)
+	if err != nil {
+		tlog.Warn.Printf("detectQuirks: Statfs on %q failed: %v", cipherdir, err)
+		return 0
+	}
+
+	logQuirk := func(s string) {
+		tlog.Info.Printf(tlog.ColorYellow + "detectQuirks: " + s + tlog.ColorReset)
+	}
+
+	// Preallocation on Btrfs is broken ( https://github.com/rfjakob/gocryptfs/issues/395 )
+	// and slow ( https://github.com/rfjakob/gocryptfs/issues/63 ).
+	//
+	// Cast to uint32 avoids compile error on arm: "constant 2435016766 overflows int32"
+	if uint32(st.Type) == BTRFS_SUPER_MAGIC {
+		logQuirk("Btrfs detected, forcing -noprealloc. See https://github.com/rfjakob/gocryptfs/issues/395 for why.")
+		q |= quirkBrokenFalloc
+	}
+	// On MacOS ExFAT, all empty files share inode number 1:
+	// https://github.com/rfjakob/gocryptfs/issues/585
+	if runtime.GOOS == "darwin" && st.Type == DARWIN_EXFAT_MAGIC {
+		logQuirk("ExFAT detected, disabling hard links. See https://github.com/rfjakob/gocryptfs/issues/585 for why.")
+		q |= quirkDuplicateIno1
+	}
+
+	return q
+}

internal/fusefrontend/root_node.go

@@ -57,6 +57,9 @@ type RootNode struct {
 	// makes go-fuse hand out separate FUSE Node IDs for each, and prevents
 	// bizarre problems when inode numbers are reused behind our back.
 	gen uint64
+	// quirks is a bitmap that enables workaround for quirks in the filesystem
+	// backing the cipherdir
+	quirks uint64
 }
 
 func NewRootNode(args Args, c *contentenc.ContentEnc, n *nametransform.NameTransform) *RootNode {
@@ -76,6 +79,7 @@ func NewRootNode(args Args, c *contentenc.ContentEnc, n *nametransform.NameTrans
 		contentEnc:    c,
 		inoMap:        inomap.New(),
 		dirCache:      dirCache{ivLen: ivLen},
+		quirks:        detectQuirks(args.Cipherdir),
 	}
 	return rn
 }

mount.go

@@ -19,8 +19,6 @@ import (
 	"syscall"
 	"time"
 
-	"golang.org/x/sys/unix"
-
 	"github.com/hanwen/go-fuse/v2/fs"
 	"github.com/hanwen/go-fuse/v2/fuse"
 
@@ -102,21 +100,6 @@ func doMount(args *argContainer) {
 			}
 		}()
 	}
-	// Preallocation on Btrfs is broken ( https://github.com/rfjakob/gocryptfs/issues/395 )
-	// and slow ( https://github.com/rfjakob/gocryptfs/issues/63 ).
-	if !args.noprealloc {
-		// darwin does not have unix.BTRFS_SUPER_MAGIC, so we define it here
-		const BTRFS_SUPER_MAGIC = 0x9123683e
-		var st unix.Statfs_t
-		err = unix.Statfs(args.cipherdir, &st)
-		// Cast to uint32 avoids compile error on arm: "constant 2435016766 overflows int32"
-		if err == nil && uint32(st.Type) == BTRFS_SUPER_MAGIC {
-			tlog.Info.Printf(tlog.ColorYellow +
-				"Btrfs detected, forcing -noprealloc. See https://github.com/rfjakob/gocryptfs/issues/395 for why." +
-				tlog.ColorReset)
-			args.noprealloc = true
-		}
-	}
 	// Initialize gocryptfs (read config file, ask for password, ...)
 	fs, wipeKeys := initFuseFrontend(args)
 	// Try to wipe secret keys from memory after unmount