syscallcompat: move quirks logic here & fix darwin

We need to look at f_fstypename acc. to
https://stackoverflow.com/a/52299141/1380267 :

> As filesystem type numbers are now assigned at runtime in
> recent versions of MacOS, you must use f_fstypename to
> determine the type.

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

8 files changed, 149 insertions, 55 deletions

diff --git a/internal/fusefrontend/file.go b/internal/fusefrontend/file.go
index 304ba7f..716a0db 100644
--- a/internal/fusefrontend/file.go
+++ b/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 && f.rootNode.quirks&quirkBrokenFalloc == 0 {
+	if !f.rootNode.args.NoPrealloc && f.rootNode.quirks&syscallcompat.QuirkBrokenFalloc == 0 {
 		err = syscallcompat.EnospcPrealloc(f.intFd(), 0, contentenc.HeaderLen)
 		if err != nil {
 			if !syscallcompat.IsENOSPC(err) {
diff --git a/internal/fusefrontend/node_helpers.go b/internal/fusefrontend/node_helpers.go
index 2f361f6..8d1749d 100644
--- a/internal/fusefrontend/node_helpers.go
+++ b/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 || rn.quirks&quirkDuplicateIno1 != 0 {
+	if rn.args.SharedStorage || rn.quirks&syscallcompat.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)
diff --git a/internal/fusefrontend/quirks.go b/internal/fusefrontend/quirks.go
deleted file mode 100644
index 2979c84..0000000
--- a/internal/fusefrontend/quirks.go
+++ /dev/null
@@ -1,52 +0,0 @@
-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
-}
diff --git a/internal/fusefrontend/root_node.go b/internal/fusefrontend/root_node.go
index 9905d66..a2de953 100644
--- a/internal/fusefrontend/root_node.go
+++ b/internal/fusefrontend/root_node.go
@@ -79,7 +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),
+		quirks:        syscallcompat.DetectQuirks(args.Cipherdir),
 	}
 	return rn
 }
diff --git a/internal/syscallcompat/quirks.go b/internal/syscallcompat/quirks.go
new file mode 100644
index 0000000..60d584d
--- /dev/null
+++ b/internal/syscallcompat/quirks.go
@@ -0,0 +1,20 @@
+package syscallcompat
+
+import (
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+const (
+	// QuirkBrokenFalloc means the falloc is broken.
+	// Preallocation on Btrfs is broken ( https://github.com/rfjakob/gocryptfs/issues/395 )
+	// and slow ( https://github.com/rfjakob/gocryptfs/issues/63 ).
+	QuirkBrokenFalloc = uint64(1 << iota)
+	// QuirkDuplicateIno1 means that we have duplicate inode numbers.
+	// On MacOS ExFAT, all empty files share inode number 1:
+	// https://github.com/rfjakob/gocryptfs/issues/585
+	QuirkDuplicateIno1
+)
+
+func logQuirk(s string) {
+	tlog.Info.Printf(tlog.ColorYellow + "DetectQuirks: " + s + tlog.ColorReset)
+}
diff --git a/internal/syscallcompat/quirks_darwin.go b/internal/syscallcompat/quirks_darwin.go
new file mode 100644
index 0000000..f4e7e71
--- /dev/null
+++ b/internal/syscallcompat/quirks_darwin.go
@@ -0,0 +1,41 @@
+package syscallcompat
+
+import (
+	"golang.org/x/sys/unix"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+func DetectQuirks(cipherdir string) (q uint64) {
+	const (
+		// From https://github.com/rfjakob/gocryptfs/issues/585#issuecomment-887370065
+		FstypenameExfat = "exfat"
+	)
+
+	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
+	}
+
+	// Convert null-terminated st.Fstypename int8 array to string
+	var buf []byte
+	for _, v := range st.Fstypename {
+		if v == 0 {
+			break
+		}
+		buf = append(buf, byte(v))
+	}
+	fstypename := string(buf)
+	tlog.Debug.Printf("DetectQuirks: Fstypename=%q\n", fstypename)
+
+	// On MacOS ExFAT, all empty files share inode number 1:
+	// https://github.com/rfjakob/gocryptfs/issues/585
+	if fstypename == FstypenameExfat {
+		logQuirk("ExFAT detected, disabling hard links. See https://github.com/rfjakob/gocryptfs/issues/585 for why.")
+		q |= QuirkDuplicateIno1
+	}
+
+	return q
+}
diff --git a/internal/syscallcompat/quirks_linux.go b/internal/syscallcompat/quirks_linux.go
new file mode 100644
index 0000000..ffdbfab
--- /dev/null
+++ b/internal/syscallcompat/quirks_linux.go
@@ -0,0 +1,36 @@
+package syscallcompat
+
+import (
+	"golang.org/x/sys/unix"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+// DetectQuirks decides if there are known quirks on the backing filesystem
+// that need to be workarounded.
+//
+// Tested by tests/root_test.TestBtrfsQuirks
+func DetectQuirks(cipherdir string) (q uint64) {
+	const (
+		// From Linux' man statfs
+		BTRFS_SUPER_MAGIC = 0x9123683e
+	)
+
+	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
+	}
+
+	// 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
+	}
+
+	return q
+}
diff --git a/tests/root_test/root_test.go b/tests/root_test/root_test.go
index 8547e4e..e68f86a 100644
--- a/tests/root_test/root_test.go
+++ b/tests/root_test/root_test.go
@@ -12,6 +12,8 @@ import (
 	"syscall"
 	"testing"
 
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
+
 	"golang.org/x/sys/unix"
 
 	"github.com/rfjakob/gocryptfs/tests/test_helpers"
@@ -311,3 +313,50 @@ func TestAcl(t *testing.T) {
 		dumpAcl()
 	}
 }
+
+// TestBtrfsQuirks needs root permissions because it creates a loop disk
+func TestBtrfsQuirks(t *testing.T) {
+	if os.Getuid() != 0 {
+		t.Skip("must run as root")
+	}
+
+	img := filepath.Join(test_helpers.TmpDir, t.Name()+".img")
+	f, err := os.Create(img)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	// minimum size for each btrfs device is 114294784
+	err = f.Truncate(200 * 1024 * 1024)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Format as Btrfs
+	cmd := exec.Command("mkfs.btrfs", img)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Log(string(out))
+		t.Fatal(err)
+	}
+
+	// Mount
+	mnt := img + ".mnt"
+	err = os.Mkdir(mnt, 0600)
+	if err != nil {
+		t.Fatal(err)
+	}
+	cmd = exec.Command("mount", img, mnt)
+	out, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Log(string(out))
+		t.Fatal(err)
+	}
+	defer syscall.Unlink(img)
+	defer syscall.Unmount(mnt, 0)
+
+	quirk := syscallcompat.DetectQuirks(mnt)
+	if quirk != syscallcompat.QuirkBrokenFalloc {
+		t.Errorf("wrong quirk: %v", quirk)
+	}
+}