syscallcompat: Drop Fchmodat emulation on macOS.

On macOS the function has a flags argument, so we don't need the
/proc/self/fd trick used on Linux.

internal/syscallcompat/emulate.go

@@ -1,7 +1,6 @@
 package syscallcompat
 
 import (
-	"os"
 	"path/filepath"
 	"sync"
 	"syscall"
@@ -31,36 +30,6 @@ func emulateMknodat(dirfd int, path string, mode uint32, dev int) error {
 	return syscall.Mknod(path, mode, dev)
 }
 
-// emulateFchmodat emulates the syscall for platforms that don't have it
-// in the kernel (darwin).
-func emulateFchmodat(dirfd int, path string, mode uint32, flags int) (err error) {
-	if !filepath.IsAbs(path) {
-		chdirMutex.Lock()
-		defer chdirMutex.Unlock()
-		cwd, err := syscall.Open(".", syscall.O_RDONLY, 0)
-		if err != nil {
-			return err
-		}
-		defer syscall.Close(cwd)
-		err = syscall.Fchdir(dirfd)
-		if err != nil {
-			return err
-		}
-		defer syscall.Fchdir(cwd)
-	}
-	// We also don't have Lchmod, so emulate it (poorly).
-	if flags&unix.AT_SYMLINK_NOFOLLOW != 0 {
-		fi, err := os.Lstat(path)
-		if err != nil {
-			return err
-		}
-		if fi.Mode()&os.ModeSymlink != 0 {
-			return nil
-		}
-	}
-	return syscall.Chmod(path, mode)
-}
-
 // emulateFchownat emulates the syscall for platforms that don't have it
 // in the kernel (darwin).
 func emulateFchownat(dirfd int, path string, uid int, gid int, flags int) (err error) {

internal/syscallcompat/emulate_test.go

@@ -29,59 +29,6 @@ func TestEmulateMknodat(t *testing.T) {
 	}
 }
 
-func TestEmulateFchmodat(t *testing.T) {
-	fd, err := os.Create(tmpDir + "/chmod")
-	if err != nil {
-		t.Fatal(err)
-	}
-	err = fd.Chmod(0654)
-	if err != nil {
-		t.Fatal(err)
-	}
-	fd.Close()
-	// Chmod a normal file
-	err = emulateFchmodat(tmpDirFd, "chmod", 0600, 0)
-	if err != nil {
-		t.Fatal(err)
-	}
-	var st syscall.Stat_t
-	err = syscall.Lstat(tmpDir+"/chmod", &st)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if st.Mode != 0100600 {
-		t.Fatalf("Wrong mode: have %o, want %o", st.Mode, 0100600)
-	}
-	// Chmod a symlink (original file should not change)
-	err = os.Symlink(tmpDir+"/chmod", tmpDir+"/chmodSymlink")
-	if err != nil {
-		t.Fatal(err)
-	}
-	err = emulateFchmodat(tmpDirFd, "chmodSymlink", 0123, unix.AT_SYMLINK_NOFOLLOW)
-	if err != nil {
-		t.Fatal(err)
-	}
-	err = syscall.Lstat(tmpDir+"/chmod", &st)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if st.Mode != 0100600 {
-		t.Fatalf("Wrong mode: have %o, want %o", st.Mode, 0100600)
-	}
-	// Test with absolute path
-	err = emulateFchmodat(-1, tmpDir+"/chmod", 0400, 0)
-	if err != nil {
-		t.Fatal(err)
-	}
-	err = syscall.Lstat(tmpDir+"/chmod", &st)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if st.Mode != 0100400 {
-		t.Fatalf("Wrong mode: have %o, want %o", st.Mode, 0100400)
-	}
-}
-
 func TestEmulateFchownat(t *testing.T) {
 	t.Skipf("TODO")
 }

internal/syscallcompat/sys_darwin.go

@@ -8,6 +8,8 @@ import (
 	"golang.org/x/sys/unix"
 
 	"github.com/hanwen/go-fuse/fuse"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
 const (
@@ -92,7 +94,12 @@ func MknodatUser(dirfd int, path string, mode uint32, dev int, context *fuse.Con
 }
 
 func Fchmodat(dirfd int, path string, mode uint32, flags int) (err error) {
-	return emulateFchmodat(dirfd, path, mode, flags)
+	// Why would we ever want to call this without AT_SYMLINK_NOFOLLOW?
+	if flags&unix.AT_SYMLINK_NOFOLLOW == 0 {
+		tlog.Warn.Printf("Fchmodat: adding missing AT_SYMLINK_NOFOLLOW flag")
+		flags |= unix.AT_SYMLINK_NOFOLLOW
+	}
+	return unix.Fchmodat(dirfd, path, mode, flags)
 }
 
 func Fchownat(dirfd int, path string, uid int, gid int, flags int) (err error) {