fusefrontend: Use the Symlinkat syscall for longname handling

internal/fusefrontend/fs.go

@@ -448,8 +448,7 @@ func (fs *FS) Symlink(target string, linkName string, context *fuse.Context) (co
 			return fuse.ToStatus(err)
 		}
 		// Create "gocryptfs.longfile." symlink
-		// TODO use syscall.Symlinkat once it is available in Go
-		err = syscall.Symlink(cTarget, cPath)
+		err = syscallcompat.Symlinkat(cTarget, int(dirfd.Fd()), cName)
 		if err != nil {
 			nametransform.DeleteLongName(dirfd, cName)
 		}

internal/syscallcompat/sys_darwin.go

@@ -148,3 +148,20 @@ func Fchownat(dirfd int, path string, uid int, gid int, flags int) (err error) {
 	defer syscall.Fchdir(cwd)
 	return syscall.Lchown(path, uid, gid)
 }
+
+// Poor man's Symlinkat.
+func Symlinkat(oldpath string, newdirfd int, newpath string) (err error) {
+	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(newdirfd)
+	if err != nil {
+		return err
+	}
+	defer syscall.Fchdir(cwd)
+	return syscall.Symlink(oldpath, newpath)
+}

internal/syscallcompat/sys_linux.go

@@ -4,6 +4,7 @@ package syscallcompat
 import (
 	"sync"
 	"syscall"
+	"unsafe"
 
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
@@ -73,3 +74,23 @@ func Dup3(oldfd int, newfd int, flags int) (err error) {
 func Fchownat(dirfd int, path string, uid int, gid int, flags int) (err error) {
 	return syscall.Fchownat(dirfd, path, uid, gid, flags)
 }
+
+// Symlinkat syscall. Unfortunately this function is not exported directly, so
+// manually call it by using the corresponding syscall number.
+func Symlinkat(oldpath string, newdirfd int, newpath string) (err error) {
+	var _p0 *byte
+	_p0, err = syscall.BytePtrFromString(oldpath)
+	if err != nil {
+		return
+	}
+	var _p1 *byte
+	_p1, err = syscall.BytePtrFromString(newpath)
+	if err != nil {
+		return
+	}
+	_, _, e1 := syscall.Syscall(syscall.SYS_SYMLINKAT, uintptr(unsafe.Pointer(_p0)), uintptr(newdirfd), uintptr(unsafe.Pointer(_p1)))
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}