local: fix permission and ownership on symlinks with --links and --metadata

Before this change, if writing to a local backend with --metadata and
--links, if the incoming metadata contained mode or ownership
information then rclone would apply the mode/ownership to the
destination of the link not the link itself.

This fixes the problem by using the link safe sycall variants
lchown/fchmodat when --links and --metadata is in use. Note that Linux
does not support setting permissions on symlinks, so rclone emits a
debug message in this case.

This also fixes setting times on symlinks on Windows which wasn't
implemented for atime, mtime and was incorrectly setting the target of
the symlink for btime.

See: https://github.com/rclone/rclone/security/advisories/GHSA-hrxh-9w67-g4cv

backend/local/setbtime_windows.go

@@ -9,15 +9,20 @@ import (
 
 const haveSetBTime = true
 
-// setBTime sets the birth time of the file passed in
-func setBTime(name string, btime time.Time) (err error) {
+// setTimes sets any of atime, mtime or btime
+// if link is set it sets a link rather than the target
+func setTimes(name string, atime, mtime, btime time.Time, link bool) (err error) {
 	pathp, err := syscall.UTF16PtrFromString(name)
 	if err != nil {
 		return err
 	}
+	fileFlag := uint32(syscall.FILE_FLAG_BACKUP_SEMANTICS)
+	if link {
+		fileFlag |= syscall.FILE_FLAG_OPEN_REPARSE_POINT
+	}
 	h, err := syscall.CreateFile(pathp,
 		syscall.FILE_WRITE_ATTRIBUTES, syscall.FILE_SHARE_WRITE, nil,
-		syscall.OPEN_EXISTING, syscall.FILE_FLAG_BACKUP_SEMANTICS, 0)
+		syscall.OPEN_EXISTING, fileFlag, 0)
 	if err != nil {
 		return err
 	}
@@ -27,6 +32,28 @@ func setBTime(name string, btime time.Time) (err error) {
 			err = closeErr
 		}
 	}()
-	bFileTime := syscall.NsecToFiletime(btime.UnixNano())
-	return syscall.SetFileTime(h, &bFileTime, nil, nil)
+	var patime, pmtime, pbtime *syscall.Filetime
+	if !atime.IsZero() {
+		t := syscall.NsecToFiletime(atime.UnixNano())
+		patime = &t
+	}
+	if !mtime.IsZero() {
+		t := syscall.NsecToFiletime(mtime.UnixNano())
+		pmtime = &t
+	}
+	if !btime.IsZero() {
+		t := syscall.NsecToFiletime(btime.UnixNano())
+		pbtime = &t
+	}
+	return syscall.SetFileTime(h, pbtime, patime, pmtime)
+}
+
+// setBTime sets the birth time of the file passed in
+func setBTime(name string, btime time.Time) (err error) {
+	return setTimes(name, time.Time{}, time.Time{}, btime, false)
+}
+
+// lsetBTime changes the birth time of the link passed in
+func lsetBTime(name string, btime time.Time) error {
+	return setTimes(name, time.Time{}, time.Time{}, btime, true)
 }