Improve error message when unlocking v2 policy is unsupported

If trying to unlock a v2-encrypted directory fails because the kernel
lacks support for v2 policies, show a better error message.  This can
happen if someone downgrades their kernel or tries to access encrypted
directories on removable storage from a computer with an older kernel.

Detecting this case is difficult since all we have to go with is EACCES
when opening the directory.  Implement a heuristic where if get EACCES,
we actually have read access to the directory, and the kernel doesn't
support v2 policies, we show the improved error message.

Before:

  # fscrypt unlock dir
  [ERROR] fscrypt unlock: open dir: permission denied

After:

  # fscrypt unlock dir
  [ERROR] fscrypt unlock: open dir: permission denied

  This may be caused by the directory using a v2 encryption policy and
  the current kernel not supporting it. If indeed the case, then this
  directory can only be used on kernel v5.4 and later. You can create
  directories accessible on older kernels by changing policy_version to
  1 in /etc/fscrypt.conf.

3 files changed, 5 insertions, 5 deletions

diff --git a/keyring/fs_keyring.go b/keyring/fs_keyring.go
index 42c1648..f0016a4 100644
--- a/keyring/fs_keyring.go
+++ b/keyring/fs_keyring.go
@@ -79,10 +79,10 @@ func checkForFsKeyringSupport(mount *filesystem.Mount) bool {
 	return true
 }
 
-// isFsKeyringSupported returns true if the kernel supports the ioctls to
+// IsFsKeyringSupported returns true if the kernel supports the ioctls to
 // add/remove fscrypt keys directly to/from the filesystem.  For support to be
 // detected, the given Mount must be for a filesystem that supports fscrypt.
-func isFsKeyringSupported(mount *filesystem.Mount) bool {
+func IsFsKeyringSupported(mount *filesystem.Mount) bool {
 	fsKeyringSupportedLock.Lock()
 	defer fsKeyringSupportedLock.Unlock()
 	if !fsKeyringSupportedKnown {
diff --git a/keyring/keyring.go b/keyring/keyring.go
index e232de3..6623943 100644
--- a/keyring/keyring.go
+++ b/keyring/keyring.go
@@ -75,11 +75,11 @@ func shouldUseFsKeyring(descriptor string, options *Options) (bool, error) {
 	// use_fs_keyring_for_v1_policies is set in /etc/fscrypt.conf and the
 	// kernel supports it.
 	if len(descriptor) == hex.EncodedLen(unix.FSCRYPT_KEY_DESCRIPTOR_SIZE) {
-		return options.UseFsKeyringForV1Policies && isFsKeyringSupported(options.Mount), nil
+		return options.UseFsKeyringForV1Policies && IsFsKeyringSupported(options.Mount), nil
 	}
 	// For v2 encryption policy keys, always use the filesystem keyring; the
 	// kernel doesn't support any other way.
-	if !isFsKeyringSupported(options.Mount) {
+	if !IsFsKeyringSupported(options.Mount) {
 		return true, ErrV2PoliciesUnsupported
 	}
 	return true, nil
diff --git a/keyring/keyring_test.go b/keyring/keyring_test.go
index 2208105..26f6036 100644
--- a/keyring/keyring_test.go
+++ b/keyring/keyring_test.go
@@ -81,7 +81,7 @@ func getTestMount(t *testing.T) *filesystem.Mount {
 // filesystem keyring and v2 encryption policies are supported.
 func getTestMountV2(t *testing.T) *filesystem.Mount {
 	mount := getTestMount(t)
-	if !isFsKeyringSupported(mount) {
+	if !IsFsKeyringSupported(mount) {
 		t.Skip("No support for fs keyring, skipping test.")
 	}
 	return mount