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.

1 files changed, 27 insertions, 0 deletions

diff --git a/filesystem/path_test.go b/filesystem/path_test.go
index eef5ce3..4152037 100644
--- a/filesystem/path_test.go
+++ b/filesystem/path_test.go
@@ -20,6 +20,8 @@ package filesystem
 
 import (
 	"fmt"
+	"io/ioutil"
+	"os"
 	"testing"
 )
 
@@ -52,3 +54,28 @@ func TestDeviceNumber(t *testing.T) {
 		t.Error("Should have failed to parse invalid device number")
 	}
 }
+
+func TestHaveReadAccessTo(t *testing.T) {
+	file, err := ioutil.TempFile("", "fscrypt_test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	file.Close()
+	defer os.Remove(file.Name())
+
+	testCases := map[os.FileMode]bool{
+		0444: true,
+		0400: true,
+		0000: false,
+		0040: false, // user bits take priority in Linux
+		0004: false, // user bits take priority in Linux
+	}
+	for mode, readable := range testCases {
+		if err := os.Chmod(file.Name(), mode); err != nil {
+			t.Error(err)
+		}
+		if HaveReadAccessTo(file.Name()) != readable {
+			t.Errorf("Expected readable=%v on mode=0%03o", readable, mode)
+		}
+	}
+}