boundos:insideBaseDirEval: return true if baseDir is "/"

insideBaseDirEval would fail if baseDir was "/".
Since "/" contains anything, and can never be a symlink,
just return true, nil if baseDir is "/".

Add a simple test for this case.

Also, while we are at it, have the returned error be a lot more
informative: show the name and the base diectory; further, have it
wrap os.ErrNotExist should anyone wish to use errors.Is at some
point.

Signed-off-by: Ronald G Minnich <rminnich@gmail.com>

osfs/os_bound.go

@@ -246,6 +246,10 @@ func (fs *BoundOS) insideBaseDir(filename string) (bool, error) {
 // a dir that is within the fs.baseDir, by first evaluating any symlinks
 // that either filename or fs.baseDir may contain.
 func (fs *BoundOS) insideBaseDirEval(filename string) (bool, error) {
+	// "/" contains all others.
+	if fs.baseDir == "/" {
+		return true, nil
+	}
 	dir, err := filepath.EvalSymlinks(filepath.Dir(filename))
 	if dir == "" || os.IsNotExist(err) {
 		dir = filepath.Dir(filename)
@@ -255,7 +259,7 @@ func (fs *BoundOS) insideBaseDirEval(filename string) (bool, error) {
 		wd = fs.baseDir
 	}
 	if filename != wd && dir != wd && !strings.HasPrefix(dir, wd+string(filepath.Separator)) {
-		return false, fmt.Errorf("path outside base dir")
+		return false, fmt.Errorf("path %q outside base dir %q:%w", filename, fs.baseDir, os.ErrNotExist)
 	}
 	return true, nil
 }

osfs/os_bound_test.go

@@ -1105,6 +1105,14 @@ func TestReadDir(t *testing.T) {
 	g.Expect(dirs).To(gomega.BeNil())
 }
 
+func TestInsideBaseDirEval(t*testing.T) {
+	g := gomega.NewWithT(t)
+	fs := BoundOS{baseDir: "/"}
+	b, err :=  fs.insideBaseDirEval("a")
+	g.Expect(b).To(gomega.BeTrue())
+	g.Expect(err).To(gomega.BeNil())
+}
+
 func TestMkdirAll(t *testing.T) {
 	g := gomega.NewWithT(t)
 	root := t.TempDir()