logcheck: fix detection of invalid * regexp in filter

Extending the user-supplied regular expression to match the entire string by
adding ^ and $ turned invalid regular expressions like * into valid
ones (`^*$`) and thus didn't flag them as error.

It's better to compile the string exactly as supplied by the user (better error
message, properly detects this case) and then checking later whether the entire
string was matched.

hack/tools/logcheck/pkg/filter.go

@@ -88,8 +88,7 @@ func (f *RegexpFilter) Set(filename string) error {
 			line.enabled[c] = enabled
 		}
 
-		// Must match entire string.
-		re, err := regexp.Compile("^" + parts[1] + "$")
+		re, err := regexp.Compile(parts[1])
 		if err != nil {
 			return fmt.Errorf("%s:%d: %v", filename, lineNr, err)
 		}
@@ -106,11 +105,25 @@ func (f *RegexpFilter) Set(filename string) error {
 // Enabled checks whether a certain check is enabled for a file.
 func (f *RegexpFilter) Enabled(check string, enabled bool, filename string) bool {
 	for _, l := range f.lines {
-		if l.match.MatchString(filename) {
+		// Must match entire string.
+		if matchFullString(filename, l.match) {
 			if e, ok := l.enabled[check]; ok {
 				enabled = e
 			}
 		}
 	}
 	return enabled
 }
+
+func matchFullString(str string, re *regexp.Regexp) bool {
+	loc := re.FindStringIndex(str)
+	if loc == nil {
+		// No match at all.
+		return false
+	}
+	if loc[1]-loc[0] < len(str) {
+		// Only matches a substring.
+		return false
+	}
+	return true
+}

hack/tools/logcheck/pkg/filter_test.go

@@ -116,7 +116,11 @@ func TestParsing(t *testing.T) {
 	}{
 		"invalid-regexp": {
 			content:     `structured [`,
-			expectError: filename + ":0: error parsing regexp: missing closing ]: `[$`",
+			expectError: filename + ":0: error parsing regexp: missing closing ]: `[`",
+		},
+		"wildcard": {
+			content:     `structured *`,
+			expectError: filename + ":0: error parsing regexp: missing argument to repetition operator: `*`",
 		},
 		"invalid-line": {
 			content: `structured .