libluna: If allowing garbage chars while decoding Base64, skip them after the padding instead of erroring out

libluna/src/Base64.cpp

@@ -97,11 +97,23 @@ namespace Base64
         char* padding = strchr(data.chars(), '=');
         if (padding)
         {
+            padding++;
             // If the string ends with padding, it must be either one or two equals signs.
-            if (padding[1] != '=' && padding[1] != '\0') return err(EINVAL);
+            if (*padding != '=' && *padding != '\0') return err(EINVAL);
-            if (padding[1])
+
+            if (*padding) padding++;
+
+            // After that, only thing allowed is newline (and garbage characters if those are permitted)
+            while (*padding)
             {
-                if (padding[strspn(&padding[2], "\n") + 2]) return err(EINVAL);
+                char c = *padding;
+                padding++;
+
+                if (c == '\n') continue;
+
+                if (_isalnum(c) || c == '+' || c == '/' || c == '=') return err(EINVAL);
+
+                if (!allow_garbage_chars) return err(EINVAL);
             }
         }
 

tests/libluna/TestBase64.cpp

@@ -130,6 +130,22 @@ TestResult test_base64_skip_garbage_chars_if_allowed()
     test_success;
 }
 
+TestResult test_base64_skip_garbage_chars_if_allowed_after_padding()
+{
+    auto rc = Base64::decode_string("YWJjZA==\n?-"_sv, true);
+    if (rc.has_error())
+    {
+        validate(rc.error() != EINVAL);
+        return rc.release_error();
+    }
+
+    auto decoded = rc.release_value();
+
+    validate(decoded.view() == "abcd"_sv);
+
+    test_success;
+}
+
 Result<void> test_main()
 {
     test_prelude;
@@ -144,6 +160,7 @@ Result<void> test_main()
     run_test(test_base64_disallow_characters_after_padding);
     run_test(test_base64_disallow_garbage_chars_by_default);
     run_test(test_base64_skip_garbage_chars_if_allowed);
+    run_test(test_base64_skip_garbage_chars_if_allowed_after_padding);
 
     return {};
 }