Don't treat files as firmware if the firmware identifier is invalid

src/libretro/config/constants.cpp

@@ -159,6 +159,13 @@ bool MelonDsDs::config::IsFirmwareImage(const retro::dirent& file, Firmware::Fir
         return false;
     }
 
+    bool isDsFirmware = loadedHeader.ConsoleType == Firmware::FirmwareConsoleType::DS || loadedHeader.ConsoleType == Firmware::FirmwareConsoleType::DSLite;
+    if (isDsFirmware && strncmp(reinterpret_cast<const char*>(loadedHeader.Identifier.data()), "MAC", 3) != 0) {
+        retro::debug("{} doesn't look like valid NDS firmware (unrecognized identifier {})", file.path, fmt::join(loadedHeader.Identifier, ""));
+        return false;
+    }
+    // TODO: Validate the checksum of the userdata region
+
     memcpy(&header, &buffer, sizeof(buffer));
     return true;
 }

test/cmake/Firmware.cmake

@@ -7,6 +7,19 @@ add_python_test(
     CORE_OPTION "melonds_sysfile_mode=builtin"
 )
 
+### Validating Firmware (see https://github.com/JesseTG/melonds-ds/issues/183)
+add_python_test(
+    NAME "Core doesn't list wrongly-sized files as NDS firmware"
+    TEST_MODULE firmware.core_rejects_wrong_sized_nds_firmware
+    NDS_SYSFILES
+)
+
+add_python_test(
+    NAME "Core doesn't list files with an invalid identifier as NDS firmware"
+    TEST_MODULE firmware.core_rejects_invalid_nds_firmware_id
+    NDS_SYSFILES
+)
+
 ### Ensuring firmware is not overwritten
 
 # See https://github.com/JesseTG/melonds-ds/issues/59

test/python/firmware/core_rejects_invalid_nds_firmware_id.py

@@ -0,0 +1,23 @@
+import os
+from libretro import Session
+
+import prelude
+
+with open(os.environ["NDS_FIRMWARE"], "rb") as f:
+    firmware = bytearray(f.read())
+
+assert len(firmware) == 262144
+firmware[0x8:0xC] = b"NNNN"
+
+badfirmwarepath = os.path.join(prelude.core_system_dir, "badfirmware.bin")
+with open(badfirmwarepath, "wb") as f:
+    f.write(firmware)
+
+session: Session
+with prelude.session() as session:
+    definition = session.options.definitions[b"melonds_firmware_nds_path"]
+    assert definition is not None, "melonds_firmware_nds_path should be defined"
+
+    assert all(v.value is None or b"badfirmware.bin" not in v.value for v in definition.values), \
+        "ARM9_BIOS should not be in melonds_firmware_nds_path values"
+    # Account for the empty entries in the values array

test/python/firmware/core_rejects_wrong_sized_nds_firmware.py

@@ -0,0 +1,17 @@
+import os
+from libretro import Session
+
+import prelude
+
+session: Session
+with prelude.session() as session:
+    definition = session.options.definitions[b"melonds_firmware_nds_path"]
+    assert definition is not None, "melonds_firmware_nds_path should be defined"
+
+    assert os.environ["ARM7_BIOS"] is not None
+
+    arm9name = os.path.basename(os.environ["ARM9_BIOS"]).encode()
+    assert arm9name is not None and arm9name != b""
+    assert all(v.value is None or arm9name not in v.value for v in definition.values), \
+        "ARM9_BIOS should not be in melonds_firmware_nds_path values"
+    # Account for the empty entries in the values array