ggwave : fix out-of-bounds access in ggwave_decode

Also, provide a memory-safe overload called ggwave_ndecode()
The overload takes an extra parameter that specifies the size of
the output buffer and thus limits the size of the Rx payload that can be
decoded and stored.

include/ggwave/ggwave.h

@@ -189,8 +189,12 @@ extern "C" {
     //   If the return value is -1 then there was an error during the decoding process.
     //   Usually can occur if there is a lot of background noise in the audio.
     //
-    //   If the return value is greater than 0, then there will be that number of bytes
-    //   decoded in the outputBuffer
+    //   If the return value is greater than 0, then there are that number of bytes decoded.
+    //
+    //   IMPORTANT:
+    //   The actual bytes written to outputBuffer are 1 more than the return value since this
+    //   function writes one extra null terminating byte at the end of the payload. This is for
+    //   convenience, since the most common type of payload is text messages.
     //
     //   Example:
     //
@@ -218,6 +222,22 @@ extern "C" {
             int dataSize,
             char * outputBuffer);
 
+    // Memory-safe overload of ggwave_decode
+    //
+    //   outputSize     - optionally specify the size of the output buffer
+    //
+    //   If the return value is -2 then the provided outputBuffer was not big enough to
+    //   store the decoded data.
+    //
+    //   See ggwave_decode for more information
+    //
+    GGWAVE_API int ggwave_ndecode(
+            ggwave_Instance instance,
+            const char * dataBuffer,
+            int dataSize,
+            char * outputBuffer,
+            int outputSize);
+
 #ifdef __cplusplus
 }
 
@@ -388,6 +408,8 @@ class GGWave {
     void setRxProtocols(const RxProtocols & rxProtocols) { m_rxProtocols = rxProtocols; }
     const RxProtocols & getRxProtocols() const { return m_rxProtocols; }
 
+    int lastRxDataLength() const { return m_lastRxDataLength; }
+
     const TxRxData & getRxData()            const { return m_rxData; }
     const RxProtocol & getRxProtocol()      const { return m_rxProtocol; }
     const RxProtocolId & getRxProtocolId()  const { return m_rxProtocolId; }

src/ggwave.cpp

@@ -116,15 +116,54 @@ int ggwave_decode(
 
     ggWave->decode(cbWaveformInp);
 
-    // todo : avoid allocation
+    // TODO : avoid allocation
     GGWave::TxRxData rxData;
 
     auto rxDataLength = ggWave->takeRxData(rxData);
     if (rxDataLength == -1) {
         // failed to decode message
         return -1;
     } else if (rxDataLength > 0) {
-        std::copy(rxData.begin(), rxData.end(), outputBuffer);
+        memcpy(outputBuffer, rxData.data(), rxDataLength + 1);
+    }
+
+    return rxDataLength;
+}
+
+extern "C"
+int ggwave_ndecode(
+        ggwave_Instance instance,
+        const char * dataBuffer,
+        int dataSize,
+        char * outputBuffer,
+        int outputSize) {
+    // TODO : avoid duplicated code
+    GGWave * ggWave = (GGWave *) g_instances[instance];
+
+    GGWave::CBWaveformInp cbWaveformInp = [&](void * data, uint32_t nMaxBytes) -> uint32_t {
+        uint32_t nCopied = std::min((uint32_t) dataSize, nMaxBytes);
+        std::copy(dataBuffer, dataBuffer + nCopied, (char *) data);
+
+        dataSize -= nCopied;
+        dataBuffer += nCopied;
+
+        return nCopied;
+    };
+
+    ggWave->decode(cbWaveformInp);
+
+    // TODO : avoid allocation
+    GGWave::TxRxData rxData;
+
+    auto rxDataLength = ggWave->takeRxData(rxData);
+    if (rxDataLength == -1) {
+        // failed to decode message
+        return -1;
+    } else if (rxDataLength + 1 > outputSize) {
+        // the outputBuffer is not big enough to store the data
+        return -2;
+    } else if (rxDataLength > 0) {
+        memcpy(outputBuffer, rxData.data(), rxDataLength + 1);
     }
 
     return rxDataLength;

tests/test-ggwave.c

@@ -22,15 +22,23 @@ int main() {
 
     int ret;
     const char * payload = "test";
-    char decoded[256];
+    char decoded[16];
 
     int n = ggwave_encode(instance, payload, 4, GGWAVE_TX_PROTOCOL_AUDIBLE_FAST, 50, NULL, 1);
     char waveform[n];
 
-    ret = ggwave_encode(instance, payload, 4, GGWAVE_TX_PROTOCOL_AUDIBLE_FAST, 50, waveform, 0);
-    CHECK(ret > 0);
+    int ne = ggwave_encode(instance, payload, 4, GGWAVE_TX_PROTOCOL_AUDIBLE_FAST, 50, waveform, 0);
+    CHECK(ne > 0);
 
-    ret = ggwave_decode(instance, waveform, sizeof(signed short)*ret, decoded);
+    // note that the output buffer has to be 1 extra byte longer than the expected payload
+    ret = ggwave_ndecode(instance, waveform, sizeof(signed short)*ne, decoded, 4);
+    CHECK(ret == -2); // fail
+
+    ret = ggwave_ndecode(instance, waveform, sizeof(signed short)*ne, decoded, 5);
+    CHECK(ret == 4);  // success
+
+    // unsafe method - will write the decoded output to the output buffer regardless of the size
+    ret = ggwave_decode(instance, waveform, sizeof(signed short)*ne, decoded);
     CHECK(ret == 4);
 
     CHECK(strcmp(decoded, payload) == 0);