Clarification request: findfirstFSync() reads 4 bytes when dataBlockSize < 4 in split FSYNC test

[awmorgan]

Hello,

I am looking for clarification on the expected behavior of the FSYNC detection logic in the OpenCSD frame deformatter when the input stream is provided in small chunks.

While analyzing subtest 2 of runHSyncFSyncTest() in frame_demux_test.cpp:

// test fsync broken across 2 input blocks
printSubTestName(2, "HSyncFSync split frame");
resetDecoder(failed);
checkDataPathValue(
    pDecoder->TraceDataIn(OCSD_OP_DATA, index, 2, buf_hsync_fsync, &processed),
    failed);

I observed the following behavior inside TraceFmtDcdImpl::findfirstFSync():

When m_in_block_size == 2, the (m_in_block_size - 3)) is 0xffffffff due to unsigned underflow, causing the loop to execute even though fewer than 4 bytes are available. Inside the loop, a 4-byte load is performed: if (*((uint32_t *)(dataPtr)) == FSYNC_PATTERN) even though the current TraceDataIn() call only provided 2 bytes.

uint32_t TraceFmtDcdImpl::findfirstFSync()
{
    uint32_t processed = 0;
    const uint32_t FSYNC_PATTERN = 0x7FFFFFFF;    // LE host pattern for FSYNC	 
    const uint8_t *dataPtr = m_in_block_base;

    while (processed < (m_in_block_size - 3))
    {
        if (*((uint32_t *)(dataPtr)) == FSYNC_PATTERN)
        {
            m_frame_synced = true;
            break;
        }
        processed++;
        dataPtr++;
    }
    return processed;

Is findfirstFSync() expected to be able to read beyond the provided dataBlockSize?

Thanks!