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[modbus] CRC scan all unknown function codes (#18483)

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
Bonne Eggleston
2026-08-18 11:09:12 -05:00
committed by GitHub
co-authored by Claude Opus 4.8
parent 92f55f721f
commit 285a508e09
5 changed files with 232 additions and 12 deletions
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esphome/components/modbus/modbus.cpp
+22 -11
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@@ -219,14 +219,25 @@ void ModbusServerHub::parse_modbus_frames() {
this->clear_rx_buffer_(LOG_STR("timeout after partial response"), true);
}
uint16_t Modbus::find_custom_frame_end_(uint16_t min_length) const {
// Custom functions could be any length - we have to rely on the CRC to determine completeness.
uint16_t Modbus::find_frame_end_by_crc_(uint16_t min_length) const {
// Unknown-length functions (user-defined codes, unimplemented management codes, unassigned values)
// could be any length - we have to rely on the CRC to determine completeness.
// If a CRC match is never found, the buffer will eventually overflow and be cleared.
const uint8_t *raw = &this->rx_buffer_[0];
const size_t size = this->rx_buffer_.size();
for (uint16_t len = min_length; len <= std::min(size, size_t(MAX_FRAME_SIZE)); len++) {
if (crc16(raw, len) == 0)
return len;
const auto max_len = static_cast<uint16_t>(std::min(size, size_t(MAX_FRAME_SIZE)));
if (min_length > max_len)
return 0;
// The Modbus CRC (poly 0xa001, refin/refout false) keeps its running state in the returned value,
// so we seed once over the first min_length bytes and extend one byte at a time instead of
// recomputing the whole prefix for every candidate length.
uint16_t crc = crc16(raw, min_length);
if (crc == 0)
return min_length;
for (uint16_t len = min_length; len < max_len; len++) {
crc = crc16(&raw[len], 1, crc);
if (crc == 0)
return len + 1;
}
return 0;
}
@@ -241,11 +252,11 @@ bool Modbus::parse_modbus_server_frame_() {
uint8_t address = this->rx_buffer_[0];
uint8_t function_code = this->rx_buffer_[1];
if (helpers::is_function_code_custom(function_code)) {
frame_length = this->find_custom_frame_end_(frame_length);
if (helpers::is_function_code_unknown_length(function_code)) {
frame_length = this->find_frame_end_by_crc_(frame_length);
if (frame_length == 0)
return size < MAX_FRAME_SIZE; // Continue to parse until we hit max size
ESP_LOGD(TAG, "User-defined function %02X found", function_code);
ESP_LOGD(TAG, "Unknown-length function %02X found", function_code);
} else {
if (crc16(&this->rx_buffer_[0], frame_length) != 0)
return false;
@@ -272,11 +283,11 @@ bool ModbusServerHub::parse_modbus_client_frame_() {
uint8_t address = this->rx_buffer_[0];
uint8_t function_code = this->rx_buffer_[1];
if (helpers::is_function_code_custom(function_code)) {
frame_length = this->find_custom_frame_end_(frame_length);
if (helpers::is_function_code_unknown_length(function_code)) {
frame_length = this->find_frame_end_by_crc_(frame_length);
if (frame_length == 0)
return size < MAX_FRAME_SIZE; // Continue to parse until we hit max size
ESP_LOGD(TAG, "User-defined function %02X found", function_code);
ESP_LOGD(TAG, "Unknown-length function %02X found", function_code);
} else {
if (crc16(&this->rx_buffer_[0], frame_length) != 0)
return false;