Validate Modbus frame size before writing the ADU buffer#985
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modbus_message wrote register values into the fixed-size adu[] buffer before any length validation; the only size check runs later in modbus_send_rtu. A register count above MODBUS_MAX_REGISTERS overran both the caller's values[] array and adu[], and even a multi-register write within the register limit could exceed MODBUS_MAX_ADU_SIZE and write past the buffer. Reject requests exceeding MODBUS_MAX_REGISTERS, and reject multi-register write frames larger than MODBUS_MAX_ADU_SIZE, before any adu[] write. Single-register reads and writes are unaffected.
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modbus_message wrote register values into the fixed-size adu[] buffer before any length validation; the only size check runs later in modbus_send_rtu. A register count above MODBUS_MAX_REGISTERS overran both the caller's values[] array and adu[], and even a multi-register write within the register limit could exceed MODBUS_MAX_ADU_SIZE and write past the buffer.
Reject requests exceeding MODBUS_MAX_REGISTERS, and reject multi-register write frames larger than MODBUS_MAX_ADU_SIZE, before any adu[] write. Single-register reads and writes are unaffected.