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Compute PDU length from MBAP Length field directly. #642

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TaikiAkita wants to merge 2 commits into from
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Compute PDU length from MBAP Length field directly. #642

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3b839c9
Compute PDU length from MBAP Length field directly.
TaikiAkita Jun 19, 2022
cb5aa38
Validate the PDU length computes from MBAP Length field.
TaikiAkita Jun 21, 2022
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16 changes: 16 additions & 0 deletions src/modbus.c
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Original file line number Diff line number Diff line change
Expand Up @@ -436,6 +436,22 @@ int _modbus_receive_msg(modbus_t *ctx, uint8_t *msg, msg_type_t msg_type)
if (length_to_read == 0) {
switch (step) {
case _STEP_FUNCTION:
/* HACK: derive the length from the Length field of the MBAP. */
if (ctx->backend->backend_type == _MODBUS_BACKEND_TYPE_TCP) {
int mbap_length_field = (
(((int)(msg[4])) << 8) |
((int)(msg[5]))
);
if (mbap_length_field < 2) {
errno = EMBBADDATA;
_error_print(ctx, "need more data");
return -1;
}
length_to_read = mbap_length_field - 2 /* the unit identifier and function code. */;
step = _STEP_DATA;
break;
}

/* Function code position */
length_to_read = compute_meta_length_after_function(
msg[ctx->backend->header_length],
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