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RAW: Problem when including length field in LENGTHTO() [message #1801091]

Sun, 13 January 2019 11:56

Harald Welte

Messages: 140
Registered: July 2017
Location: Berlin, Germany

Senior Member

I've been trying to express USB Standard descriptors using the TITAN RAW codec (TITAN 6.5.0) and stumbled upon some behavior that I cannot explain.

Let's take the device descriptor as an example. I'm using '12011001FF11FF088A131700780000000101'O as a real-world example of the USB-attached fingerprint reader built into my laptop.

The following definition provides a working decode:

type integer uint8_t (0..255) with { variant "unsigned 8 bit" };
type octetstring OCT1 length(1) with { variant "FIELDLENGTH(1)" };
type octetstring OCT2 length(2) with { variant "FIELDLENGTH(2)" };
type record USB_DeviceDescriptor {
        uint8_t         bLength,
        OCT1            bDescriptorType,
        OCT2            bcdUSB,
        OCT1            bDeviceClass,
        OCT1            bDeviceSubClass,
        OCT1            bDeviceProtocol,
        uint8_t         bMaxPacketSize0,
        OCT2            idVendor,
        OCT2            idProduct,
        OCT2            bcdDevice,
        uint8_t         iManufacturer,
        uint8_t         iProduct,
        uint8_t         iSerialNumber,
        uint8_t         bNumConfigurations
};

Which renders the following correctly decoded output:

{ bLength := 18, bDescriptorType := '01'O, bcdUSB := '0110'O, bDeviceClass := 'FF'O, bDeviceSubClass := '11'O, bDeviceProtocol := 'FF'O, bMaxPacketSize0 := 8, idVendor := '138A'O, idProduct := '0017'O, bcdDevice := '0078'O, iManufacturer := 0, iProduct := 0, iSerialNumber := 1, bNumConfigurations := 1 }

however, if I amend the type definition with LENGTHTO(), like

type record USB_DeviceDescriptor {
        uint8_t         bLength,
        OCT1            bDescriptorType,
        OCT2            bcdUSB,
        OCT1            bDeviceClass,
        OCT1            bDeviceSubClass,
        OCT1            bDeviceProtocol,
        uint8_t         bMaxPacketSize0,
        OCT2            idVendor,
        OCT2            idProduct,
        OCT2            bcdDevice,
        uint8_t         iManufacturer,
        uint8_t         iProduct,
        uint8_t         iSerialNumber,
        uint8_t         bNumConfigurations
} with { variant (bLength) "LENGTHTO(bLength,bDescriptorType,bcdUSB,bDeviceClass,bcdUSB,bDeviceClass,
                                     bDeviceSubClass,bDeviceProtocol,bMaxPacketSize0,idVendor,idProduct,
                                     bcdDevice,iManufacturer,iProduct,iSerialNumber,bNumConfigurations)" };

then decoding fails with:

Dynamic test case error: While RAW-decoding type '@USB_Types.USB_DeviceDescriptor': There are not enough bits in the buffer to decode type @USB_Types.USB_DeviceDescriptor.iProduct (needed: 8, found: 0).

This is odd. The Titan Techincal Reference Guide clearly states that including the length field in LENGTHTO() is permitted. Yet, when I do so, it somehow fails to decode the message - despite the length value being 18 and the message being exactly 18 bytes long.

Even amending the uint8_t definition with variant "FIELDLENGTH(8)" doesn't change anything.

Any ideas? Am I missing something? Thanks!

[Updated on: Sun, 13 January 2019 12:05]

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Re: RAW: Problem when including length field in LENGTHTO() [message #1801092 is a reply to message #1801091]

Sun, 13 January 2019 12:19

Harald Welte

Messages: 140
Registered: July 2017
Location: Berlin, Germany

Senior Member

Nevermind the noise. The problem was that I listed some fields twice in:

} with { variant (bLength) "LENGTHTO(bLength,bDescriptorType,bcdUSB,bDeviceClass,bcdUSB,bDeviceClass,
                                     bDeviceSubClass,bDeviceProtocol,bMaxPacketSize0,idVendor,idProduct,
                                     bcdDevice,iManufacturer,iProduct,iSerialNumber,bNumConfigurations)" };

Specifically, bcdUSB and bDeviceClass were repeated.

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