Potential bug: Presentation underflow

[rnd-ash]

Hey!

As you know, my own CBFParser shares a lot of similar code with CaesarSuite.

However, as CBFParser is built in Rust and not C#, I thought I would alert you to a potential bug (Not tested in Caesarsuite), which trips Rusts debug build underflow detection. The code in question is shared between CBFParser and CaesarSuite.

Parsing CPC301T.CBF panics like so:

Error:
 × Main thread panicked.
 ├─▶ at C:\Users\___\.cargo\git\checkouts\openvehiclediag-
 │   39994c2d0095f930\09719e6\CBFParser\src\diag\preparation.rs:139:35
 ╰─▶ attempt to subtract with overflow
 help: set the `RUST_BACKTRACE=1` environment variable to display a backtrace.

This panic is caused by this line:
https://github.com/rnd-ash/OpenVehicleDiag/blob/egui_ecu_diag/CBFParser/src/diag/preparation.rs#L139

result_bit_size =(((byte_count & 0xFF) - (self.bit_pos / 8)) * 8) as i32;

So essentially, byte_count & 0xFF is somehow smaller than bit_pos, which triggers the underflow detection, breaking the parser.

I noticed that CaesarSuite shares almost identical code here:
https://github.com/jglim/CaesarSuite/blob/main/Caesar/Caesar/DiagPreparation.cs#L246

int resultByteSize = (ParentDiagService.RequestBytes.Length & 0xFF) - (BitPosition / 8);

So I've raised this issue here in case this is something that CaesarSuite does not handle correctly and instead continues silently (I don't program in C# so not sure if it has a similar Over/underflow detection system :) )

[jglim]

The library is built without /unsafe so it automatically includes runtime bounds checking that is provided by .net. In the case of a malformed file that leads to this issue, the runtime will make angry noises and exit without memory corruption or losing control flow.

For now, I can't replicate this as I do not have a copy of CPC301T.CBF. I will not be making any changes, but will keep an eye out for this particular case.

Thanks for looking out for this project ^^

[N0cynym]

If you have a Xentry installation you'll find this file at this path Program Files\Mercedes-Benz\DAS\comdat\evobus\cbf