Add snake demo for Sonata board

[AlexJones0]

I've implemented the Snake demo for the Sonata board to show off the CHERI capabilities.

Currently it is implemented as a fully functional game of Snake, where the snake attempting to exit the boundaries of the display will trigger a CHERI Capability violation, which is used to detect the end of the game. The bottom and right side of the screen trigger bounds violations as we try to access outside of a 2D array representing the game. The top and left side of the screen trigger tag violations due to using a negative index on an array, which is undefined behavior. The snake colliding with itself or winning the game currently just ends the game normally, without any CHERI capability violation.

The demo should be quite customisable - you can change things like the game speed, size and appearance using the constants defined near the top of the file to tweak it as you like.

Would appreciate any feedback regarding where improvements can be made in the code - I'm still quite unfamiliar with many of the features of C++.

[HU90m]

Only did a quick skim, but looks good!

nix run .#clang-lint fix

[AlexJones0]

I've fixed up all the raised comments now and changed a couple things based on feedback:

• The snake can now move in any direction when it is only 1 tile long, as this was causing some confusion
• Rather than ending the game normally, a CHERI capability violation now occurs when the snake collides with itself. This is just done using an arbitrary out of bounds check - we originally had this working with a PLoad violation but that was causing an issue with the hacky way that we're modifying the PCC and restoring the registers to return back to the program from the error handling, so it was decided to leave that for now.
• Rather than the original xor_shift functionality, the program now uses the superior PRNG entropy driver from https://github.com/lowRISC/cheriot-rtos/tree/ethernet (specifically lowRISC/cheriot-rtos@32cd341).
  • This involved bumping the cheriot-rtos submodule to that commit because the relevant PR (Implement Ethernet/entropy driver for Sonata board CHERIoT-Platform/cheriot-rtos#257) has not yet been merged.
• Added some extra clarifying comments on specific parts of code that required some more explanation.

From investigation the CI quality check appears to be failing because nix run .#clang-lint fix is not equipped to deal with changes to the cheriot-rtos submodule. I could just leave the simple randomness using xor_shift but I figure it's better to use the entropy driver to avoid the possibility of forgetting to replace this in the future when that is properly merged.

[AlexJones0]

we originally had this working with a PLoad violation but that was causing an issue with the hacky way that we're modifying the PCC and restoring the registers to return back to the program from the error handling, so it was decided to leave that for now

Just in case anybody needs it for future reference this is how I was manually triggering the CHERI PLoad capability violation:

Capability<Tile *> gameCapability{gameSpace};
gameCapability.permissions() &= PermissionSet::omnipotent().without(Permission::Load);
return gameCapability[nextPosition.y][nextPosition.x] == Tile::SNAKE;