Tockloader and Tockloader-rs produce different outputs

[Joan-DwD]

I noticed a discrepancy between the Rust and Python Tockloade-rs where, for the init_fn_offset value, the Rust version (tockloader-rs/src/tbf_parser/types.rs at line 989) adds the full header size to the offset while the Python version (tockloader/tbfh.py at line 74) does not, and a similar issue exists for the protected_size value in the same files (Rust at line 959 and Python at line 75), which together cause a mismatch in the outputs between the two tools.

[lschuermann]

Hey @Joan-DwD, thanks for raising this issue! At this point, tockloader-rs is not widely used across the core Tock maintainers yet, so tockloader is the canonical tool we'd recommend users to use. But this discrepancy is a good opportunity for us to cross-check both implementations, and what our documentation says about these header fields.

Tagging @bradjc and @george-cosma as the most active maintainers of both tockloader and tockloader-rs respectively.

[eva-cosma]

On tockloader-rs we use a modified version of tock-tbf.

Just to leave a link to the relevant line here:

https://github.com/tock/tock/blob/72f743f88ce7081aa967e194ee391eddeb49e737/libraries/tock-tbf/src/types.rs#L762

In tockloader i also noticed quite an interesting comment. It suggests that the documentation implies something while the kernel uses init_fn_offset in a way that deviates from that documentation:

tockloader/tockloader/tbfh.py

Line 1170 in 7774669

##### NOTE! Based on how things are implemented in the Tock

[lschuermann]

I knew this sounded familiar: at least for the protected_region, we fixed the documentation to match the behavior that is actually implemented in the kernel: tock/tock#3515

[bradjc]

Yes this was not very well specified initially, and different tools ended up doing different things. The language in PR 3515 is what we settled on and is the correct behavior.

[eva-cosma]

It's not immediately clear to me then what the correct definition for it should be, as the mentioned PR refers to protected_size not init_fn_size.

What I am trying to get to the bottom of, is what should both tools display? What is currently displayed by tockloader (without adding the size of the header), or what is returned by tock-tbf?

Quick edit: i forgot this issue also mentions protected_size, oops! My question still stands. Or are the two values linked? @Joan-DwD do you have any insight that might help to clarify?

[Joan-DwD]

Regarding the implementation of get_init_fn_offset (start of the flash region, not the binary userspace), I don't understand why only the header size was added and not the protected trailer size as well. My guess is that init_fn_offset is calculated from the end of the TBF header, therefore, only the header size is added.

The protected region consists of two sub-regions in the Tock Binary Format: header_size (technically the base header size) and protected_trailer_size. Hence, both of them are added in the protected size method.
Tockloader-rs output appears sound.

@george-cosma

[bradjc]

It would be helpful to know what commands you are running and what the output looks like.