Attestation: free JSON from the Wasm module heap

[JamesMenetrey]

Dear developers,

The JSON evidence is allocated on the module instance heap, but no API was given to dispose of this memory buffer.
The sample mentions using the function free, which (I think) behaves differently depending on the execution context.
Typically, using the attestation API in a loop in a complex program (AOT) leads to a sudden stop of the program (without error output).

After investigation, I discovered that calling free in the Wasm app to dispose of the JSON evidence was the issue.
While the logic of memory allocation is quite complex to understand and have a full picture of it, my understanding is that calling free from the Wasm app may be different than calling the module_free macro in the runtime.
Since the JSON evidence is allocated using module_malloc, the corresponding free logic must be called accordingly.

This fix provides a new function called librats_dispose_evidence_json, enabling to free the JSON evidence directly from the Wasm app.

[wenyongh]

LGTM

[JamesMenetrey]

Thanks! Quick question to follow up on this fix: could you please briefly describe how the malloc/free functions are mapped from the Wasm apps to the runtime? Is there a general handler for these functions we can inspect in the runtime to understand how the runtime handles them?

Thanks!

[wenyongh]

@JamesMenetrey please refer to
https://github.com/bytecodealliance/wasm-micro-runtime/blob/main/doc/memory_tune.md
and
https://bytecodealliance.github.io/wamr.dev/blog/understand-the-wamr-heaps/