Implementing an interposer

I wanted to use wasmtime-py to implement a tool similar to ltrace that intercepts WASI calls. Here’s the key part of my code:

def trace(name, orig):
	def trace_fn(*args):
		rets = orig(*args)
		print("{}{!r}->{!r}\n".format(name, args, rets))
		return rets
	return trace_fn
wasi = wasmtime.WasiInstance(store, "wasi_snapshot_preview1", wasi_cfg)
imports = []
for import_ in module.imports():
	if import_.module() == "wasi_snapshot_preview1":
		export = wasi.bind(import_)
		import_type = import_.type().func_type()
		wrapper = wasmtime.Func(store, import_type, trace(import_.name(), export))
		imports.append(wrapper.as_extern())
instance = wasmtime.Instance(module, imports)
instance.get_export("_start")()

However, this crashes almost instantly with a nested trap. I think this happens because the WASI implementation is internally using something like access_caller=True in order to access the memory, since the pointer-like WASI arguments are just indexes into that. But it seems like there’s no way to pass the Caller along.

Is there a good way to implement such functionality regardless?

[alexcrichton]

I don’t believe the C API currently ever initializes the log levels, but that seems plausible to add! I’ll file a PR for that

[alexcrichton]

Ok!