Limit the number of frames in backtrace collection

[adambratschikaye]

Add Config::wasm_backtrace_max_frames option to limit the number of frames collected in backtraces and set the default at 20. This helps prevent expensive work from very deep call stacks.

Setting the value to 0 is the same as disabling backtraces and so this change deprecates Config::wasm_backtrace.

Addresses #5052

[fitzgen]

Thanks! A couple nitpicks inline below before we merge this.

[github-actions]

Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to
complete this check list:

• If you added a new Config method, you wrote extensive documentation for
  it.

  Details

  Our documentation should be of the following form:

  Short, simple summary sentence.
  
  More details. These details can be multiple paragraphs. There should be
  information about not just the method, but its parameters and results as
  well.
  
  Is this method fallible? If so, when can it return an error?
  
  Can this method panic? If so, when does it panic?
  
  # Example
  
  Optional example here.
  

• If you added a new Config method, or modified an existing one, you
  ensured that this configuration is exercised by the fuzz targets.

  Details

  For example, if you expose a new strategy for allocating the next instance
  slot inside the pooling allocator, you should ensure that at least one of our
  fuzz targets exercises that new strategy.

  Often, all that is required of you is to ensure that there is a knob for this
  configuration option in wasmtime_fuzzing::Config (or one
  of its nested structs).

  Rarely, this may require authoring a new fuzz target to specifically test this
  configuration. See our docs on fuzzing for more details.

• If you are enabling a configuration option by default, make sure that it
  has been fuzzed for at least two weeks before turning it on by default.

---

Details

To modify this label's message, edit the .github/label-messager/wasmtime-config.md file.

To add new label messages or remove existing label messages, edit the
.github/label-messager.json configuration file.

Learn more.

[adambratschikaye]

Regarding the note on fuzzing - I guess we don't need to do anything because fuzzing always used the default of enabling backtraces and that isn't changing.

[fitzgen]

Thanks!

[adambratschikaye]

@fitzgen Looks like there was a fuzz testing failure and a docs failure when merging. They should be fixed now.

[github-actions]

Subscribe to Label Action

cc @fitzgen

Details

This issue or pull request has been labeled: "fuzzing", "wasmtime:api", "wasmtime:config"

Thus the following users have been cc'd because of the following labels:

• fitzgen: fuzzing

To subscribe or unsubscribe from this label, edit the .github/subscribe-to-label.json configuration file.

Learn more.

[fitzgen]

Thanks! Just one more small thing before we land this. Thanks for your patience!

[fitzgen]

Rather than hard-coding a second copy of the default, which means this test will start failing if we ever change the default, can we explicitly set a limit and rely on that in the test here? Even better would be to generate an arbitrary limit as part of the wasmtime_fuzzing::generators::Stacks struct, so we fuzz different stack trace limits as well.

Should just require adding a pub limit: Option<NonZeroUsize>, field here:

wasmtime/crates/fuzzing/src/generators/stacks.rs

Lines 17 to 23 in f641800

	/// Generate a Wasm module that keeps track of its current call stack, to
	/// compare to the host.
	#[derive(Debug)]
	pub struct Stacks {
	funcs: Vec<Function>,
	inputs: Vec<u8>,
	}

And then adding something like let limit = NonZeroUsize::new(u.int_in_range(0..=256)?); inside here:

wasmtime/crates/fuzzing/src/generators/stacks.rs

Lines 40 to 47 in f641800

	impl<'a> Arbitrary<'a> for Stacks {
	fn arbitrary(u: &mut Unstructured<'a>) -> Result<Self> {
	let funcs = Self::arbitrary_funcs(u)?;
	let n = u.len().min(200);
	let inputs = u.bytes(n)?.to_vec();
	Ok(Stacks { funcs, inputs })
	}
	}

And finally creating the config with the limit and an engine from that config inside this current function, rather than using a default config and default engine:

let mut config = Config::new();
config.wasm_backtrace_max_frames(stacks.limit);
let engine = Engine::new(&config)?;

[adambratschikaye]

No problem, I made those changes.

[fitzgen]

Thanks!

[adambratschikaye]

Thanks @fitzgen