Stream: wasmtime

Topic: Help: memory fault on a simple ToString implementation

Nick Pashkov (Nov 26 2024 at 00:14):

I am working on a little scripting interface I want the users of my program have.
I am using AssemblyScript which is compiled to wasm and I am defining host functions in Rust using wasmtime.

To test it out, I am defining a Vector2 class like this in AssemblyScript:

export declare class Vector2 {
  public x: number;
  public y: number;
  constructor(x: number, y: number);
  public ToString(): string;
}

And this is how I define the ToString function in the host:

linker.func_wrap("env", "Vector2#ToString", |mut caller: Caller<'_, WasiCtx>, ptr: i32| {
    let memory = caller.get_export("memory").unwrap().into_memory().unwrap();
    let mut x_bytes = [0u8; 8];
    let mut y_bytes = [0u8; 8];

    memory
        .read(caller.as_context(), ptr as _, &mut x_bytes)
        .unwrap();

    memory
        .read(caller.as_context(), (ptr + 8) as _, &mut y_bytes)
        .unwrap();

    let x = f64::from_le_bytes(x_bytes);
    let y = f64::from_le_bytes(y_bytes);
    let result_str = format!("[{}, {}]", x, y);
    let len = result_str.as_bytes().len() as i32;
    println!("Result str {} len {}", result_str, len);
    len
}).unwrap();

Everything is going well until I try to console.log the value like this:

import { Vector2 } from "./env";

function main(): void {
  let newVec = new Vector2(1, 2);
  let val = newVec.ToString();
  console.log(val);
}

main();

Gives an error like this

called Result::unwrap() on an Err value: error while executing at wasm backtrace:
0: 0x186 - <unknown>!~lib/rt/common/OBJECT#get:rtSize
1: 0x191 - <unknown>!~lib/string/String#get:length
2: 0xc42 - <unknown>!~lib/wasi_process/writeString
3: 0xda7 - <unknown>!~lib/wasi_process/WritableStream#write<~lib/string/String>
4: 0xdb6 - <unknown>!~lib/wasi_console/wasi_console.log
5: 0xde4 - <unknown>!scripts/index/main
6: 0xe00 - <unknown>!start:scripts/index
7: 0xe92 - <unknown>!~start

Caused by:
0: memory fault at wasm address 0x100000002 in linear memory of size 0x10000
1: wasm trap: out of bounds memory access

I am sure this has to do with how I am accessing the memory, but what is weird is that if I don't console.log the value everything works.

I would appreciate any insights on this issue!

Alex Crichton (Nov 26 2024 at 01:18):

It looks like the error here is happening before your host function is invoked, so this may be a bug in the assemblyscript runtime or compiler. Removing console.log or changing the source can do all sorts of things to compiled output, so if it's a compiler bug then that makes sense as a possible trigger and/or cover-up.

Nick Pashkov (Nov 26 2024 at 05:19):

Hey @Alex Crichton thanks for your reply, the host function is actually invoked because I can see the println statement actually printing something right before the error:

Result str [1, 2] len 6
thread 'main' panicked at pixa/src/scripting/mod.rs:171:14:
called Result::unwrap() on an Err value: error while executing at wasm backtrace:
0: 0x186 - <unknown>!~lib/rt/common/OBJECT#get:rtSize
1: 0x191 - <unknown>!~lib/string/String#get:length
2: 0xc42 - <unknown>!~lib/wasi_process/writeString
3: 0xda7 - <unknown>!~lib/wasi_process/WritableStream#write<~lib/string/String>
4: 0xdb6 - <unknown>!~lib/wasi_console/wasi_console.log
5: 0xde4 - <unknown>!scripts/index/main
6: 0xe00 - <unknown>!start:scripts/index
7: 0xe92 - <unknown>!~start

Caused by:
0: memory fault at wasm address 0x100000002 in linear memory of size 0x10000
1: wasm trap: out of bounds memory access

Looks like just when it is trying to get the length of the console.log parameter, something happens, also worth adding is that logging a normal string works, and so does accessing each one of the vector components:

let newVec = new Vector2(1, 2);
  // Works fine, prints 1.0
  console.log(newVec.x.toString());
  // Works fine, prints 2.0
  console.log(newVec.x.toString());
  // Works as well
  console.log("Hello world");
  // Gives memory trap error
  console.log(newVec.ToString());

Maybe there is something wrong with the host function definition?

Nick Pashkov (Nov 26 2024 at 05:23):

Worth adding also the constructor host function which I defined like this

self.linker
            .func_wrap(
                "env",
                "Vector2#constructor",
                |mut caller: Caller<'_, ScriptStore>, _ptr: i32, x: f64, y: f64| {
                    let memory = caller.get_export("memory").unwrap().into_memory().unwrap();
                    let offset = allocate_memory(16); // 2*8

                    let mut store_context = caller.as_context_mut();

                    memory
                        .write(&mut store_context, offset, &x.to_le_bytes())
                        .expect("Failed to write x to memory");

                    memory
                        .write(&mut store_context, offset + 8, &y.to_le_bytes())
                        .expect("Failed to write y to memory");

                    offset as i32
                },
            )
            .unwrap();

allocate_memory is a function to keep track of the memory offset:

lazy_static! {
    static ref MEMORY_OFFSET: Mutex<usize> = Mutex::new(0);
}

fn allocate_memory(size: usize) -> usize {
    let mut offset = MEMORY_OFFSET.lock().unwrap();
    let current_offset = *offset;
    *offset += size;
    current_offset
}

Nick Pashkov (Nov 29 2024 at 00:03):

I finally fixed it, to anybody else wondering, I stumbled across this issue: https://github.com/AssemblyScript/assemblyscript/issues/2099
Basically I was not running the _start function, which I understand will initialize all the memory.
So indeed this issue is not related to wasmtime but only AssemblyScript specific things

Last updated: Dec 23 2024 at 13:07 UTC