是否可以在 WebAssembly 中使用 Rust 进行动态链接？

Question

我正在使用 wasm-bindgen 在 Rust 中为 web 制作图灵完备的 DSL。我希望能够从 Web 下载任意 WASM 代码，然后在我的 DSL 中使用该文件中的函数。我想到的是某种与 dlopen 等效的动态链接。

我不知道如何实际实现这一点。

通过阅读WebAssembly docs，我得到的印象是它确实应该是可能的，但我的知识不足以理解本文档中的过程细节。

在 wasm-bindgen 参考中有一个chapter 详细说明了如何从 WebAssembly 模块内部实例化 WebAssembly 模块！，但这似乎是通过 JavaScript 来实现的，这似乎不是最理想的，而不是 WebAssembly文档描述。

在 js-sys 中，可以从任意字符串创建 JavaScript 函数，但这本质上是从 JavaScript 端调用 Function(/* some arbitrary string */)，这似乎不是最理想的，而不是 WebAssembly 文档所描述的。

是否有可能或有其他更合适的方式来实现我的目标？

Answer 1

不要在生产代码中使用它（它是纸牌屋），我只是将我的研究分享给其他也在围绕这个主题进行修补的人。这将允许您在运行时任意更改绑定。今天似乎在每个优化级别都能正常工作，但谁能知道它明天是否能工作。如需真正的支持，请参阅 sbc100 的回答。

/// If at any point you call this function directly, it will probably do exactly what its
/// implementation here is, as it should compile to a "call" instruction when you directly call.
/// That instruction does not appear to be impacted by changes in the function table.
pub fn replace_me() {
    // The function body must be unique, otherwise the optimizer will deduplicate
    // it and you create unintended impacts. This is never called, it's just unique.
    force_call_indirect_for_function_index(replace_me as u32);
}

/// We'll replace the above function with this function for all "call indirect" instructions.
pub fn return_50() -> u64 {
    50
}

/// This allows us to force "call indirect". Both no_mangle and inline(never) seem to be required.
/// You could simply strip every invocation of this function from your final wasm binary, since
/// it takes one value and returns one value. It's here to stop optimizations around the function
/// invocation by index.
#[inline(never)]
#[no_mangle]
fn force_call_indirect_for_function_index(function_index: u32) -> u32 {
    function_index
}

/// Inline this or make it generic or whatever you want for ease of use, this is your calling code.
/// Note that the function index you use does not need to have the same signature as the function it
/// is replaced with.
///
/// This seems to compile to:
/// i32.const, call force_call_indirect_for_function_index, call indirect.
///
/// So stripping force_call_indirect_for_function_index invocations would make this as efficient
/// as possible for a dynamically linked wasm call I think.
fn call_replace_me_indirectly() -> u64 {
    unsafe {
        std::mem::transmute::<u32, fn() -> u64>(force_call_indirect_for_function_index(
            replace_me as u32,
        ))()
    }
}

/// Replaces replace_me with return_50 in the wasm function table. I've tested that this works with
/// Functions exported from other wasm modules. For this example, I'll use a function defined in
/// this module (return_50).
fn replace_replace_me() {
    let function_table: js_sys::WebAssembly::Table = wasm_bindgen::function_table()
        .dyn_into::<js_sys::WebAssembly::Table>()
        .expect("I'm going to find you...");
    let function = function_table
        .get(return_50 as u32)
        .expect("I know you're in there...");
    function_table
        .set(replace_me as u32, &function)
        .expect("It's not unsafe, but is it undefined behavior?");
}

/// Mangles "replace_me" call indirection invocations, and returns 50.
pub fn watch_me() -> u64 {
    replace_replace_me();
    call_replace_me_indirectly()
}

Answer 2

llvm/lld 对 WebAssembly 的动态链接支持是 still a work in progress。我想 Rust 中的动态链接目前在更普遍的 llvm/lld 中的动态链接支持上被阻止。