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libdyld

README.md

What's this?

A runtime linker for WebAssembly Dynamic Linking.

TODO

  • Have a cache of loaded modules (struct module) so that they can be actually shared among programs. A simple mechanism for a limited list of libraries (eg. libc) might be good enough.

Portability notes

While this library is implemented using the toywasm API directly, it should be straightforward to port it to other runtime APIs like wasm-c-api except a few things listed below.

dylink.0 custom section

The wasm-c-api doesn't provide a way to query the contents of custom sections. You need something runtime-specific.

PLT trampoline

Maybe simply creating host functions (with wasm_func_new_with_env) and making it call the corresponding wasm function (with wasm_func_call) is enough for most cases. The approach is not safe if the call is made with tail call instructions though. Also, with the exception-handling proposal, such a host function need to propagate execeptions approprately. As of writing this, wasm-c-api doesn't have an API to deal with exceptions.

For toywasm, we use the restart mechanism to call the function to maintain the tail call guarantee. As it doesn't leave host frames, it doesn't interfere exceptions either. See dyld_plt.c.

WASI and other host functions, including our dlopen-like API

The import/export API of wasm-c-api is a bit low-level and cumbersome to use.

Also, the API to link host functions like WASI varies among runtimes. Eg. wasmer-c-api-wasi, wasmtime-c-api-wasi

Diagnostic

If you see the error like the following when trying to load a PIE executable, please make sure your executable imports env.memory as specified in WebAssembly Dynamic Linking. (eg. via --import-memory wasm-ld option.)

No entry for import env:__indirect_function_table

This error message is admittedly cryptic and probably needs some explanations. In short, it's the result of an attempt to load an PIE executable as a non-PIE executable.

We support both of PIE and non-PIE executables. To determine if the executable is PIE or not, we make a guess by checking if the executable imports env.memory or not. If it does, we assume PIE. Otherwise, non-PIE. We provide env.__indirect_function_table and other resources only for PIE executables. (Non-PIE executables should provide them by exporting them.)