wasmtime/compile/code_builder/

compile_time_builtins.rs

use super::*;

impl<'a> CodeBuilder<'a> {
    pub(crate) fn get_compile_time_builtins(&self) -> &HashMap<Cow<'a, str>, Cow<'a, [u8]>> {
        &self.compile_time_builtins
    }

    pub(super) fn compose_compile_time_builtins<'b>(
        &self,
        main_wasm: &'b [u8],
    ) -> Result<Cow<'b, [u8]>> {
        if self.get_compile_time_builtins().is_empty() {
            return Ok(main_wasm.into());
        }

        let imports = self.check_imports_for_compile_time_builtins(&main_wasm)?;
        if imports.is_empty() {
            drop(imports);
            return Ok(main_wasm.into());
        }

        let tempdir = tempfile::TempDir::new().context("failed to create a temporary directory")?;
        let deps = tempdir.path().join("_deps");
        std::fs::create_dir(&deps)
            .with_context(|| format!("failed to create directory: {}", deps.display()))?;

        let main_wasm_path = tempdir.path().join("_main.wasm");
        std::fs::write(&main_wasm_path, &main_wasm)
            .with_context(|| format!("failed to write to file: {}", main_wasm_path.display()))?;

        let mut config = wasm_compose::config::Config::default();
        for (name, bytes) in self.get_compile_time_builtins() {
            let name: &str = &*name;
            if !imports.contains(&name) {
                continue;
            }

            let mut path = deps.join(Path::new(name));
            path.set_extension("wasm");

            std::fs::write(&path, &bytes)
                .with_context(|| format!("failed to write to file: {}", path.display()))?;

            config
                .dependencies
                .insert(name.to_string(), wasm_compose::config::Dependency { path });
        }

        let composer = wasm_compose::composer::ComponentComposer::new(&main_wasm_path, &config);
        let composed = composer.compose()?;
        Ok(composed.into())
    }

    /// Check that the main Wasm doesn't import unsafe intrinsics, keeping the
    /// TCB to just the compile-time builtins' implementation.
    ///
    /// Returns the Wasm's top-level instance imports for `wasm-compose`
    /// configuration.
    fn check_imports_for_compile_time_builtins<'b>(
        &self,
        main_wasm: &'b [u8],
    ) -> Result<crate::hash_set::HashSet<&'b str>, Error> {
        let intrinsics_import = self.unsafe_intrinsics_import.as_deref().ok_or_else(|| {
            anyhow!("must configure the unsafe-intrinsics import when using compile-time builtins")
        })?;

        let mut instance_imports = crate::hash_set::HashSet::new();
        let parser = wasmparser::Parser::new(0);
        let mut level = 0;

        for payload in parser.parse_all(main_wasm) {
            match payload? {
                wasmparser::Payload::Version { .. } => {
                    level += 1;
                }
                wasmparser::Payload::End(_) => {
                    level -= 1;
                }
                wasmparser::Payload::ComponentImportSection(imports) if level == 1 => {
                    for imp in imports.into_iter() {
                        let imp = imp?;
                        // Ideally we would simply choose a new import name that
                        // doesn't conflict with the main Wasm's imports and
                        // plumb that through to the compile-time builtins
                        // regardless of the import name that they use, but
                        // unfortunately the `wasm-compose` API is not powerful
                        // enough for us to do all that.
                        ensure!(
                            imp.name.0 != intrinsics_import,
                            "main Wasm cannot import the unsafe intrinsics (`{intrinsics_import}`) \
                             when using compile-time builtins"
                        );

                        if let wasmparser::ComponentTypeRef::Instance(_) = imp.ty {
                            instance_imports.insert(imp.name.0);
                        }
                    }
                }
                _ => {}
            }
        }

        Ok(instance_imports)
    }

    /// Define a compile-time builtin component, via its Wasm bytes.
    ///
    /// Compile-time builtins enable you to build safe, zero-copy, and (with
    /// [inlining][crate::Config::compiler_inlining])
    /// zero-function-call-overhead Wasm APIs for accessing host data, buffers,
    /// and objects.
    ///
    /// A compile-time builtin is a component that is
    ///
    /// * authored by the host (Wasmtime embedder),
    ///
    /// * whose implementation (though not necessarily its interface!) is
    ///   host-specific,
    ///
    /// * has access to unsafe intrinsics (and is therefore part of the host's
    ///   [trusted compute base]), and
    ///
    /// * is linked into guest Wasm programs at compile-time.
    ///
    /// Any imports satisfied by a compile-time builtin during compilation will
    /// not show up in the resulting component's
    /// [imports][crate::component::types::Component::imports], and they can no
    /// longer be customized by a [`Linker`][crate::component::Linker]
    /// definition at instantiation time.[^0]
    ///
    /// [^0]: If linking compile-time builtins into a component at compile-time
    /// reminds you of [component composition], that is not a coincidence:
    /// component composition is used under the covers as part of compile-time
    /// builtins' implementation.
    ///
    /// Comparing compile-time builtins with
    /// [`Linker`][crate::component::Linker]s is informative:
    ///
    /// * Both mechanisms define APIs to satisfy a Wasm program's imports.
    ///
    /// * A `Linker` satisfies those imports at instantiation-time, while
    ///   compile-time builtins do it during compilation.
    ///
    /// * APIs defined by a `Linker` are implemented in Rust, and hosts can
    ///   build safe, sandboxed Wasm APIs on top of raw, un-sandboxed primitives
    ///   via Rust's `unsafe`. APIs defined by compile-time builtins are
    ///   implemented as Wasm components, and hosts can build safe, sandboxed
    ///   Wasm APIs on top of raw, un-sandboxed primitives via [unsafe
    ///   intrinsics][CodeBuilder::expose_unsafe_intrinsics].
    ///
    /// * Imports satisfied via `Linker`-defined APIs are implemented with
    ///   [PLT/GOT]-style function table lookups and indirect calls in the
    ///   Wasm's compiled native code. On the other hand, Wasmtime implements
    ///   calls to imports satisfied via compile-time builtins with direct calls
    ///   in the Wasm's compiled native code. Wasmtime's compiler can also
    ///   [inline][crate::Config::compiler_inlining] these direct calls,
    ///   removing function call overheads and enabling further, cascading
    ///   compiler optimizations.
    ///
    /// If you are familiar with Wasm on the Web, you can think of compile-time
    /// builtins as the rough equivalent of [the `js-string-builtins` proposal]
    /// but for arbitrary host-defined APIs in a Wasmtime embedding environment
    /// rather than JS string APIs in a Web browser environment.
    ///
    /// [trusted compute base]: https://en.wikipedia.org/wiki/Trusted_computing_base
    /// [the `js-string-builtins` proposal]: https://github.com/WebAssembly/js-string-builtins/blob/main/proposals/js-string-builtins/Overview.md
    /// [component composition]: https://component-model.bytecodealliance.org/composing-and-distributing/composing.html
    /// [PLT/GOT]: https://reverseengineering.stackexchange.com/a/1993
    ///
    /// # Safety
    ///
    /// Compile-time builtins are part of your [trusted compute base] and should
    /// be authored by trusted, first-party developers with extreme care. You
    /// should never use compile-time builtins authored by untrusted,
    /// third-party developers.
    ///
    /// Compile-time builtins are given access to Wasmtime's [unsafe
    /// intrinsics][CodeBuilder::expose_unsafe_intrinsics], and the same safety
    /// invariants and portability concerns apply. However, when compile-time
    /// builtins are defined on a `CodeBuilder`, unsafe intrinsics are *only*
    /// exposed to the compile-time builtins, and they are *not* exposed to the
    /// main guest Wasm program. This means that — assuming your compile-time
    /// builtins only exposing safe APIs, encapsulating the intrinsics'
    /// unsafety, and modulo bugs in your implementation of those safe APIs —
    /// that the main guest Wasm program is not part of your trusted compute
    /// base.
    ///
    /// # Example
    ///
    /// See the example in [CodeBuilder::expose_unsafe_intrinsics].
    pub unsafe fn compile_time_builtins_binary(
        &mut self,
        name: impl Into<Cow<'a, str>>,
        wasm_bytes: impl Into<Cow<'a, [u8]>>,
    ) -> &mut Self {
        self.compile_time_builtins
            .insert(name.into(), wasm_bytes.into());
        self
    }

    /// Equivalent of [`CodeBuilder::compile_time_builtins_binary`] that also
    /// accepts the WebAssembly text format.
    ///
    /// This method will configure the WebAssembly binary to be compiled and
    /// used to satisfy the `name` instance import. The input `wasm_bytes` may
    /// either be the wasm text format or the binary format. If the `wat` crate
    /// feature is enabled, which is enabled by default, then the text format
    /// will automatically be converted to the binary format.
    ///
    /// # Errors
    ///
    /// This method will also return an error if `wasm_bytes` is the wasm text
    /// format and the text syntax is not valid.
    ///
    /// # Safety
    ///
    /// See [`CodeBuilder::compile_time_builtins_binary`].
    ///
    /// # Example
    ///
    /// See the example in [CodeBuilder::expose_unsafe_intrinsics], which uses
    /// compile-time builtins.
    pub unsafe fn compile_time_builtins_binary_or_text(
        &mut self,
        name: impl Into<Cow<'a, str>>,
        wasm_bytes: impl Into<Cow<'a, [u8]>>,
        wasm_path: Option<&Path>,
    ) -> Result<&mut Self> {
        let wasm_bytes = wasm_bytes.into();

        #[cfg(feature = "wat")]
        if let Cow::Owned(wasm_bytes) = wat::parse_bytes(&wasm_bytes).map_err(|mut e| {
            if let Some(path) = wasm_path {
                e.set_path(path);
            }
            e
        })? {
            // SAFETY: Same as our unsafe contract.
            return Ok(unsafe { self.compile_time_builtins_binary(name, wasm_bytes) });
        }

        // SAFETY: Same as our unsafe contract.
        Ok(unsafe { self.compile_time_builtins_binary(name, wasm_bytes) })
    }

    /// Like [`CodeBuilder::compile_time_builtins_binary`], but reads the `file`
    /// specified for the bytes that will define the compile-time builtin.
    ///
    /// # Safety
    ///
    /// See [`CodeBuilder::compile_time_builtins_binary`].
    ///
    /// # Example
    ///
    /// See the example in [CodeBuilder::expose_unsafe_intrinsics], which uses
    /// compile-time builtins.
    pub unsafe fn compile_time_builtins_binary_file(
        &mut self,
        name: impl Into<Cow<'a, str>>,
        file: &Path,
    ) -> Result<&mut Self> {
        let wasm_bytes = std::fs::read(file)
            .with_context(|| format!("failed to read file: {}", file.display()))?;
        // SAFETY: Same as our unsafe contract.
        Ok(unsafe { self.compile_time_builtins_binary(name, wasm_bytes) })
    }

    /// Equivalent of [`CodeBuilder::compile_time_builtins_binary_file`] that
    /// also accepts the WebAssembly text format.
    ///
    /// This method is will read the file at the given path and interpret the
    /// contents to determine if it's the Wasm text format or binary format. The
    /// file extension is not consulted. The text format is automatically
    /// converted to the binary format if the crate feature `wat` is active.
    ///
    /// # Errors
    ///
    /// In addition to the errors returned by
    /// [`CodeBuilder::compile_time_builtins_binary_file`] this may also fail if
    /// the text format is read and the syntax is invalid.
    ///
    /// # Safety
    ///
    /// See [`CodeBuilder::compile_time_builtins_binary`].
    ///
    /// # Example
    ///
    /// See the example in [CodeBuilder::expose_unsafe_intrinsics], which uses
    /// compile-time builtins.
    pub unsafe fn compile_time_builtins_binary_or_text_file(
        &mut self,
        name: impl Into<Cow<'a, str>>,
        file: &Path,
    ) -> Result<&mut Self> {
        #[cfg(feature = "wat")]
        {
            let wasm = wat::parse_file(file)
                .with_context(|| format!("error parsing file: {}", file.display()))?;
            // SAFETY: Same as our unsafe contract.
            Ok(unsafe { self.compile_time_builtins_binary(name, wasm) })
        }

        #[cfg(not(feature = "wat"))]
        {
            // SAFETY: Same as our unsafe contract.
            unsafe { self.compile_time_builtins_binary_file(name, file) }
        }
    }
}