wasmtime_wizer/

lib.rs

//! Wizer: the WebAssembly pre-initializer!
//!
//! See the [`Wizer`] struct for details.

#![deny(missing_docs)]
#![cfg_attr(docsrs, feature(doc_cfg))]

mod info;
mod instrument;
mod parse;
mod rewrite;
mod snapshot;

#[cfg(feature = "wasmtime")]
mod wasmtime;
#[cfg(feature = "wasmtime")]
pub use wasmtime::*;
#[cfg(feature = "component-model")]
mod component;
#[cfg(feature = "component-model")]
pub use component::*;

pub use crate::info::ModuleContext;
pub use crate::snapshot::SnapshotVal;
use anyhow::Context;
use std::collections::{HashMap, HashSet};

const DEFAULT_KEEP_INIT_FUNC: bool = false;

/// Wizer: the WebAssembly pre-initializer!
///
/// Don't wait for your Wasm module to initialize itself, pre-initialize it!
/// Wizer instantiates your WebAssembly module, executes its initialization
/// function, and then serializes the instance's initialized state out into a
/// new WebAssembly module. Now you can use this new, pre-initialized
/// WebAssembly module to hit the ground running, without making your users wait
/// for that first-time set up code to complete.
///
/// ## Caveats
///
/// * The initialization function may not call any imported functions. Doing so
///   will trigger a trap and `wizer` will exit.
///
/// * The Wasm module may not import globals, tables, or memories.
///
/// * Reference types are not supported yet. This is tricky because it would
///   allow the Wasm module to mutate tables, and we would need to be able to
///   snapshot the new table state, but funcrefs and externrefs don't have
///   identity and aren't comparable in the Wasm spec, which makes snapshotting
///   difficult.
#[derive(Clone, Debug)]
#[cfg_attr(feature = "clap", derive(clap::Parser))]
pub struct Wizer {
    /// The Wasm export name of the function that should be executed to
    /// initialize the Wasm module.
    #[cfg_attr(
        feature = "clap",
        arg(short = 'f', long, default_value = "wizer-initialize")
    )]
    init_func: String,

    /// Any function renamings to perform.
    ///
    /// A renaming specification `dst=src` renames a function export `src` to
    /// `dst`, overwriting any previous `dst` export.
    ///
    /// Multiple renamings can be specified. It is an error to specify more than
    /// one source to rename to a destination name, or to specify more than one
    /// renaming destination for one source.
    ///
    /// This option can be used, for example, to replace a `_start` entry point
    /// in an initialized module with an alternate entry point.
    ///
    /// When module linking is enabled, these renames are only applied to the
    /// outermost module.
    #[cfg_attr(
        feature = "clap",
        arg(
            short = 'r',
            long = "rename-func",
            alias = "func-rename",
            value_name = "dst=src",
            value_parser = parse_rename,
        ),
    )]
    func_renames: Vec<(String, String)>,

    /// After initialization, should the Wasm module still export the
    /// initialization function?
    ///
    /// This is `false` by default, meaning that the initialization function is
    /// no longer exported from the Wasm module.
    #[cfg_attr(
        feature = "clap",
        arg(long, require_equals = true, value_name = "true|false")
    )]
    keep_init_func: Option<Option<bool>>,
}

#[cfg(feature = "clap")]
fn parse_rename(s: &str) -> anyhow::Result<(String, String)> {
    let parts: Vec<&str> = s.splitn(2, '=').collect();
    if parts.len() != 2 {
        anyhow::bail!("must contain exactly one equals character ('=')");
    }
    Ok((parts[0].into(), parts[1].into()))
}

#[derive(Default)]
struct FuncRenames {
    /// For a given export name that we encounter in the original module, a map
    /// to a new name, if any, to emit in the output module.
    rename_src_to_dst: HashMap<String, String>,
    /// A set of export names that we ignore in the original module (because
    /// they are overwritten by renamings).
    rename_dsts: HashSet<String>,
}

impl FuncRenames {
    fn parse(renames: &[(String, String)]) -> anyhow::Result<FuncRenames> {
        let mut ret = FuncRenames {
            rename_src_to_dst: HashMap::new(),
            rename_dsts: HashSet::new(),
        };
        if renames.is_empty() {
            return Ok(ret);
        }

        for (dst, src) in renames {
            if ret.rename_dsts.contains(dst) {
                anyhow::bail!("Duplicated function rename dst {dst}");
            }
            if ret.rename_src_to_dst.contains_key(src) {
                anyhow::bail!("Duplicated function rename src {src}");
            }
            ret.rename_dsts.insert(dst.clone());
            ret.rename_src_to_dst.insert(src.clone(), dst.clone());
        }

        Ok(ret)
    }
}

impl Wizer {
    /// Construct a new `Wizer` builder.
    pub fn new() -> Self {
        Wizer {
            init_func: "wizer-initialize".to_string(),
            func_renames: vec![],
            keep_init_func: None,
        }
    }

    /// The export name of the initializer function.
    ///
    /// Defaults to `"wizer-initialize"`.
    pub fn init_func(&mut self, init_func: impl Into<String>) -> &mut Self {
        self.init_func = init_func.into();
        self
    }

    /// Returns the initialization function that will be run for wizer.
    pub fn get_init_func(&self) -> &str {
        &self.init_func
    }

    /// Add a function rename to perform.
    pub fn func_rename(&mut self, new_name: &str, old_name: &str) -> &mut Self {
        self.func_renames
            .push((new_name.to_string(), old_name.to_string()));
        self
    }

    /// After initialization, should the Wasm module still export the
    /// initialization function?
    ///
    /// This is `false` by default, meaning that the initialization function is
    /// no longer exported from the Wasm module.
    pub fn keep_init_func(&mut self, keep: bool) -> &mut Self {
        self.keep_init_func = Some(Some(keep));
        self
    }

    /// First half of [`Self::run`] which instruments the provided `wasm` and
    /// produces a new wasm module which should be run by a runtime.
    ///
    /// After the returned wasm is executed the context returned here and the
    /// state of the instance should be passed to [`Self::snapshot`].
    pub fn instrument<'a>(&self, wasm: &'a [u8]) -> anyhow::Result<(ModuleContext<'a>, Vec<u8>)> {
        // Make sure we're given valid Wasm from the get go.
        self.wasm_validate(&wasm)?;

        let mut cx = parse::parse(wasm)?;

        // When wizening core modules directly some imports aren't supported,
        // so check for those here.
        for import in cx.imports() {
            match import.ty {
                wasmparser::TypeRef::Global(_) => {
                    anyhow::bail!("imported globals are not supported")
                }
                wasmparser::TypeRef::Table(_) => {
                    anyhow::bail!("imported tables are not supported")
                }
                wasmparser::TypeRef::Memory(_) => {
                    anyhow::bail!("imported memories are not supported")
                }
                wasmparser::TypeRef::Func(_) => {}
                wasmparser::TypeRef::FuncExact(_) => {}
                wasmparser::TypeRef::Tag(_) => {}
            }
        }

        let instrumented_wasm = instrument::instrument(&mut cx);
        self.debug_assert_valid_wasm(&instrumented_wasm);

        Ok((cx, instrumented_wasm))
    }

    /// Second half of [`Self::run`] which takes the [`ModuleContext`] returned
    /// by [`Self::instrument`] and the state of the `instance` after it has
    /// possibly executed its initialization function.
    ///
    /// This returns a new WebAssembly binary which has all state
    /// pre-initialized.
    pub async fn snapshot(
        &self,
        mut cx: ModuleContext<'_>,
        instance: &mut impl InstanceState,
    ) -> anyhow::Result<Vec<u8>> {
        // Parse rename spec.
        let renames = FuncRenames::parse(&self.func_renames)?;

        let snapshot = snapshot::snapshot(&cx, instance).await;
        let rewritten_wasm = self.rewrite(&mut cx, &snapshot, &renames);

        self.debug_assert_valid_wasm(&rewritten_wasm);

        Ok(rewritten_wasm)
    }

    fn debug_assert_valid_wasm(&self, wasm: &[u8]) {
        if !cfg!(debug_assertions) {
            return;
        }
        if let Err(error) = self.wasm_validate(&wasm) {
            #[cfg(feature = "wasmprinter")]
            let wat = wasmprinter::print_bytes(&wasm)
                .unwrap_or_else(|e| format!("Disassembling to WAT failed: {}", e));
            #[cfg(not(feature = "wasmprinter"))]
            let wat = "`wasmprinter` cargo feature is not enabled".to_string();
            panic!("instrumented Wasm is not valid: {error:?}\n\nWAT:\n{wat}");
        }
    }

    fn wasm_validate(&self, wasm: &[u8]) -> anyhow::Result<()> {
        log::debug!("Validating input Wasm");

        wasmparser::Validator::new_with_features(wasmparser::WasmFeatures::all())
            .validate_all(wasm)
            .context("wasm validation failed")?;

        for payload in wasmparser::Parser::new(0).parse_all(wasm) {
            match payload? {
                wasmparser::Payload::CodeSectionEntry(code) => {
                    let mut ops = code.get_operators_reader()?;
                    while !ops.eof() {
                        match ops.read()? {
                            // Table mutations aren't allowed as wizer has no
                            // way to record a snapshot of a table at this time.
                            // The only table mutations allowed are those from
                            // active element segments which can be
                            // deterministically replayed, so disallow all other
                            // forms of mutating a table.
                            //
                            // Ideally Wizer could take a snapshot of a table
                            // post-instantiation and then ensure that after
                            // running initialization the table didn't get
                            // mutated, allowing these instructions, but that's
                            // also not possible at this time.
                            wasmparser::Operator::TableCopy { .. } => {
                                anyhow::bail!("unsupported `table.copy` instruction")
                            }
                            wasmparser::Operator::TableInit { .. } => {
                                anyhow::bail!("unsupported `table.init` instruction")
                            }
                            wasmparser::Operator::TableSet { .. } => {
                                anyhow::bail!("unsupported `table.set` instruction")
                            }
                            wasmparser::Operator::TableGrow { .. } => {
                                anyhow::bail!("unsupported `table.grow` instruction")
                            }
                            wasmparser::Operator::TableFill { .. } => {
                                anyhow::bail!("unsupported `table.fill` instruction")
                            }

                            // Wizer has no way of dynamically determining which
                            // element or data segments were dropped during
                            // execution so instead disallow these instructions
                            // entirely. Like above it'd be nice to allow them
                            // but just forbid their execution during the
                            // initialization function, but that can't be done
                            // easily at this time.
                            wasmparser::Operator::ElemDrop { .. } => {
                                anyhow::bail!("unsupported `elem.drop` instruction")
                            }
                            wasmparser::Operator::DataDrop { .. } => {
                                anyhow::bail!("unsupported `data.drop` instruction")
                            }

                            // Wizer can't snapshot GC references, so disallow
                            // any mutation of GC references. This prevents, for
                            // example, reading something from a table and then
                            // mutating it.
                            wasmparser::Operator::StructSet { .. } => {
                                anyhow::bail!("unsupported `struct.set` instruction")
                            }
                            wasmparser::Operator::ArraySet { .. } => {
                                anyhow::bail!("unsupported `array.set` instruction")
                            }
                            wasmparser::Operator::ArrayFill { .. } => {
                                anyhow::bail!("unsupported `array.fill` instruction")
                            }
                            wasmparser::Operator::ArrayCopy { .. } => {
                                anyhow::bail!("unsupported `array.copy` instruction")
                            }
                            wasmparser::Operator::ArrayInitData { .. } => {
                                anyhow::bail!("unsupported `array.init_data` instruction")
                            }
                            wasmparser::Operator::ArrayInitElem { .. } => {
                                anyhow::bail!("unsupported `array.init_elem` instruction")
                            }

                            _ => continue,
                        }
                    }
                }
                wasmparser::Payload::GlobalSection(globals) => {
                    for g in globals {
                        let g = g?.ty;
                        if !g.mutable {
                            continue;
                        }
                        match g.content_type {
                            wasmparser::ValType::I32
                            | wasmparser::ValType::I64
                            | wasmparser::ValType::F32
                            | wasmparser::ValType::F64
                            | wasmparser::ValType::V128 => {}
                            wasmparser::ValType::Ref(_) => {
                                anyhow::bail!(
                                    "unsupported mutable global containing a reference type"
                                )
                            }
                        }
                    }
                }
                _ => {}
            }
        }

        Ok(())
    }

    fn get_keep_init_func(&self) -> bool {
        match self.keep_init_func {
            Some(keep) => keep.unwrap_or(true),
            None => DEFAULT_KEEP_INIT_FUNC,
        }
    }
}

/// Abstract ability to load state from a WebAssembly instance after it's been
/// instantiated and some exports have run.
pub trait InstanceState {
    /// Loads the global specified by `name`, returning a `SnapshotVal`.
    ///
    /// # Panics
    ///
    /// This function panics if `name` isn't an exported global or if the type
    /// of the global doesn't fit in `SnapshotVal`.
    fn global_get(&mut self, name: &str) -> impl Future<Output = SnapshotVal> + Send;

    /// Loads the contents of the memory specified by `name`, returning the
    /// entier contents as a `Vec<u8>`.
    ///
    /// # Panics
    ///
    /// This function panics if `name` isn't an exported memory.
    fn memory_contents(
        &mut self,
        name: &str,
        contents: impl FnOnce(&[u8]) + Send,
    ) -> impl Future<Output = ()> + Send;
}