c9a0ba81a0
* Implement interrupting wasm code, reimplement stack overflow This commit is a relatively large change for wasmtime with two main goals: * Primarily this enables interrupting executing wasm code with a trap, preventing infinite loops in wasm code. Note that resumption of the wasm code is not a goal of this commit. * Additionally this commit reimplements how we handle stack overflow to ensure that host functions always have a reasonable amount of stack to run on. This fixes an issue where we might longjmp out of a host function, skipping destructors. Lots of various odds and ends end up falling out in this commit once the two goals above were implemented. The strategy for implementing this was also lifted from Spidermonkey and existing functionality inside of Cranelift. I've tried to write up thorough documentation of how this all works in `crates/environ/src/cranelift.rs` where gnarly-ish bits are. A brief summary of how this works is that each function and each loop header now checks to see if they're interrupted. Interrupts and the stack overflow check are actually folded into one now, where function headers check to see if they've run out of stack and the sentinel value used to indicate an interrupt, checked in loop headers, tricks functions into thinking they're out of stack. An interrupt is basically just writing a value to a location which is read by JIT code. When interrupts are delivered and what triggers them has been left up to embedders of the `wasmtime` crate. The `wasmtime::Store` type has a method to acquire an `InterruptHandle`, where `InterruptHandle` is a `Send` and `Sync` type which can travel to other threads (or perhaps even a signal handler) to get notified from. It's intended that this provides a good degree of flexibility when interrupting wasm code. Note though that this does have a large caveat where interrupts don't work when you're interrupting host code, so if you've got a host import blocking for a long time an interrupt won't actually be received until the wasm starts running again. Some fallout included from this change is: * Unix signal handlers are no longer registered with `SA_ONSTACK`. Instead they run on the native stack the thread was already using. This is possible since stack overflow isn't handled by hitting the guard page, but rather it's explicitly checked for in wasm now. Native stack overflow will continue to abort the process as usual. * Unix sigaltstack management is now no longer necessary since we don't use it any more. * Windows no longer has any need to reset guard pages since we no longer try to recover from faults on guard pages. * On all targets probestack intrinsics are disabled since we use a different mechanism for catching stack overflow. * The C API has been updated with interrupts handles. An example has also been added which shows off how to interrupt a module. Closes #139 Closes #860 Closes #900 * Update comment about magical interrupt value * Store stack limit as a global value, not a closure * Run rustfmt * Handle review comments * Add a comment about SA_ONSTACK * Use `usize` for type of `INTERRUPTED` * Parse human-readable durations * Bring back sigaltstack handling Allows libstd to print out stack overflow on failure still. * Add parsing and emission of stack limit-via-preamble * Fix new example for new apis * Fix host segfault test in release mode * Fix new doc example
Cranelift Code Generator
A Bytecode Alliance project
Cranelift is a low-level retargetable code generator. It translates a target-independent intermediate representation into executable machine code.
For more information, see the documentation.
For an example of how to use the JIT, see the SimpleJIT Demo, which implements a toy language.
For an example of how to use Cranelift to run WebAssembly code, see Wasmtime, which implements a standalone, embeddable, VM using Cranelift.
Status
Cranelift currently supports enough functionality to run a wide variety of programs, including all the functionality needed to execute WebAssembly MVP functions, although it needs to be used within an external WebAssembly embedding to be part of a complete WebAssembly implementation.
The x86-64 backend is currently the most complete and stable; other architectures are in various stages of development. Cranelift currently supports both the System V AMD64 ABI calling convention used on many platforms and the Windows x64 calling convention. The performance of code produced by Cranelift is not yet impressive, though we have plans to fix that.
The core codegen crates have minimal dependencies, support no_std mode (see below), and do not require any host floating-point support, and do not use callstack recursion.
Cranelift does not yet perform mitigations for Spectre or related security issues, though it may do so in the future. It does not currently make any security-relevant instruction timing guarantees. It has seen a fair amount of testing and fuzzing, although more work is needed before it would be ready for a production use case.
Cranelift's APIs are not yet stable.
Cranelift currently requires Rust 1.37 or later to build.
Contributing
If you're interested in contributing to Cranelift: thank you! We have a [contributing guide] which will help you getting involved in the Cranelift project.
Planned uses
Cranelift is designed to be a code generator for WebAssembly, but it is general enough to be useful elsewhere too. The initial planned uses that affected its design are:
- WebAssembly compiler for the SpiderMonkey engine in Firefox.
- Backend for the IonMonkey JavaScript JIT compiler in Firefox.
- Debug build backend for the Rust compiler.
- Wasmtime non-Web wasm engine.
Building Cranelift
Cranelift uses a conventional Cargo build process.
Cranelift consists of a collection of crates, and uses a Cargo
Workspace,
so for some cargo commands, such as cargo test, the --all is needed
to tell cargo to visit all of the crates.
test-all.sh at the top level is a script which runs all the cargo
tests and also performs code format, lint, and documentation checks.
Building with no_std
The following crates support `no_std`, although they do depend on liballoc:
- cranelift-entity
- cranelift-bforest
- cranelift-codegen
- cranelift-frontend
- cranelift-native
- cranelift-wasm
- cranelift-module
- cranelift-preopt
- cranelift
To use no_std mode, disable the std feature and enable the core feature. This currently requires nightly rust.
For example, to build `cranelift-codegen`:
cd cranelift-codegen
cargo build --no-default-features --features core
Or, when using cranelift-codegen as a dependency (in Cargo.toml):
[dependency.cranelift-codegen]
...
default-features = false
features = ["core"]
no_std support is currently "best effort". We won't try to break it, and we'll accept patches fixing problems, however we don't expect all developers to build and test no_std when submitting patches. Accordingly, the ./test-all.sh script does not test no_std.
There is a separate ./test-no_std.sh script that tests the no_std support in packages which support it.
It's important to note that cranelift still needs liballoc to compile. Thus, whatever environment is used must implement an allocator.
Also, to allow the use of HashMaps with no_std, an external crate called hashmap_core is pulled in (via the core feature). This is mostly the same as std::collections::HashMap, except that it doesn't have DOS protection. Just something to think about.
Log configuration
Cranelift uses the log crate to log messages at various levels. It doesn't
specify any maximal logging level, so embedders can choose what it should be;
however, this can have an impact of Cranelift's code size. You can use log
features to reduce the maximum logging level. For instance if you want to limit
the level of logging to warn messages and above in release mode:
[dependency.log]
...
features = ["release_max_level_warn"]
Editor Support
Editor support for working with Cranelift IR (clif) files: