ca9f33b6d9
This commit rewrites the runtime crate to provide safety in the face of recursive calls to the guest. The basic principle is that `GuestMemory` is now a trait which dynamically returns the pointer/length pair. This also has an implicit contract (hence the `unsafe` trait) that the pointer/length pair point to a valid list of bytes in host memory "until something is reentrant". After this changes the various suite of `Guest*` types were rewritten. `GuestRef` and `GuestRefMut` were both removed since they cannot safely exist. The `GuestPtrMut` type was removed for simplicity, and the final `GuestPtr` type subsumes `GuestString` and `GuestArray`. This means that there's only one guest pointer type, `GuestPtr<'a, T>`, where `'a` is the borrow into host memory, basically borrowing the `GuestMemory` trait object itself. Some core utilities are exposed on `GuestPtr`, but they're all 100% safe. Unsafety is now entirely contained within a few small locations: * Implementations of the `GuestType` for primitive types (e.g. `i8`, `u8`, etc) use `unsafe` to read/write memory. The `unsafe` trait of `GuestMemory` though should prove that they're safe. * `GuestPtr<'_, str>` has a method which validates utf-8 contents, and this requires `unsafe` internally to read all the bytes. This is guaranteed to be safe however given the contract of `GuestMemory`. And that's it! Everything else is a bunch of safe combinators all built up on the various utilities provided by `GuestPtr`. The general idioms are roughly the same as before, with various tweaks here and there. A summary of expected idioms are: * For small values you'd `.read()` or `.write()` very quickly. You'd pass around the type itself. * For strings, you'd pass `GuestPtr<'_, str>` down to the point where it's actually consumed. At that moment you'd either decide to copy it out (a safe operation) or you'd get a raw view to the string (an unsafe operation) and assert that you won't call back into wasm while you're holding that pointer. * Arrays are similar to strings, passing around `GuestPtr<'_, [T]>`. Arrays also have a `iter()` method which yields an iterator of `GuestPtr<'_, T>` for convenience. Overall there's still a lot of missing documentation on the runtime crate specifically around the safety of the `GuestMemory` trait as well as how the utilities/methods are expected to be used. Additionally there's utilities which aren't currently implemented which would be easy to implement. For example there's no method to copy out a string or a slice, although that would be pretty easy to add. In any case I'm curious to get feedback on this approach and see what y'all think!
88 lines
2.5 KiB
Rust
88 lines
2.5 KiB
Rust
use proptest::prelude::*;
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use wiggle_runtime::{GuestError, GuestMemory, GuestPtr};
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use wiggle_test::{impl_errno, HostMemory, MemArea, WasiCtx};
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wiggle::from_witx!({
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witx: ["tests/strings.witx"],
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ctx: WasiCtx,
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});
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impl_errno!(types::Errno);
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impl strings::Strings for WasiCtx {
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fn hello_string(&self, a_string: &GuestPtr<str>) -> Result<u32, types::Errno> {
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let s = a_string.as_raw().expect("should be valid string");
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unsafe {
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println!("a_string='{}'", &*s);
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Ok((*s).len() as u32)
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}
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}
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}
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fn test_string_strategy() -> impl Strategy<Value = String> {
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"\\p{Greek}{1,256}"
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}
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#[derive(Debug)]
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struct HelloStringExercise {
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test_word: String,
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string_ptr_loc: MemArea,
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return_ptr_loc: MemArea,
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}
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impl HelloStringExercise {
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pub fn strat() -> BoxedStrategy<Self> {
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(test_string_strategy(),)
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.prop_flat_map(|(test_word,)| {
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(
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Just(test_word.clone()),
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HostMemory::mem_area_strat(test_word.len() as u32),
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HostMemory::mem_area_strat(4),
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)
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})
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.prop_map(|(test_word, string_ptr_loc, return_ptr_loc)| Self {
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test_word,
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string_ptr_loc,
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return_ptr_loc,
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})
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.prop_filter("non-overlapping pointers", |e| {
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MemArea::non_overlapping_set(&[&e.string_ptr_loc, &e.return_ptr_loc])
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})
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.boxed()
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}
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pub fn test(&self) {
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let ctx = WasiCtx::new();
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let host_memory = HostMemory::new();
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// Populate string in guest's memory
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let ptr = host_memory.ptr::<str>((self.string_ptr_loc.ptr, self.test_word.len() as u32));
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for (slot, byte) in ptr.as_bytes().iter().zip(self.test_word.bytes()) {
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slot.expect("should be valid pointer")
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.write(byte)
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.expect("failed to write");
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}
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let res = strings::hello_string(
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&ctx,
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&host_memory,
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self.string_ptr_loc.ptr as i32,
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self.test_word.len() as i32,
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self.return_ptr_loc.ptr as i32,
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);
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assert_eq!(res, types::Errno::Ok.into(), "hello string errno");
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let given = host_memory
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.ptr::<u32>(self.return_ptr_loc.ptr)
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.read()
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.expect("deref ptr to return value");
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assert_eq!(self.test_word.len() as u32, given);
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}
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}
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proptest! {
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#[test]
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fn hello_string(e in HelloStringExercise::strat()) {
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e.test()
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}
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}
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