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Rewrite for recursive safety

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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!
This commit is contained in:
Alex Crichton
2020-03-04 10:21:34 -08:00
parent 3764204250
commit ca9f33b6d9
28 changed files with 751 additions and 2013 deletions
Download Patch File Download Diff File Expand all files Collapse all files

70
tests/strings.rs
View File

@@ -1,5 +1,5 @@
use proptest::prelude::*;
use wiggle_runtime::{GuestError, GuestPtrMut, GuestString};
use wiggle_runtime::{GuestError, GuestMemory, GuestPtr};
use wiggle_test::{impl_errno, HostMemory, MemArea, WasiCtx};
wiggle::from_witx!({
@@ -10,11 +10,12 @@ wiggle::from_witx!({
impl_errno!(types::Errno);
impl strings::Strings for WasiCtx {
fn hello_string(&self, a_string: &GuestString<'_>) -> Result<u32, types::Errno> {
let as_ref = a_string.as_ref().expect("deref ptr should succeed");
let as_str = as_ref.as_str().expect("valid UTF-8 string");
println!("a_string='{}'", as_str);
Ok(as_str.len() as u32)
fn hello_string(&self, a_string: &GuestPtr<str>) -> Result<u32, types::Errno> {
let s = a_string.as_raw().expect("should be valid string");
unsafe {
println!("a_string='{}'", &*s);
Ok((*s).len() as u32)
}
}
}
@@ -26,7 +27,6 @@ fn test_string_strategy() -> impl Strategy<Value = String> {
struct HelloStringExercise {
test_word: String,
string_ptr_loc: MemArea,
string_len_loc: MemArea,
return_ptr_loc: MemArea,
}
@@ -38,63 +38,43 @@ impl HelloStringExercise {
Just(test_word.clone()),
HostMemory::mem_area_strat(test_word.len() as u32),
HostMemory::mem_area_strat(4),
HostMemory::mem_area_strat(4),
)
})
.prop_map(
|(test_word, string_ptr_loc, string_len_loc, return_ptr_loc)| Self {
test_word,
string_ptr_loc,
string_len_loc,
return_ptr_loc,
},
)
.prop_map(|(test_word, string_ptr_loc, return_ptr_loc)| Self {
test_word,
string_ptr_loc,
return_ptr_loc,
})
.prop_filter("non-overlapping pointers", |e| {
MemArea::non_overlapping_set(&[
&e.string_ptr_loc,
&e.string_len_loc,
&e.return_ptr_loc,
])
MemArea::non_overlapping_set(&[&e.string_ptr_loc, &e.return_ptr_loc])
})
.boxed()
}
pub fn test(&self) {
let mut ctx = WasiCtx::new();
let mut host_memory = HostMemory::new();
let mut guest_memory = host_memory.guest_memory();
// Populate string length
*guest_memory
.ptr_mut(self.string_len_loc.ptr)
.expect("ptr mut to string len")
.as_ref_mut()
.expect("deref ptr mut to string len") = self.test_word.len() as u32;
let ctx = WasiCtx::new();
let host_memory = HostMemory::new();
// Populate string in guest's memory
{
let mut next: GuestPtrMut<'_, u8> = guest_memory
.ptr_mut(self.string_ptr_loc.ptr)
.expect("ptr mut to the first byte of string");
for byte in self.test_word.as_bytes() {
*next.as_ref_mut().expect("deref mut") = *byte;
next = next.elem(1).expect("increment ptr by 1");
}
let ptr = host_memory.ptr::<str>((self.string_ptr_loc.ptr, self.test_word.len() as u32));
for (slot, byte) in ptr.as_bytes().iter().zip(self.test_word.bytes()) {
slot.expect("should be valid pointer")
.write(byte)
.expect("failed to write");
}
let res = strings::hello_string(
&mut ctx,
&mut guest_memory,
&ctx,
&host_memory,
self.string_ptr_loc.ptr as i32,
self.string_len_loc.ptr as i32,
self.test_word.len() as i32,
self.return_ptr_loc.ptr as i32,
);
assert_eq!(res, types::Errno::Ok.into(), "hello string errno");
let given = *guest_memory
let given = host_memory
.ptr::<u32>(self.return_ptr_loc.ptr)
.expect("ptr to return value")
.as_ref()
.read()
.expect("deref ptr to return value");
assert_eq!(self.test_word.len() as u32, given);
}