native-lib: allow passing structs as arguments

Cargo.toml

@@ -39,7 +39,7 @@ features = ['unprefixed_malloc_on_supported_platforms']
 [target.'cfg(unix)'.dependencies]
 libc = "0.2"
 # native-lib dependencies
-libffi = { version = "4.0.0", optional = true }
+libffi = { version = "4.1.1", optional = true }
 libloading = { version = "0.8", optional = true }
 serde = { version = "1.0.219", features = ["derive"], optional = true }
 

src/shims/native_lib/ffi.rs

@@ -0,0 +1,113 @@
+use libffi::low::CodePtr;
+use libffi::middle::{Arg as ArgPtr, Cif, Type as FfiType};
+
+/// Perform the actual FFI call.
+///
+/// SAFETY: The `FfiArg`s passed must have been correctly instantiated (i.e. their
+/// type layout must match the data they point to), and the safety invariants of
+/// the foreign function being called must be upheld (if any).
+pub unsafe fn call<'a, R: libffi::high::CType>(fun: CodePtr, args: Vec<FfiArg<'a>>) -> R {
+    let mut arg_tys = vec![];
+    let mut arg_ptrs = vec![];
+    for arg in args {
+        arg_tys.push(arg.ty);
+        arg_ptrs.push(arg.ptr)
+    }
+    let cif = Cif::new(arg_tys, R::reify().into_middle());
+    unsafe { cif.call(fun, &arg_ptrs) }
+}
+
+/// A wrapper type for `libffi::middle::Type` which also holds a pointer to the data.
+pub struct FfiArg<'a> {
+    /// The type layout information for the pointed-to data.
+    ty: FfiType,
+    /// A pointer to the data described in `ty`.
+    ptr: ArgPtr,
+    /// Lifetime of the actual pointed-to data.
+    _p: std::marker::PhantomData<&'a [u8]>,
+}
+
+impl<'a> FfiArg<'a> {
+    fn new(ty: FfiType, ptr: ArgPtr) -> Self {
+        Self { ty, ptr, _p: std::marker::PhantomData }
+    }
+}
+
+/// An owning form of `FfiArg`.
+/// We introduce this enum instead of just calling `Arg::new` and storing a list of
+/// `libffi::middle::Arg` directly, because the `libffi::middle::Arg` just wraps a reference to
+/// the value it represents and we need to store a copy of the value, and pass a reference to
+/// this copy to C instead.
+#[derive(Debug, Clone)]
+pub enum CArg {
+    /// Primitive type.
+    Primitive(CPrimitive),
+    /// Struct with its computed type layout and bytes.
+    Struct(FfiType, Box<[u8]>),
+}
+
+impl CArg {
+    /// Convert a `CArg` to the required FFI argument type.
+    pub fn arg_downcast<'a>(&'a self) -> FfiArg<'a> {
+        match self {
+            CArg::Primitive(cprim) => cprim.arg_downcast(),
+            // FIXME: Using `&items[0]` to reference the whole array is definitely
+            // unsound under SB, but we're waiting on
+            // https://github.com/libffi-rs/libffi-rs/commit/112a37b3b6ffb35bd75241fbcc580de40ba74a73
+            // to land in a release so that we don't need to do this.
+            CArg::Struct(cstruct, items) => FfiArg::new(cstruct.clone(), ArgPtr::new(&items[0])),
+        }
+    }
+}
+
+impl From<CPrimitive> for CArg {
+    fn from(prim: CPrimitive) -> Self {
+        Self::Primitive(prim)
+    }
+}
+
+#[derive(Debug, Clone)]
+/// Enum of supported primitive arguments to external C functions.
+pub enum CPrimitive {
+    /// 8-bit signed integer.
+    Int8(i8),
+    /// 16-bit signed integer.
+    Int16(i16),
+    /// 32-bit signed integer.
+    Int32(i32),
+    /// 64-bit signed integer.
+    Int64(i64),
+    /// isize.
+    ISize(isize),
+    /// 8-bit unsigned integer.
+    UInt8(u8),
+    /// 16-bit unsigned integer.
+    UInt16(u16),
+    /// 32-bit unsigned integer.
+    UInt32(u32),
+    /// 64-bit unsigned integer.
+    UInt64(u64),
+    /// usize.
+    USize(usize),
+    /// Raw pointer, stored as C's `void*`.
+    RawPtr(*mut std::ffi::c_void),
+}
+
+impl CPrimitive {
+    /// Convert a primitive to the required FFI argument type.
+    fn arg_downcast<'a>(&'a self) -> FfiArg<'a> {
+        match self {
+            CPrimitive::Int8(i) => FfiArg::new(FfiType::i8(), ArgPtr::new(i)),
+            CPrimitive::Int16(i) => FfiArg::new(FfiType::i16(), ArgPtr::new(i)),
+            CPrimitive::Int32(i) => FfiArg::new(FfiType::i32(), ArgPtr::new(i)),
+            CPrimitive::Int64(i) => FfiArg::new(FfiType::i64(), ArgPtr::new(i)),
+            CPrimitive::ISize(i) => FfiArg::new(FfiType::isize(), ArgPtr::new(i)),
+            CPrimitive::UInt8(i) => FfiArg::new(FfiType::u8(), ArgPtr::new(i)),
+            CPrimitive::UInt16(i) => FfiArg::new(FfiType::u16(), ArgPtr::new(i)),
+            CPrimitive::UInt32(i) => FfiArg::new(FfiType::u32(), ArgPtr::new(i)),
+            CPrimitive::UInt64(i) => FfiArg::new(FfiType::u64(), ArgPtr::new(i)),
+            CPrimitive::USize(i) => FfiArg::new(FfiType::usize(), ArgPtr::new(i)),
+            CPrimitive::RawPtr(i) => FfiArg::new(FfiType::pointer(), ArgPtr::new(i)),
+        }
+    }
+}