From 24c9507c2d6529699edca9b2479d7e4f1a3415e6 Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Mon, 27 Feb 2023 22:28:40 +0100
Subject: Add minimal runtime to execute code

---
 Cargo.toml |  1 +
 src/lib.rs |  1 +
 src/rt.rs  | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 53 insertions(+)
 create mode 100644 src/rt.rs

diff --git a/Cargo.toml b/Cargo.toml
index 86f2da9..7bbbcfc 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,3 +4,4 @@ version = "0.1.0"
 edition = "2021"
 
 [dependencies]
+nix = "0.26.2"
diff --git a/src/lib.rs b/src/lib.rs
index 3368e3d..4a7b73e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,4 +1,5 @@
 pub mod prelude;
+pub mod rt;
 
 mod imm;
 mod insn;
diff --git a/src/rt.rs b/src/rt.rs
new file mode 100644
index 0000000..fc8c930
--- /dev/null
+++ b/src/rt.rs
@@ -0,0 +1,51 @@
+use core::ffi::c_void;
+use nix::sys::mman::{mmap, munmap, MapFlags, ProtFlags};
+
+/// A simple `mmap`ed runtime with executable pages.
+pub struct Runtime {
+    buf: *mut c_void,
+    len: usize,
+}
+
+impl Runtime {
+    /// Create a new [Runtime].
+    pub fn new(code: &[u8]) -> Runtime {
+        // Allocate a single page.
+        let len = core::num::NonZeroUsize::new(4096).unwrap();
+        let buf = unsafe {
+            mmap(
+                None,
+                len,
+                ProtFlags::PROT_WRITE | ProtFlags::PROT_READ | ProtFlags::PROT_EXEC,
+                MapFlags::MAP_PRIVATE | MapFlags::MAP_ANONYMOUS,
+                0, /* fd */
+                0, /* off */
+            )
+            .unwrap()
+        };
+        {
+            // Copy over code.
+            assert!(code.len() < len.get());
+            unsafe { std::ptr::copy_nonoverlapping(code.as_ptr(), buf.cast(), len.get()) };
+        }
+
+        Runtime {
+            buf,
+            len: len.get(),
+        }
+    }
+
+    /// Reinterpret the block of code as `F`.
+    #[inline]
+    pub unsafe fn as_fn<F>(&self) -> F {
+        unsafe { std::mem::transmute_copy(&self.buf) }
+    }
+}
+
+impl Drop for Runtime {
+    fn drop(&mut self) {
+        unsafe {
+            munmap(self.buf, self.len).expect("Failed to munmap Runtime");
+        }
+    }
+}
-- 
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