fix(kernel): Fix calculation of handle offset when splitting re-used memory, add kernel test (#659)

.github/workflows/kernel.yml

@@ -39,6 +39,7 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         if: ${{ env.GIT_EXIT_CODE }} != 0
         with:
+          author: "zshipko <zshipko@users.noreply.github.com>"
           title: "update(kernel): extism-runtime.wasm in ${{ github.event.pull_request.head.ref }}"
           body: "Automated PR to update `runtime/src/extism-runtime.wasm` in PR #${{ github.event.number }}"
           base: "${{ github.event.pull_request.head.ref }}"

kernel/Cargo.toml

@@ -5,6 +5,9 @@ edition = "2021"
 
 [dependencies]
 
+[dev-dependencies]
+wasm-bindgen-test = "0.3.39"
+
 [features]
 default = ["bounds-checking"]
 bounds-checking = []

kernel/src/bin/extism-runtime.rs

@@ -3,7 +3,7 @@
 
 pub use extism_runtime_kernel::*;
 
-#[cfg(target_arch = "wasm32")]
+#[cfg(all(target_arch = "wasm32", not(test)))]
 #[panic_handler]
 fn panic(_info: &core::panic::PanicInfo) -> ! {
     core::arch::wasm32::unreachable()

kernel/src/lib.rs

@@ -228,7 +228,7 @@ impl MemoryRoot {
             if status == MemoryStatus::Free as u8 && b.size >= length as usize {
                 // Split block if there is too much excess
                 if b.size - length as usize >= 128 {
-                    b.size -= length as usize;
+                    b.size -= length as usize + core::mem::size_of::<MemoryBlock>();
                     b.used = 0;
 
                     let block1 = b.data.as_mut_ptr().add(b.size) as *mut MemoryBlock;
@@ -574,3 +574,48 @@ pub unsafe fn error_get() -> Handle {
 pub unsafe fn memory_bytes() -> u64 {
     MemoryRoot::new().length.load(Ordering::Acquire)
 }
+
+#[cfg(test)]
+mod test {
+    use crate::*;
+    use wasm_bindgen_test::*;
+
+    // See https://github.com/extism/extism/pull/659
+    #[wasm_bindgen_test]
+    fn test_659() {
+        unsafe {
+            // Warning: These offsets will need to change if we adjust the kernel memory layout at all
+            reset();
+            assert_eq!(alloc(1065), 77);
+            assert_eq!(alloc(288), 1154);
+            assert_eq!(alloc(128), 1454);
+            assert_eq!(length(1154), 288);
+            assert_eq!(length(1454), 128);
+            free(1454);
+            assert_eq!(alloc(213), 1594);
+            length_unsafe(1594);
+            assert_eq!(alloc(511), 1819);
+            assert_eq!(alloc(4), 1454);
+            assert_eq!(length(1454), 4);
+            assert_eq!(length(1819), 511);
+            assert_eq!(alloc(13), 2342);
+            assert_eq!(length(2342), 13);
+            assert_eq!(alloc(336), 2367);
+            assert_eq!(alloc(1077), 2715);
+            assert_eq!(length(2367), 336);
+            assert_eq!(length(2715), 1077);
+            free(2715);
+            assert_eq!(alloc(1094), 3804);
+            length_unsafe(3804);
+
+            // Allocate 4 bytes, expect to receive address 3788
+            assert_eq!(alloc(4), 3788);
+
+            assert_eq!(alloc(4), 3772);
+            assert_eq!(length(3772), 4);
+
+            // Address 3788 has not been freed yet, so expect it to have 4 bytes allocated
+            assert_eq!(length(3788), 4);
+        }
+    }
+}

kernel/test.sh

@@ -0,0 +1,5 @@
+# install wasm-bindgen-cli to get wasm-bindgen-runner if it is not installed yet
+which wasm-bindgen-test-runner 1>/dev/null || cargo install -f wasm-bindgen-cli
+
+# run tests with the wasm-bindgen-runner
+CARGO_TARGET_WASM32_UNKNOWN_UNKNOWN_RUNNER=wasm-bindgen-test-runner cargo test --release --target=wasm32-unknown-unknown