doc,test: mem protection must be observed in ffi

doc/api/ffi.md

@@ -533,6 +533,8 @@ native memory directly. The caller must guarantee that:
 * `length` stays within the allocated native region.
 * no native code frees or repurposes that memory while JavaScript still uses
   the `Buffer`.
+* Memory protection is observed. For example, read-only memory pages must not
+  be written to.
 
 If these guarantees are not met, reading or writing the `Buffer` can corrupt
 memory or crash the process.

test/ffi/ffi-test-common.js

@@ -79,6 +79,10 @@ const fixtureSymbols = {
   array_set_f64: { parameters: ['pointer', 'u64', 'f64'], result: 'void' },
 };
 
+if (!common.isWindows) {
+  fixtureSymbols.readonly_memory = { parameters: [], result: 'pointer' };
+}
+
 function cString(value) {
   return Buffer.from(`${value}\0`);
 }

test/ffi/fixture_library/ffi_test_library.c

@@ -2,10 +2,10 @@
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
-
 #ifdef _WIN32
 #define FFI_EXPORT __declspec(dllexport)
 #else
+#include <sys/mman.h>
 #define FFI_EXPORT
 #endif
 
@@ -378,3 +378,12 @@ FFI_EXPORT void array_set_f64(double* arr, size_t index, double value) {
 
   arr[index] = value;
 }
+
+#ifndef _WIN32
+FFI_EXPORT void* readonly_memory() {
+  // TODO(bengl) Add a Windows version of this.
+
+  void* p = mmap(0, 4096, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  return p;
+}
+#endif

test/ffi/test-ffi-readonly-write.js

@@ -0,0 +1,31 @@
+// Flags: --experimental-ffi
+'use strict';
+const { skipIfFFIMissing, isWindows, skip } = require('../common');
+const assert = require('node:assert');
+const { spawnSync } = require('node:child_process');
+const { test } = require('node:test');
+const { fixtureSymbols, libraryPath } = require('./ffi-test-common');
+
+skipIfFFIMissing();
+if (isWindows) {
+  skip('This test currently relies on POSIX APIs');
+}
+
+test('writing to readonly memory via buffer fails', () => {
+  const symbols = JSON.stringify(fixtureSymbols);
+  const libPath = JSON.stringify(libraryPath);
+  const { stdout, status } = spawnSync(process.execPath, [
+    '--experimental-ffi',
+    '-p',
+    `
+    const ffi = require('node:ffi');
+    const { functions } = ffi.dlopen(${libPath}, ${symbols})
+    const p = functions.readonly_memory();
+    const b = ffi.toBuffer(p, 4096, false);
+    b[0] = 42;
+    console.log('success');
+    `,
+  ]);
+  assert.notStrictEqual(status, 0);
+  assert.strictEqual(stdout.length, 0);
+});