From b9638db386c1112b1cc54900612d17e451f9d5d0 Mon Sep 17 00:00:00 2001
From: Maruthan G <maruthang4@gmail.com>
Date: Thu, 2 Jul 2026 20:31:41 +0530
Subject: [PATCH] buffer: add regression test for copy beyond 2 GiB

The 32-bit truncation in Buffer.prototype.copy for buffers larger than
2 GiB (refs: #55422) was fixed as part of routing copy through V8's
CopyArrayBufferBytes API, but no regression test covers offsets and
byte counts above 2**32. Add an opt-in test (NODE_TEST_LARGE_BUFFER=1)
that exercises copies across the uint32 boundary.

Refs: https://github.com/nodejs/node/issues/55422
Signed-off-by: Maruthan G <maruthang4@gmail.com>
---
 test/parallel/test-buffer-copy-4gb.js | 76 +++++++++++++++++++++++++++
 1 file changed, 76 insertions(+)
 create mode 100644 test/parallel/test-buffer-copy-4gb.js

diff --git a/test/parallel/test-buffer-copy-4gb.js b/test/parallel/test-buffer-copy-4gb.js
new file mode 100644
index 00000000000000..89e5132e4219d2
--- /dev/null
+++ b/test/parallel/test-buffer-copy-4gb.js
@@ -0,0 +1,76 @@
+'use strict';
+
+// This tests that Buffer.prototype.copy correctly handles copies whose
+// source/target offsets or byte counts exceed 2**32. Refs:
+// https://github.com/nodejs/node/issues/55422
+const common = require('../common');
+
+// Cannot test on 32-bit machines because the test relies on creating
+// buffers larger than 4 GiB.
+common.skipIf32Bits();
+
+// Allocating 4+ GiB buffers in CI environments is expensive and may also
+// fail under sanitizer / memory-constrained builds. Gate the test behind
+// an explicit opt-in env var to avoid timeouts/OOMs in normal CI runs.
+if (!process.env.NODE_TEST_LARGE_BUFFER) {
+  common.skip('Skipping: requires NODE_TEST_LARGE_BUFFER=1 (allocates >4GiB)');
+}
+
+const assert = require('assert');
+
+const threshold = 0xFFFFFFFF; // 2**32 - 1
+const overflow = threshold + 5; // 2**32 + 4 — exercises offsets > 2**32
+
+let largeBuffer;
+try {
+  // Allocate a buffer slightly larger than 2**32 so we can copy to and from
+  // offsets above the uint32 boundary.
+  largeBuffer = Buffer.alloc(overflow);
+} catch (e) {
+  if (e.code === 'ERR_MEMORY_ALLOCATION_FAILED' ||
+      /Array buffer allocation failed/.test(e.message)) {
+    common.skip('insufficient memory for >4GiB Buffer.alloc');
+  }
+  throw e;
+}
+
+// Sentinel byte at an index above 2**32. Before the fix, copy() truncates the
+// length to 32 bits and the sentinel never gets written, so reading it back
+// would yield 0.
+const sentinelIndex = threshold + 2;
+largeBuffer[sentinelIndex] = 0xAB;
+
+// Test 1: Buffer.prototype.copy with sourceEnd > 2**32 should copy the bytes
+// past the 4 GiB boundary instead of silently truncating.
+{
+  const target = Buffer.alloc(overflow);
+  const copied = largeBuffer.copy(target, 0, 0, overflow);
+  // copy() must report copying the full byte range, and the byte beyond
+  // 2**32 must be copied rather than truncated.
+  assert.strictEqual(copied, overflow);
+  assert.strictEqual(target[sentinelIndex], 0xAB);
+}
+
+// Test 2: Buffer.prototype.copy with targetStart > 2**32 should write at the
+// large offset rather than wrapping back to a low address.
+{
+  const target = Buffer.alloc(overflow);
+  const src = Buffer.from([0xCD]);
+  const copied = src.copy(target, threshold + 1, 0, 1);
+  assert.strictEqual(copied, 1);
+  // targetStart > 2**32 must not wrap to a low offset.
+  assert.strictEqual(target[threshold + 1], 0xCD);
+  // The low offset that uint32 truncation would have written to must remain
+  // untouched.
+  assert.strictEqual(target[(threshold + 1) >>> 0], 0);
+}
+
+// Test 3: Buffer.concat with a single >4 GiB buffer should preserve the
+// trailing bytes, exercising the original repro from the issue.
+{
+  largeBuffer.fill(0x6F); // 'o'
+  const result = Buffer.concat([largeBuffer]);
+  assert.strictEqual(result.length, overflow);
+  // The final byte beyond 2**32 must survive concat.
+  assert.strictEqual(result[overflow - 1], 0x6F);
+}