robobun 2ebf6c16b6 Fix bounds check in Buffer writeBigInt64/writeBigUInt64 methods (#23781) ...

## Summary

Fixed an unsigned integer underflow in the bounds check for
`writeBigInt64LE`, `writeBigInt64BE`, `writeBigUInt64LE`, and
`writeBigUInt64BE` methods.

## Problem

When `byteLength < 8`, the bounds check `offset > byteLength - 8` would
cause unsigned integer underflow (since both are `size_t`), resulting in
a large positive number that would pass the check. This allowed
out-of-bounds writes and caused ASAN use-after-poison errors.

**Reproduction:**
```js
const buf = Buffer.from("Hello World");
const slice = buf.slice(0, 5);
slice.writeBigUInt64BE(4096n, 10000); // ASAN error!
```

## Solution

Added an explicit `byteLength < 8` check before the subtraction to
prevent the underflow. The fix is applied to all four functions:
- `writeBigInt64LE` (src/bun.js/bindings/JSBuffer.cpp:2464)
- `writeBigInt64BE` (src/bun.js/bindings/JSBuffer.cpp:2504)
- `writeBigUInt64LE` (src/bun.js/bindings/JSBuffer.cpp:2543)
- `writeBigUInt64BE` (src/bun.js/bindings/JSBuffer.cpp:2582)

## Test plan

- Added comprehensive regression tests covering all edge cases
- Verified the original reproduction case now throws a proper RangeError
instead of crashing
- All tests pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>

2025-10-18 16:52:07 -07:00

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