View8

View8 is a static analysis tool designed to decompile serialized V8 bytecode objects (JSC files) into high-level readable code. To parse and disassemble these serialized objects, View8 utilizes a patched compiled V8 binary. As a result, View8 produces a textual output similar to JavaScript.

Requirements

• Python 3.x
• Disassembler binary. Available versions:
• V8 Version 9.4.146.24 (Used in Node V16.x)
• V8 Version 10.2.154.26 (Used in Node V18.x)
• V8 Version 11.3.244.8 (Used in Node V20.x)

For compiled versions, visit the releases page.

Usage

Command-Line Arguments

• input_file: The input file name.
• output_file: The output file name.
• --path, -p: Path to disassembler binary (optional).
• --disassembled, -d: Indicate if the input file is already disassembled (optional).
• --export_format, -e: Specify the export format(s). Options are v8_opcode, translated, and decompiled. Multiple options can be combined (optional, default: decompiled).

Basic Usage

To decompile a V8 bytecode file and export the decompiled code:

python view8.py input_file output_file

Disassembler Path

By default, view8 detects the V8 bytecode version of the input file (using VersionDetector.exe) and automatically searches for a compatible disassembler binary in the Bin folder. This can be changed by specifing a different disassembler binary, use the --path (or -p) option:

python view8.py input_file output_file --path /path/to/disassembler

Processing Disassembled Files

To skip the disassembling process and provide an already disassembled file as the input, use the --disassembled (or -d) flag:

python view8.py input_file output_file --disassembled

Export Formats

Specify the export format(s) using the --export_format (or -e) option. You can combine multiple formats:

• v8_opcode
• translated
• decompiled

For example, to export both V8 opcodes and decompiled code side by side:

python view8.py input_file output_file -e v8_opcode decompiled

By default, the format used is decompiled.

VersionDetector.exe

The V8 bytecode version is stored as a hash at the beginning of the file. Below are the options available for VersionDetector.exe:

• -h: Retrieves a version and returns its hash.
• -d: Retrieves a hash (little-endian) and returns its corresponding version using brute force.
• -f: Retrieves a file and returns its version.

Get V8 Version

The v8 version of a .jsc file can be found using one of the following methods:

• VersionDetector.exe
• https://j4k0xb.github.io/v8-version-analyzer
• If the Node.js binary is available: ./path_to_node -p process.versions.v8
• If the Electron binary is available:
  • Linux/Mac: ELECTRON_RUN_AS_NODE=1 ./path_to_electron_app -p process.versions.v8
  • Windows: set ELECTRON_RUN_AS_NODE=1 && path_to_electron_app -p process.versions.v8

Sometimes there isn't a matching v8 version because it has been edited. In this case, just select the closest one before.

Building The Disassembler

Guide/disassembler/patch based on v8dasm and https://github.com/v8/v8/tree/10.6.194.26.

1. Check out your v8 version: https://v8.dev/docs/source-code

2. Apply the patch:

   git apply -3 v8.patch

   It's expected that a few merge conflicts occur for different versions, resolve them manually.

3. Create a build configuration:

   ./tools/dev/v8gen.py x64.release

4. Edit the build flags in out.gn/x64.release/args.gn:

   dcheck_always_on = false
   is_component_build = false
   is_debug = false
   target_cpu = "x64"
   use_custom_libcxx = false
   v8_monolithic = true
   v8_use_external_startup_data = false
   
   v8_static_library = true
   v8_enable_disassembler = true
   v8_enable_object_print = true

   • For Node: add v8_enable_pointer_compression = false

5. Build the static library:

   ninja -C out.gn/x64.release v8_monolith

6. Compile the disassembler:

   (For very old v8 versions, v8dasm_legacy.cpp might be needed instead).

   • For Node:

     clang++ v8dasm.cpp -g -std=c++17 -Iinclude -Lout.gn/x64.release/obj -lv8_libbase -lv8_libplatform -lv8_monolith -o v8dasm

   • For Electron:

     clang++ v8dasm.cpp -g -std=c++17 -Iinclude -Lout.gn/x64.release/obj -lv8_libbase -lv8_libplatform -lv8_monolith -o v8dasm -DV8_COMPRESS_POINTERS