WebAssembly Debugger for JavaScriptCore

A comprehensive debugging solution that enables LLDB debugging of WebAssembly code running in JavaScriptCore's IPInt (In-Place Interpreter) tier through the GDB Remote Serial Protocol.

Related Documentation:
This document: JSC debug server implementation (both Standalone and RWI modes)
RWI_ARCHITECTURE.md: WebKit integration architecture (RWI mode details)
Debugger-Mutator-Protocol.md: Thread synchronization protocol and control flow diagrams

What is this project?

This project implements a WebAssembly debugger server that bridges the gap between LLDB (the LLVM debugger) and WebAssembly code execution in JavaScriptCore. It allows developers to:

Set breakpoints in WebAssembly functions
Step through WebAssembly bytecode instruction by instruction
Inspect WebAssembly locals, globals, and memory
View call stacks across WebAssembly function calls
Disassemble WebAssembly bytecode in real-time

The implementation follows the GDB Remote Serial Protocol standard with wasm extension.

Design and Architecture

High-Level Architecture

┌─────────────────────────────────────────────────────────────────┐
│                        LLDB Debugger                            │
│  ┌─────────────────┐  ┌─────────────────┐  ┌─────────────────┐  │
│  │   Breakpoints   │  │  Symbol Lookup  │  │ Execution Ctrl  │  │
│  │   Management    │  │   & Modules     │  │   & Stepping    │  │
│  └─────────────────┘  └─────────────────┘  └─────────────────┘  │
└─────────────────────────────┬───────────────────────────────────┘
                              │ GDB Remote Protocol (TCP:1234)
                              │
┌─────────────────────────────▼───────────────────────────────────┐
│                      WasmDebugServer                            │
│  ┌─────────────────┐  ┌─────────────────┐  ┌─────────────────┐  │
│  │Execution Handler│  │ Memory Handler  │  │ Query Handler   │  │
│  │(Breakpoints)    │  │ (WASM Memory)   │  │(Capabilities)   │  │
│  └─────────────────┘  └─────────────────┘  └─────────────────┘  │
│  ┌─────────────────┐  ┌─────────────────┐  ┌─────────────────┐  │
│  │ Module Manager  │  │Breakpoint Mgr   │  │                 │  │
│  │ (Virtual Addrs) │  │(Helper Class)   │  │                 │  │
│  └─────────────────┘  └─────────────────┘  └─────────────────┘  │
└─────────────────────────────┬───────────────────────────────────┘
                              │ Module Tracking & Execution Hooks
                              │
┌─────────────────────────────▼───────────────────────────────────┐
│                JavaScriptCore WebAssembly Engine                │
│  ┌─────────────────┐  ┌─────────────────┐  ┌─────────────────┐  │
│  │JSWebAssemblyMod │  │ IPInt Execution │  │ Debug Info      │  │
│  │(Module Tracking)│  │ (Interpreter)   │  │ (PC Mapping)    │  │
│  └─────────────────┘  └─────────────────┘  └─────────────────┘  │
└─────────────────────────────────────────────────────────────────┘

Core Components

1. WasmDebugServer - Main Debug Server

Location: WasmDebugServer.h/cpp
Purpose: Central coordinator implementing GDB Remote Protocol
Two Modes:
- Standalone Mode: TCP socket server (default port 1234) for JSC shell debugging
- RWI Mode: IPC-based communication for WebKit/WebContent debugging (see RWI_ARCHITECTURE.md)
Key Features:
- Protocol packet parsing and response generation
- Contains all protocol handlers and helper classes

2. Protocol Handlers - GDB Command Processing

ExecutionHandler: Breakpoints, continue, step, interrupt
MemoryHandler: Memory read/write operations
QueryHandler: Capability negotiation and queries

3. Helper Classes - Supporting Components

ModuleManager: Virtual address space management and module tracking
BreakpointManager: Breakpoint storage and management
VirtualAddress: 64-bit virtual address encoding for LLDB compatibility

Virtual Address Space Design

The debugger uses a sophisticated virtual address encoding system to present WebAssembly modules and memory to LLDB:

Address Format (64-bit):
- Bits 63-62: Address Type (2 bits)
- Bits 61-32: ID (30 bits) - ModuleID for code, InstanceID for memory  
- Bits 31-0:  Offset (32 bits)

Address Types:
- 0x00 (Memory): Instance linear memory
- 0x01 (Module): Module code/bytecode
- 0x02 (Invalid): Invalid/unmapped regions
- 0x03 (Invalid2): Invalid/unmapped regions

Virtual Memory Layout:
- 0x0000000000000000 - 0x3FFFFFFFFFFFFFFF: Memory regions
- 0x4000000000000000 - 0x7FFFFFFFFFFFFFFF: Module regions  
- 0x8000000000000000 - 0xFFFFFFFFFFFFFFFF: Invalid regions

Testing

Unit Tests

The debugger includes comprehensive unit tests that validate debug info generation for WebAssembly opcodes:

# Run unit tests via WebKit build system
./Tools/Scripts/run-javascriptcore-tests --testwasmdebugger

Opcode Coverage (Base OpType):

[DONE] Special Ops (FOR_EACH_WASM_SPECIAL_OP)
[DONE] Control Flow Ops (FOR_EACH_WASM_CONTROL_FLOW_OP)
[DONE] Unary Ops (FOR_EACH_WASM_UNARY_OP)
[DONE] Binary Ops (FOR_EACH_WASM_BINARY_OP)
[DONE] Memory Load Ops (FOR_EACH_WASM_MEMORY_LOAD_OP)
[DONE] Memory Store Ops (FOR_EACH_WASM_MEMORY_STORE_OP)

Extended Opcode Coverage:

[TODO] Ext1OpType (FOR_EACH_WASM_EXT1_OP)
[PARTIAL] ExtGCOpType (FOR_EACH_WASM_GC_OP) - 2 control flow ops fully tested (BrOnCast, BrOnCastFail), 29 non-control-flow ops have stub tests
[TODO] ExtAtomicOpType (FOR_EACH_WASM_EXT_ATOMIC_OP)
[TODO] ExtSIMDOpType (FOR_EACH_WASM_EXT_SIMD_OP)

Integration Tests

The JSTests/wasm/debugger includes a comprehensive test framework with auto-discovery, parallel execution, and process isolation capabilities:

# Run comprehensive test framework with LLDB and wasm debugger
python3 JSTests/wasm/debugger/test-wasm-debugger.py

For details, see JSTests/wasm/debugger/README.md.

Manual Testing

Standalone Mode (JSC Shell):

Terminal 1 - Start JSC with debugger:

cd JSTests/wasm/debugger/resources/add
VM=<Path-To-WebKitBuild>/Debug && DYLD_FRAMEWORK_PATH=$VM lldb $VM/jsc -- --verboseWasmDebugger=1 --wasm-debugger --useConcurrentJIT=0 main.js

Terminal 2 - Connect LLDB:

lldb -o 'log enable gdb-remote packets' -o 'process connect --plugin wasm connect://localhost:1234'

RWI Mode (WebKit/WebContent):

See RWI_ARCHITECTURE.md for complete setup instructions including:

Starting Safari/MiniBrowser with __XPC_JSC_enableWasmDebugger=1 JSC_enableWasmDebugger=1 flag
Using WasmDebuggerRWIClient to relay LLDB commands
Debugging WebContent processes via Remote Web Inspector

Known Issues and Future Improvements

Per-Page Debuggable Targets

Issue: Ideally each page (URL) hosted by a WebContent process should appear as a separate debuggable target in lldb platform process list, so users can attach to a specific website. Currently there is one WasmDebuggerDebuggable per WebContent process, so when Safari places multiple pages into the same process (and therefore the same VM), all their hostnames are aggregated into a single target entry (e.g. "earth.google.com, github.com").
Why aggregation is acceptable for now: A single WebContent process runs a single WasmDebugServer that owns all Wasm execution for every page it hosts. Splitting that into per-page debuggables would create multiple LLDB sessions backed by the same VM state, which is architecturally incorrect. Aggregation correctly reflects that one LLDB attach covers all pages in the process.
Future improvement: If the architecture evolves so that each page gets its own isolated VM (and thus its own WasmDebugServer), replace the aggregated URL with a per-page WasmDebuggerDebuggable so each URL appears as a distinct, independently attachable target.

WASM Stack Value Type Support

Issue: Current implementation only supports WASM local variable inspection, missing WASM stack value types
Current Support: Local variables with types (parameters and locals in function scope)
Missing Support: Stack values with types
Solution: Extend debugging protocol to expose WASM operand stack contents with proper type information
Benefits: Complete variable inspection during debugging, better understanding of WASM execution state

Extended Opcode Test Coverage

Issue: Current unit tests only cover base OpType opcodes; ExtGCOpType has partial coverage with stub implementations
Complete Coverage:
- ExtGCOpType control flow: BrOnCast, BrOnCastFail (fully tested)
Partial Coverage:
- ExtGCOpType non-control-flow: 29 opcodes have stub tests that need proper implementation
Missing Coverage:
- Ext1OpType (table operations, saturated truncation)
- ExtAtomicOpType (atomic operations)
- ExtSIMDOpType (SIMD operations)

Client Session Management

Issue: Client disconnect, kill, and quit commands only stop the client session for debugging purposes
Location: WasmDebugServer.cpp:348-349
Solution: Introduce various stop states and proper termination handling

X86_64 Support

Issue: The WebAssembly debugger is currently restricted to ARM64 platforms only
Known Problems on Other Platforms:
- x86_64: VMTraps race condition causes register corruption during interrupt handling
- ARM32, iOS: Untested

Protocol References

The following references correspond to the numbered citations used throughout the WebAssembly debugger implementation: