# gwatch - Global Variable Watcher

A tool to monitor writes to global integer variables in Linux binaries using ptrace single-stepping.

## Features

- Monitors specific global integer variables in a running program
- Detects and reports all writes to the watched variable
- Supports PIE (Position Independent Executable) binaries
- Passes command-line arguments to the target program
- Uses DWARF debug information to locate variables
- Tab-delimited output format for easy parsing

## Building

```bash
cmake -B build
cmake --build build
```

## Usage

```bash
gwatch --var <symbol> --exec <path> [-- arg1 arg2 ... argN]
```

**Arguments:**
- `--var <symbol>`: Name of the global variable to watch (required)
- `--exec <path>`: Path to the executable to monitor (required)
- `-- arg1 arg2 ...`: Optional arguments to pass to the target program

**Note:** The target binary must be compiled with debug symbols (`-g` flag).

**Examples:**

```bash
# Watch global_counter in test_access
./build/gwatch --var global_counter --exec ./test_access

# Watch with program arguments
./build/gwatch --var global_counter --exec ./test_with_args -- hello world 123
```

## Output Format

When watchpoints trigger, gwatch outputs access events in the following format:

**For writes:**
```
<symbol>\twrite\t<old_value>-><new_value>
```

**For reads:**
```
<symbol>\tread\t<value>
```

Example output:
```
global_counter	read	0
global_counter	write	0->42
global_counter	read	42
global_counter	write	42->52
```

## Implementation Details

### Variable Detection
- Uses libdwarf to parse DWARF debug information
- Identifies global integer variables (int, long, short, char, unsigned variants)
- Handles const/volatile type qualifiers
- Retrieves variable address and size from debug info

### Single-Stepping Approach
The tool uses `PTRACE_SINGLESTEP` to monitor memory access:
- Executes the target program one instruction at a time
- After each instruction, reads the watched variable's value from memory
- Detects writes by comparing current value to previous value
- Reports changes in the specified tab-delimited format

### PIE Binary Support
For Position Independent Executables, the tool:
1. Reads `/proc/[pid]/maps` to find the base load address (lowest mapping)
2. Adds the load address to the DWARF-provided offset
3. Monitors the calculated runtime address

### Write Detection
When monitoring the variable:
1. The current value is read from the traced process memory using `ptrace(PTRACE_PEEKDATA)`
2. If the value has changed since the last check, it's reported as a write
3. Values are formatted according to the variable size (1, 2, 4, or 8 bytes)
4. Sign extension is applied for smaller integer types

## Performance Considerations

The tool uses single-stepping which executes one CPU instruction at a time. This is:
- **Very thorough**: Catches every write to the watched variable
- **Slow**: Adds significant overhead compared to native execution
- **Reliable**: Works consistently across different systems and configurations

For programs with many instructions (100K+ steps), there will be noticeable slowdown.

## Testing

Test programs are provided:
- `test_access.c`: Program that reads and writes to `global_counter`
- `test_with_args.c`: Program that accepts command-line arguments and modifies `global_counter`

Compile test programs with:
```bash
gcc -g -O0 -o test_access test_access.c
gcc -g -O0 -o test_with_args test_with_args.c
```

Run tests:
```bash
# Basic test
./build/gwatch --var global_counter --exec ./test_access

# Test with arguments
./build/gwatch --var global_counter --exec ./test_with_args -- hello world 123
```

## Future Work

- Optimize performance (possibly using hardware watchpoints if system supports it)
- Add support for watching non-integer types (floats, structs, etc.)
- Support multiple simultaneous variables
- Add read detection (currently only writes are reported)
- Output instruction pointer (RIP) for each access
- Better error reporting and diagnostics
- Optional: Skip library code and focus on main program only