---
name: write-patch-as-yaml
description: Write patch analysis results as YAML file beside the binary using IDA Pro MCP. Use this skill after identifying a patch target and generating a signature for it to persist the results in a standardized YAML format.
---

# Write Patch as YAML

Persist a single patch analysis result to a YAML file beside the binary using IDA Pro MCP.

## Prerequisites

Before using this skill, you should have:
1. Identified the patch name and determined `patch_bytes`
2. Generated a unique signature using `/generate-signature-for-patch`

## Required Parameters

| Parameter | Description | Example |
|-----------|-------------|---------|
| `patch_name` | Descriptive name of the patch | `ServerMovementUnlock` |
| `patch_sig` | Unique byte signature locating the instruction to patch | `0F 86 AF 00 00 00 0F 57 C0 0F 2E C2` |
| `patch_bytes` | Replacement bytes to write at the patch location | `E9 B0 00 00 00 90` |

## Optional Parameters

| Parameter | Description | Example |
|-----------|-------------|---------|
| `patch_sig_disp` | Byte displacement from signature start to the target instruction. `0` or `None` means signature starts at the target instruction. Non-zero means backward expansion was used by `/generate-signature-for-patch`. (use `None` to omit) | `5` |

## Method

```python
mcp__ida-pro-mcp__py_eval code="""
import idaapi
import os
import yaml

# === REQUIRED: Replace these values ===
patch_name = "<patch_name>"             # e.g., "ServerMovementUnlock"
patch_sig = "<patch_sig>"               # e.g., "0F 86 AF 00 00 00 0F 57 C0 0F 2E C2"
patch_bytes = "<patch_bytes>"           # e.g., "E9 B0 00 00 00 90"
# ======================================

# === OPTIONAL: Set to None to omit from output ===
patch_sig_disp = <patch_sig_disp>       # e.g., 5 or None (0 also omitted)
# =================================================

# Get binary path and determine platform
input_file = idaapi.get_input_file_path()
dir_path = os.path.dirname(input_file)

if input_file.endswith('.dll'):
    platform = 'windows'
else:
    platform = 'linux'

# Build data dictionary conditionally
data = {}

data['patch_name'] = patch_name
data['patch_sig'] = patch_sig

if patch_sig_disp is not None and patch_sig_disp > 0:
    data['patch_sig_disp'] = patch_sig_disp

data['patch_bytes'] = patch_bytes

yaml_path = os.path.join(dir_path, f"{patch_name}.{platform}.yaml")
with open(yaml_path, 'w', encoding='utf-8') as f:
    yaml.dump(data, f, default_flow_style=False, sort_keys=False, allow_unicode=True)
print(f"Written to: {yaml_path}")
"""
```

## Output File Naming Convention

The output YAML filename follows this pattern:
- `<patch_name>.<platform>.yaml`

Examples:
- `server.dll` → `ServerMovementUnlock.windows.yaml`
- `libserver.so` / `libserver.so` → `ServerMovementUnlock.linux.yaml`

## Output YAML Format

Full output (with `patch_sig_disp` provided):
```yaml
patch_name: ServerMovementUnlock
patch_sig: 48 85 C0 74 ?? E8 ?? ?? ?? ?? 0F 86 AF 00 00 00
patch_sig_disp: 5
patch_bytes: E9 B0 00 00 00 90
```

Standard output (without backward expansion, `patch_sig_disp` is 0 or omitted):
```yaml
patch_name: ServerMovementUnlock
patch_sig: 0F 86 AF 00 00 00 0F 57 C0 0F 2E C2
patch_bytes: E9 B0 00 00 00 90
```

Each field:
- `patch_name` - Descriptive name of the patch
- `patch_sig` - Unique byte signature locating the instruction to patch
- `patch_sig_disp` (optional) - Byte displacement from signature start to the target instruction. Only present when non-zero (backward expansion was used). Runtime: scan for `patch_sig`, then add `patch_sig_disp` to get the target instruction address.
- `patch_bytes` - Replacement bytes to write at the patch location

## Platform Detection

The skill automatically detects the platform based on file extension:
- `.dll` → Windows
- `.so` → Linux

## Example Usage

### Standard patch (forward-only signature)

```python
patch_name = "ServerMovementUnlock"
patch_sig = "0F 86 AF 00 00 00 0F 57 C0 0F 2E C2"
patch_bytes = "E9 B0 00 00 00 90"
patch_sig_disp = None
```

### Patch with backward-expanded signature

```python
patch_name = "DisableSteamBanCheck"
patch_sig = "48 85 C0 74 ?? E8 ?? ?? ?? ?? 0F 84 AA 00 00 00"
patch_bytes = "90 90 90 90 90 90"
patch_sig_disp = 5
```

### NOP a call instruction

```python
patch_name = "SkipPrecacheCall"
patch_sig = "E8 AA BB CC DD 48 8B 5C 24 30"
patch_bytes = "90 90 90 90 90"
patch_sig_disp = None
```

## Notes

- The YAML file is written to the same directory as the input binary
- When `patch_sig_disp` is `None` or `0`, the `patch_sig_disp` field is omitted from the output entirely (signature starts at the target instruction)
- `patch_sig` should be a signature generated by `/generate-signature-for-patch`
- `patch_bytes` must have the same byte count as the original instruction being patched
- `patch_sig_disp` is the byte displacement from signature start to the target instruction, only needed when backward expansion was used
