write-structoffset-as-yaml

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

Write Struct Offset as YAML

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

Prerequisites

Before using this skill, you should have:

1. Identified the struct name and member name
2. Determined the member offset (and optionally size)
3. Generated a unique signature using /generate-signature-for-structoffset

Required Parameters

Parameter	Description	Example
struct_name	Name of the struct/class	CBaseEntity
member_name	Name of the struct member	m_skeletonInstance
offset	Hex offset of the member from struct start	0x278

Optional Parameters

Parameter	Description	Example
size	Size of the member in bytes (use None to omit)	8
offset_sig	Unique byte signature locating an instruction that contains the offset (use None to omit)	8B 93 E0 04 00 00
offset_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-structoffset. (use None to omit)	8

Method

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

# === REQUIRED: Replace these values ===
struct_name = "<struct_name>"           # e.g., "CBaseEntity"
member_name = "<member_name>"           # e.g., "m_skeletonInstance"
offset = <offset>                       # e.g., 0x278
# ======================================

# === OPTIONAL: Set to None to omit from output ===
size = <size>                           # e.g., 8 or None
offset_sig = <offset_sig>              # e.g., "8B 93 E0 04 00 00" or None
offset_sig_disp = <offset_sig_disp>    # e.g., 8 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['struct_name'] = struct_name
data['member_name'] = member_name
data['offset'] = hex(offset)

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

if offset_sig is not None:
    data['offset_sig'] = offset_sig

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

yaml_path = os.path.join(dir_path, f"{struct_name}_{member_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:

• <struct_name>_<member_name>.<platform>.yaml

Examples:

• server.dll → CBaseEntity_m_skeletonInstance.windows.yaml
• libserver.so / libserver.so → CBaseEntity_m_skeletonInstance.linux.yaml

Output YAML Format

Full output (with size, offset_sig, and offset_sig_disp provided):

struct_name: CBaseEntity
member_name: m_skeletonInstance
offset: 0x278
size: 8
offset_sig: FF 50 ?? 48 85 C0 74 ?? 48 8B 80 A0 03 00 00 48 83 C4 28 C3
offset_sig_disp: 8

Output without backward expansion (offset_sig_disp is 0 or omitted):

struct_name: CBaseEntity
member_name: m_skeletonInstance
offset: 0x278
size: 8
offset_sig: 8B 93 78 02 00 00

Minimal output (with size=None, offset_sig=None):

struct_name: CBaseEntity
member_name: m_skeletonInstance
offset: 0x278

Each field:

• struct_name - Name of the struct/class
• member_name - Name of the struct member
• offset - Hex offset from struct start
• size (optional) - Size in bytes
• offset_sig (optional) - Unique byte signature of an instruction containing the offset (e.g., 8B 93 E0 04 00 00 for mov edx, [rbx+4E0h])
• offset_sig_disp (optional) - Byte displacement from signature start to the target instruction. Only present when non-zero (backward expansion was used). Runtime: scan for offset_sig, then add offset_sig_disp to get the target instruction address.

Platform Detection

The skill automatically detects the platform based on file extension:

• .dll → Windows
• .so → Linux

Example Usage

With all parameters

struct_name = "CBaseEntity"
member_name = "m_skeletonInstance"
offset = 0x278
size = 8
offset_sig = "8B 93 78 02 00 00"
offset_sig_disp = None

With backward-expanded signature

struct_name = "CSkeletonInstance"
member_name = "m_animationController"
offset = 0x3A0
size = 8
offset_sig = "FF 50 40 48 85 C0 74 0C 48 8B 80 A0 03 00 00 48 83 C4 28 C3"
offset_sig_disp = 8

Without optional parameters

struct_name = "CBaseEntity"
member_name = "m_skeletonInstance"
offset = 0x278
size = None
offset_sig = None
offset_sig_disp = None

With only size

struct_name = "CBaseEntity"
member_name = "m_iHealth"
offset = 0x408
size = 4
offset_sig = None

With only signature

struct_name = "CBaseEntity"
member_name = "m_nActualMoveType"
offset = 0x4E0
size = None
offset_sig = "8B 93 E0 04 00 00"

Notes

• All offsets are written in hexadecimal format with lowercase 0x prefix
• The YAML file is written to the same directory as the input binary
• When size is None or 0, the size field is omitted from the output entirely
• When offset_sig is None, the offset_sig field is omitted from the output entirely
• When offset_sig_disp is None or 0, the offset_sig_disp field is omitted from the output entirely (signature starts at the target instruction)
• offset_sig should be a signature generated by /generate-signature-for-structoffset
• offset_sig_disp is the byte displacement from signature start to the target instruction, only needed when backward expansion was used