generate-signature-for-globalvar

name	generate-signature-for-globalvar
description	Generate and validate unique byte signatures for global variable using IDA Pro MCP. Use this skill when you need to create a pattern-scanning signature for a global variable that can reliably locate it across binary updates. Triggers: global variable signature, signature for global variable

Generate Signature for Global Variable

Generate a unique hex byte signature that locates an instruction accessing a global variable. Users can then resolve the actual global variable address at runtime by parsing the instruction's RIP-relative offset.

Core Concept

Since global variable addresses change between binary updates, we don't signature the GV itself. Instead, we:

1. Find an instruction that references the global variable (mov/lea/cmp/etc.)
2. Generate a signature to locate that instruction
3. At runtime, parse the instruction to resolve the actual GV address

RIP-Relative Addressing (x86-64)

In x86-64, most global variable accesses use RIP-relative addressing:

GV_Address = Instruction_Address + Instruction_Length + RIP_Offset

Where:

• Instruction_Address = address found by pattern scan
• Instruction_Length = total bytes of the instruction (opcode + ModR/M + offset)
• RIP_Offset = signed 32-bit displacement (last 4 bytes of instruction)

Prerequisites

• Global variable address. qword_XXXXXX for example.
• IDA Pro MCP connection

Method

1. Find an instruction that has ref to this global variable

• Find an instruction that has reference to this global variable, for example:

48 8B 1D XX XX XX XX mov rbx, cs:qword_XXXXXX.

2. Analyze and Generate Signature (LLM Task)

YOU (the LLM) must analyze the bytes and disassembly to create a signature.

When analyzing, consider:

• Opcodes (instruction bytes): Usually stable, keep as-is
• Immediates/Offsets: Often change between builds, use ?? wildcards
• Register encodings: Usually stable unless compiler changes register allocation
• Relocation addresses: Always use ?? wildcards (4 bytes for 32-bit, 8 for 64-bit)
• Displacement values in memory operands: May change, consider wildcarding, ESPECIALLY with E8 call, E9 jmp, jnz, jz, je, jne, other conditional branching...

Example analysis:

.text:00000001804F3DF3 48 8B 1D 2E 93 88 01                                mov     rbx, cs:IGameSystem_InitAllSystems_pFirst
.text:00000001804F3DFA 48 85 DB                                            test    rbx, rbx
.text:00000001804F3DFD 0F 84 A4 00 00 00                                   jz      loc_1804F3EA7
.text:00000001804F3E03 BD FF FF 00 00                                      mov     ebp, 0FFFFh

Expected Result: 48 8B 1D ?? ?? ?? ?? 48 85 DB 0F ?? ?? ?? ?? BD FF FF 00 00

Generate a signature that is:

• Long enough to be unique (typically 16-32 bytes)
• Uses ?? for bytes that may change between binary updates
• Includes the GV-accessing instruction (the instruction with RIP-relative offset to the GV)

IMPORTANT: Record the offset from signature start to the GV-accessing instruction. This is needed for runtime resolution.

3. Validate Signature Uniqueness

Test that YOUR generated signature matches ONLY this function:

mcp__ida-pro-mcp__py_eval code="""
import idaapi
import ida_bytes
import ida_segment

func_addr = <func_addr>

# YOUR GENERATED SIGNATURE HERE (space-separated hex with ?? for wildcards)
signature_str = "<YOUR_SIGNATURE>"  # e.g., "48 89 5C 24 08 48 89 74 24 10 57 48 83 EC 20 48 8B F9 E8 ?? ?? ?? ??"

# IDA pattern syntax uses single '?' wildcard tokens (not '??').
pattern_str = " ".join("?" if p == "??" else p for p in signature_str.split())

# Get .text segment bounds
seg = ida_segment.get_segm_by_name(".text")
start = seg.start_ea
end = seg.end_ea

# Compile pattern and search using IDA's native binary search (fast, avoids get_bytes() timeouts)
pat = ida_bytes.compiled_binpat_vec_t()
ida_bytes.parse_binpat_str(pat, start, pattern_str, 16)

flags = ida_bytes.BIN_SEARCH_FORWARD | ida_bytes.BIN_SEARCH_NOBREAK
matches = []

res = ida_bytes.bin_search3(start, end, pat, flags)
ea = res[0] if isinstance(res, tuple) else res  # IDA 9 may return (ea, idx)
while ea != idaapi.BADADDR:
    matches.append(ea)
    res = ida_bytes.bin_search3(ea + 1, end, pat, flags)
    ea = res[0] if isinstance(res, tuple) else res

print(f"Signature matches: {len(matches)}")
for m in matches:
    print(hex(m))

if len(matches) == 1 and matches[0] == func_addr:
    print("SUCCESS: Signature is unique and matches target function!")
elif len(matches) == 1 and matches[0] != func_addr:
    print("WARNING: Signature matches but at different address ! You should re-generate a valid signature that exactly matches the {hex(func_addr)} !")
elif len(matches) > 1:
    print("FAILED: Signature not unique, need longer pattern.")
elif len(matches) == 0:
    print("FAILED: Found nothing with this signature. You should re-generate a valid signature that exactly matches the {hex(func_addr)} !")

"""

4. Iterate if Needed

If the signature is not unique:

1. Extend the signature length, maybe include some preceding padding, or even bytes from next function, to make it unique.
2. Re-validate until unique

5. Continue with Unfinished Tasks

If we are called by a task from a task list / parent SKILL, restore and continue with the unfinished tasks.

Output Format

Provide the following information for runtime GV resolution:

Required Output Fields

1. gv_sig: Space-separated hex bytes with ?? for wildcards
2. gv_sig_va: The virtual address that the signature matches
3. gv_inst_offset: Offset (in bytes) from signature start to the GV-accessing instruction
4. gv_inst_length: Total length of the GV-accessing instruction (for RIP calculation)
5. gv_inst_disp: Position of the 4-byte RIP-relative offset within the instruction

Example Output

gv_sig: "48 8B 1D ?? ?? ?? ?? 48 85 DB 0F 84 ?? ?? ?? ?? BD FF FF 00 00"
gv_sig_va: 0x1804f3df3     # The virtual address that the signature matches
gv_inst_offset: 0          # GV instruction starts at signature start
gv_inst_length: 7          # 48 8B 1D XX XX XX XX = 7 bytes
gv_inst_disp:   3          # Displacement offset start at position 3 (after 48 8B 1D)

Runtime Resolution Formula

At runtime, after pattern scan finds the signature at address scan_result:

// C++ example
uint8_t* inst_addr = scan_result + inst_offset;
int32_t rip_offset = *(int32_t*)(inst_addr + inst_disp);
void* gv_address = inst_addr + inst_length + rip_offset;

# Python example
import struct
inst_addr = scan_result + inst_offset
rip_offset = struct.unpack('<i', memory[inst_addr + inst_disp : inst_addr + inst_disp + 4])[0]
gv_address = inst_addr + inst_length + rip_offset

Signature Analysis Guidelines for LLM

When analyzing function bytes, consider these instruction patterns:

Bytes to Keep (Usually Stable)

• Function prologues: 48 89 5C 24 (mov [rsp+x], rbx), 55 (push rbp), 48 83 EC (sub rsp)
• Opcode bytes: The actual instruction opcodes
• ModR/M bytes: Register-to-register operations
• Fixed immediate values: Constants that are part of the algorithm

Bytes to Wildcard (May Change)

• Relative call/jump offsets: 4 bytes after E8 (call) or E9 (jmp) - these are relative addresses
• Absolute addresses: Any pointer/address in code
• Stack offsets in large functions: May change with local variable layout
• String/data references: Offsets to .rdata/.data sections

Common Patterns

Pattern	Meaning	Wildcard?
E8 XX XX XX XX	Relative call	Wildcard the 4 offset bytes
48 8D 0D XX XX XX XX	LEA with RIP-relative	Wildcard the 4 offset bytes
FF 15 XX XX XX XX	Indirect call via IAT	Wildcard the 4 offset bytes
48 8B 05 XX XX XX XX	MOV from global	Wildcard the 4 offset bytes

GV-Accessing Instruction Reference (x86-64)

Use this table to determine instr_length and offset_position for common GV access patterns:

Opcode Pattern	Example Disasm	instr_length	offset_position	Notes
48 8B 05 XX XX XX XX	mov rax, [rip+XX]	7	3	Load 64-bit from GV
48 8B 0D XX XX XX XX	mov rcx, [rip+XX]	7	3	Load 64-bit from GV
48 8B 15 XX XX XX XX	mov rdx, [rip+XX]	7	3	Load 64-bit from GV
48 8B 1D XX XX XX XX	mov rbx, [rip+XX]	7	3	Load 64-bit from GV
48 8B 35 XX XX XX XX	mov rsi, [rip+XX]	7	3	Load 64-bit from GV
48 8B 3D XX XX XX XX	mov rdi, [rip+XX]	7	3	Load 64-bit from GV
4C 8B 05 XX XX XX XX	mov r8, [rip+XX]	7	3	Load 64-bit from GV (r8-r15)
48 8D 05 XX XX XX XX	lea rax, [rip+XX]	7	3	Load address of GV
48 8D 0D XX XX XX XX	lea rcx, [rip+XX]	7	3	Load address of GV
48 8D 15 XX XX XX XX	lea rdx, [rip+XX]	7	3	Load address of GV
48 8D 1D XX XX XX XX	lea rbx, [rip+XX]	7	3	Load address of GV
48 89 05 XX XX XX XX	mov [rip+XX], rax	7	3	Store 64-bit to GV
48 89 0D XX XX XX XX	mov [rip+XX], rcx