generate-signature-for-patch

name

generate-signature-for-patch

description

Generate and validate unique byte signatures for instructions that need to be runtime-patched using IDA Pro MCP. Use this skill when you need a signature to locate a specific instruction for patching (e.g., force/skip a branch, NOP a call, change an immediate operand). Triggers: patch signature, signature for patch, patch instruction signature, nop signature, jump patch signature, skip branch signature, force branch signature

Generate Signature for Patch

Generate a unique hex byte signature that locates an instruction to be patched at runtime, along with the replacement patch_bytes.

Core Concept

For patch signatures, we signature the instruction to be patched. The signature uniquely identifies the location so a runtime patcher can find and overwrite the original bytes with patch_bytes.

Hard requirements:

1. The target instruction must be fully fixed (no wildcard bytes at all).
2. Instructions other than the target instruction may use wildcarding.
3. Signature length grows by complete instruction boundaries and stops at the shortest unique prefix.
4. patch_bytes are determined by the LLM based on the desired patch effect, not by the script.
5. len(patch_bytes) must equal len(original_instruction_bytes) (pad with 0x90 NOP if the replacement is shorter).

Strategy:

• Forward-only expansion: Expand only forward (after target instruction). The signature may extend beyond the current function boundary into CC padding or the next function. patch_sig_disp is always 0 — the signature always starts at the target instruction.

Prerequisites

• Target instruction address (the instruction to be patched)
• Desired patch effect description (e.g., "skip if-branch", "NOP out function call", "change immediate value")
• IDA Pro MCP connection

Method

1. Determine patch_bytes (LLM Step — before running the script)

Examine the target instruction and its context using IDA Pro MCP, then determine the appropriate patch_bytes.

Common patch patterns:

• Skip conditional branch (near jcc 0F 8x rel32 → jmp rel32): Replace first 6 bytes with E9 <new_rel32> 90. Compute new_rel32 = original branch target − (patch_addr + 5).
• Skip conditional branch (short jcc 7x rel8 → jmp short): Replace 7x rel8 with EB rel8.
• Force conditional branch to fall through: NOP the entire jcc instruction (90 90 ...).
• NOP a call rel32: Replace E8 xx xx xx xx with 90 90 90 90 90.
• NOP a call [reg+disp]: Replace all bytes with 90.
• Change immediate operand: Modify the immediate bytes in-place.

2. Generate and Validate Signature (Single Step)

Use a single py_eval call that:

• Collects instruction bytes from the target instruction forward, tests uniqueness.
• Forward-only expansion (no backward expansion — patch_sig_disp is always 0).
• Enforces no wildcard on the target instruction.
• Computes both VA and RVA for the target instruction.
• Outputs the shortest unique signature as patch_sig with metadata.

mcp__ida-pro-mcp__py_eval code="""
import idaapi, ida_bytes, idautils, ida_ua, ida_segment, json

def main():
    target_inst = <inst_addr>
    min_sig_bytes = 6
    max_sig_bytes = 96
    max_instructions = 64

    image_base = idaapi.get_imagebase()

    f = idaapi.get_func(target_inst)
    if not f:
        print(json.dumps({
            "inst_va": hex(target_inst),
            "error": "target instruction is not inside a known function",
            "status": "failed"
        }))
        return

    insn0 = idautils.DecodeInstruction(target_inst)
    if not insn0 or insn0.size <= 0:
        print(json.dumps({
            "inst_va": hex(target_inst),
            "error": "failed to decode target instruction",
            "status": "failed"
        }))
        return

    raw0 = ida_bytes.get_bytes(target_inst, insn0.size)
    if not raw0:
        print(json.dumps({
            "inst_va": hex(target_inst),
            "error": "failed to read target instruction bytes",
            "status": "failed"
        }))
        return

    seg = ida_segment.get_segm_by_name(".text")
    if seg:
        search_start, search_end = seg.start_ea, seg.end_ea
    else:
        search_start, search_end = idaapi.cvar.inf.min_ea, idaapi.cvar.inf.max_ea

    # --- Helper: wildcard non-target instructions ---
    def wildcard_instruction(addr, insn_obj, raw_bytes):
        wild = set()
        for op in insn_obj.ops:
            ot = int(op.type)
            if ot == int(idaapi.o_void):
                continue
            if ot in (int(idaapi.o_imm), int(idaapi.o_near), int(idaapi.o_far), int(idaapi.o_mem), int(idaapi.o_displ)):
                offb = int(getattr(op, "offb", 0))
                if offb > 0 and offb < insn_obj.size:
                    dsz = ida_ua.get_dtype_size(getattr(op, "dtype", getattr(op, "dtyp", 0)))
                    if dsz <= 0:
                        dsz = insn_obj.size - offb
                    for i in range(offb, min(insn_obj.size, offb + dsz)):
                        wild.add(i)

                offo = int(getattr(op, "offo", 0))
                if offo > 0 and offo < insn_obj.size:
                    dsz2 = ida_ua.get_dtype_size(getattr(op, "dtype", getattr(op, "dtyp", 0)))
                    if dsz2 <= 0:
                        dsz2 = insn_obj.size - offo
                    for i in range(offo, min(insn_obj.size, offo + dsz2)):
                        wild.add(i)

        # Branch/call rel targets are volatile.
        b0 = raw_bytes[0]
        if b0 in (0xE8, 0xE9, 0xEB):
            for i in range(1, insn_obj.size):
                wild.add(i)
        elif b0 == 0x0F and insn_obj.size >= 2 and (raw_bytes[1] & 0xF0) == 0x80:
            for i in range(2, insn_obj.size):
                wild.add(i)
        elif 0x70 <= b0 <= 0x7F:
            for i in range(1, insn_obj.size):
                wild.add(i)

        tokens = []
        for idx in range(insn_obj.size):
            tokens.append("??" if idx in wild else f"{raw_bytes[idx]:02X}")
        return tokens

    # --- Helper: test uniqueness of a token list, expecting match at expected_addr ---
    def test_unique(tokens, expected_addr):
        if all(t == "??" for t in tokens):
            return False
        pattern_str = " ".join("?" if t == "??" else t for t in tokens)
        pat = ida_bytes.compiled_binpat_vec_t()
        ida_bytes.parse_binpat_str(pat, search_start, pattern_str, 16)
        flags = ida_bytes.BIN_SEARCH_FORWARD | ida_bytes.BIN_SEARCH_NOBREAK

        matches = []
        res = ida_bytes.bin_search3(search_start, search_end, pat, flags)
        ea = res[0] if isinstance(res, tuple) else res
        while ea != idaapi.BADADDR and len(matches) < 2:
            matches.append(ea)
            res = ida_bytes.bin_search3(ea + 1, search_end, pat, flags)
            ea = res[0] if isinstance(res, tuple) else res

        return len(matches) == 1 and matches[0] == expected_addr

    # ====================================================================
    # Forward-only expansion (signature starts at target_inst)
    # May extend beyond the current function into CC padding or next function.
    # ====================================================================
    limit_end = target_inst + max_sig_bytes
    fwd_tokens = []
    fwd_boundaries = []
    cursor = target_inst
    inst_count = 0
    target_inst_len = None

    while (
        cursor < search_end
        and cursor < limit_end
        and len(fwd_tokens) < max_sig_bytes
        and inst_count < max_instructions
    ):
        insn = idautils.DecodeInstruction(cursor)
        if not insn or insn.size <= 0:
            break
        raw = ida_bytes.get_bytes(cursor, insn.size)
        if not raw:
            break

        if cursor == target_inst:
            # Target instruction: fully fixed, no wildcards.
            target_inst_len = insn.size
            for idx in range(insn.size):
                if len(fwd_tokens) < max_sig_bytes:
                    fwd_tokens.append(f"{raw[idx]:02X}")
        else:
            toks = wildcard_instruction(cursor, insn, raw)
            for t in toks:
                if len(fwd_tokens) < max_sig_bytes:
                    fwd_tokens.append(t)

        fwd_boundaries.append(len(fwd_tokens))
        cursor += insn.size
        inst_count += 1

    if target_inst_len is None:
        print(json.dumps({
            "inst_va": hex(target_inst),
            "error": "no signature bytes collected",
            "status": "failed"
        }))
        return

    min_boundary = max(min_sig_bytes, target_inst_len)

    # Try expanding at each instruction boundary until unique
    phase1_sig = None
    phase1_boundary = 0
    for boundary in fwd_boundaries:
        if boundary < min_boundary:
            continue
        prefix = fwd_tokens[:boundary]
        if test_unique(prefix, target_inst):
            phase1_sig = " ".join(prefix)
            phase1_boundary = boundary
            break

    if phase1_sig:
        print(json.dumps({
            "patch_sig": phase1_sig,
            "sig_bytes": phase1_boundary,
            "patch_sig_va": hex(target_inst),
            "patch_sig_disp": 0,
            "patch_inst_length": target_inst_len,
            "patch_va": hex(target_inst),
            "patch_rva": hex(target_inst - image_base),
            "original_bytes": " ".join(f"{b:02X}" for b in raw0),
            "status": "success"
        }))
        return

    # Forward-only expansion exhausted without finding a unique signature.
    print(json.dumps({
        "patch_va": hex(target_inst),
        "patch_rva": hex(target_inst - image_base),
        "original_bytes": " ".join(f"{b:02X}" for b in raw0),
        "total_fwd_tokens": len(fwd_tokens),
        "sig_full_fwd": " ".join(fwd_tokens),
        "error": "no unique signature found with forward-only expansion",
        "status": "failed"
    }))

main()
"""

Result handling:

• status == "success" -> Use patch_sig directly as final signature. Proceed to Step 3.
• status == "failed" -> See Step 4.

3. Verify patch_bytes (Optional but Recommended)

After generating the signature, verify the LLM-determined patch_bytes by applying them in IDA and checking the disassembly, then restore the original bytes.

Step 3a: Apply patch and inspect

Use mcp__ida-pro-mcp__patch to write patch_bytes at patch_va:

mcp__ida-pro-mcp__patch addr="<patch_va>" data="<patch_bytes hex>"

Then use mcp__ida-pro-mcp__decompile or mcp__ida-pro-mcp__disasm to verify the patch effect matches the desired behavior.

Step 3b: Restore original bytes

After verification, always restore the original bytes:

mcp__ida-pro-mcp__patch addr="<patch_va>" data="<original_bytes hex>"

If the patch effect does not match expectations, revise patch_bytes and repeat from Step 1.

4. Iterate if Needed

If Step 2 returns status: "failed":

1. Increase max_sig_bytes (e.g. from 96 to 192) and re-run Step 2.
2. Increase max_instructions (e.g. from 64 to 128) if function instructions are short.
3. If still not unique, consider patching a different instruction that achieves the same effect and re-run.

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

Required:

• patch_sig: Space-separated hex bytes with ?? for wildcards.
• patch_bytes: Space-separated hex bytes to write at the patch location.

Recommended metadata:

• patch_sig_va: VA of signature start (always equals patch_va since patch_sig_disp is always 0).
• patch_sig_disp: Always 0 — signature always starts at the target instruction.
• patch_inst_length: Length of the target instruction in bytes.
• patch_va: VA of the instruction to be patched.
• patch_rva: RVA of the instruction to be patched (VA − image base).
• original_bytes: Original bytes of the target instruction (for restore/rollback).

Example Output

Patch effect: skip conditional branch jbe → unconditional jmp (the if block becomes dead code).

patch_sig: "0F 86 AF 00 00 00 0F 57 C0 0F 2E C2"
patch_va: 0x180A00E2F
patch_rva: 0xA00E2F
patch_sig_disp: 0
patch_inst_length: 6
original_bytes: "0F 86 AF 00 00 00"
patch_bytes: "E9 B0 00 00 00 90"

In this example:

• Original instruction: jbe loc_180A00EE4 (6 bytes: 0F 86 AF 00 00 00)
• Patch converts it to jmp loc_180A00EE4 + nop: E9 B0 00 00 00 90
• new_rel32 = 0x180A00EE4 − (0x180A00E2F + 5) = 0xB0 → B0 00 00 00
• The branch now always jumps, making the if-block dead code