make-signature

name	make-signature
description	Generate a byte signature for a function that survives binary updates

/probe:sig

Generate a byte signature for a function that survives binary updates.

Arguments

• address (required): Address of the function (hex)
• module (optional): Module name for context

Why signatures matter

Binary updates change function addresses but rarely change the instruction sequence. A byte signature captures the function's unique bytes with wildcards for parts that change (relocations, offsets). This lets you find the function again after every update.

Steps

1. Disassemble the function start:

   probe_disassemble address=<addr> count=15
   

   Look at the first 15-20 instructions. The function prologue and early logic are usually the most unique.

2. Auto-generate a signature:

   probe_generate_signature address=<addr> size=32
   

   Returns a pattern with ?? wildcards for bytes that are likely to change between builds.

3. Test uniqueness:

   probe_test_signature address=<addr> module=<module>
   

   A good signature has exactly 1 match. If it has multiple matches, you need a longer or more specific signature.

4. If not unique, extend the signature:

   • Read more bytes: probe_generate_signature address=<addr> size=64
   • Or manually craft a signature using the disassembly — pick instruction bytes that are distinctive

5. Verify the signature finds the right function:

   probe_pattern_scan pattern=<signature> module=<module>
   

   The result should be the original function address.

What to wildcard (use ??)

Always wildcard:

• RIP-relative displacements (4 bytes after 48 8B 05, 48 8D 0D, E8, etc.) — these change every build
• Stack frame sizes that depend on local variables — compiler may reorder
• Immediate operands that reference build-specific constants

Never wildcard:

• Opcodes (48 8B, 48 89, E8, etc.) — these define the instruction type
• Register encodings — which registers the function uses rarely changes
• ModR/M bytes — encode addressing modes, very stable

Manual signature crafting

If auto-generation fails, read the raw bytes and apply wildcards manually:

Address     Bytes                      Instruction
7FF6A000    48 89 5C 24 08            mov [rsp+8], rbx      ; stable
7FF6A005    57                         push rdi               ; stable
7FF6A006    48 83 EC 20               sub rsp, 0x20          ; stack size might change
7FF6A00A    48 8B D9                  mov rbx, rcx           ; stable
7FF6A00D    E8 XX XX XX XX            call SomeFunc          ; wildcard the call target
7FF6A012    48 85 C0                  test rax, rax          ; stable
7FF6A015    74 XX                     je short label         ; wildcard the branch offset

Resulting signature:

48 89 5C 24 08 57 48 83 EC ?? 48 8B D9 E8 ?? ?? ?? ?? 48 85 C0 74 ??

Good vs bad signatures

Good:

48 89 5C 24 08 57 48 83 EC 20 48 8B D9 E8 ?? ?? ?? ?? 48 85 C0

• Specific prologue pattern (mov [rsp+8], rbx + push rdi)
• Stable register usage
• Unique call sequence

Bad:

48 89 5C 24 ?? 48 83 EC ??

• Too short — hundreds of functions start with mov [rsp+?], reg + sub rsp
• Too many wildcards — matches almost anything

Rule of thumb: 16-32 bytes with 2-4 wildcards is usually enough. If you need more than 48 bytes, the function might not be unique enough — consider combining with a module name filter.

Tips

• Function prologues vary by calling convention — __fastcall (default x64) typically saves non-volatile registers and allocates stack
• Leaf functions (no calls) often have no prologue at all — they just start with the first instruction
• Inlined functions don't have their own prologue — the signature must capture the surrounding context
• Virtual functions can be found via RTTI vtable index instead of a byte signature — this is often more reliable since vtable index doesn't change