detect-params

name	detect-params
description	Detect function parameters from prologue and register usage — infer types, count, and calling convention
argument-hint	<address>

/probe:detect-params

Detect the parameters of a function by analyzing its prologue, register usage, and instruction patterns. Infers parameter count, types, and calling convention for Windows x64 (__fastcall).

Arguments

• address (required): Address of the function start (hex). Can be absolute or module+offset.

Steps

1. Disassemble the function

probe.exe "disasm func <address>"

If disasm func fails or returns too many instructions (>500), fall back to a fixed count:

probe.exe "disasm <address> 80"

Record the full disassembly output. You will scan it for register usage patterns.

2. Analyze the prologue for saved registers

Look at the first 5-10 instructions for the function prologue pattern:

Non-volatile register saves (tells you how many locals the function needs):

mov [rsp+0x08], rcx    ; saves arg1 to shadow space
mov [rsp+0x10], rdx    ; saves arg2 to shadow space
mov [rsp+0x18], r8     ; saves arg3 to shadow space
push rbx               ; saves non-volatile register
sub rsp, 0x40          ; allocate stack frame

Key prologue patterns:

• mov [rsp+0x08], rcx early -- function saves first arg, confirms RCX is used
• mov [rsp+0x10], rdx early -- function saves second arg, confirms RDX is used
• movaps [rsp+...], xmm6 or movups -- saves non-volatile XMM registers (function uses floats internally)
• Large sub rsp, 0x?? -- many locals or large stack-allocated structs

3. Scan for first usage of each argument register

Walk through the disassembly and find the first occurrence of each argument register. The key rule: if a register is read before it is written, it is a parameter.

Integer argument registers (Windows x64 ABI):

Register	Arg #	Notes
RCX/ECX/CX/CL	1	Also this pointer for member functions
RDX/EDX/DX/DL	2
R8/R8D/R8W/R8B	3
R9/R9D/R9W/R9B	4

Float argument registers (shadow the same slots):

Register	Arg #	Notes
XMM0	1	Replaces RCX when arg1 is float/double
XMM1	2	Replaces RDX when arg2 is float/double
XMM2	3	Replaces R8 when arg3 is float/double
XMM3	4	Replaces R9 when arg4 is float/double

For each register, classify the first instruction that references it:

Read (parameter confirmed):

• mov rax, [rcx] -- reads from RCX (pointer dereference)
• test rcx, rcx -- tests RCX (null check)
• cmp edx, 0x10 -- compares RDX (integer comparison)
• mov [rsp+...], rcx -- saves RCX (shadow space store)
• movss xmm4, xmm0 -- copies XMM0 (float parameter save)

Write (NOT a parameter -- register is being assigned):

• xor ecx, ecx -- zeroes RCX
• mov rcx, rax -- overwrites RCX
• lea rcx, [rip+...] -- loads address into RCX (preparing a call)

Stop scanning a register once you find its first read or write. If the first usage is a write, that register slot is NOT a parameter.

4. Infer parameter types

For each confirmed parameter register, determine the type from how it is used:

Pointer type (most common for RCX):

• mov rax, [rcx] -- dereferences as pointer (vtable load = this pointer)
• mov eax, [rcx+0x354] -- struct field access at known offset
• lea rax, [rcx+0x10] -- address arithmetic on pointer
• test rcx, rcx followed by je -- null check (definitely a pointer)
• call [rax+0x??] after mov rax, [rcx] -- virtual function call (RCX is object pointer)

Integer type:

• cmp edx, <immediate> -- range check
• and edx, 0xFF -- masking to byte
• shl edx, 2 -- bit shift (index calculation)
• add edx, eax -- arithmetic
• mov [rax+...], edx -- stored as 32-bit value

Boolean type:

• test dl, dl -- boolean test
• movzx eax, dl -- zero-extend byte
• cmp cl, 0 / cmp cl, 1 -- explicit boolean check

Float type:

• movss [rsp+...], xmm0 -- saved as float
• comiss xmm0, xmm1 -- float comparison
• mulss xmm0, [rcx+...] -- float multiply
• addss xmm0, xmm1 -- float add
• cvtsi2ss xmm0, eax -- int-to-float conversion (function receives int, converts)

String type (const char):*

• RCX/RDX passed to a function that eventually calls strlen, strcmp, or prints it
• The register value is used with movzx eax, byte [rcx] -- reading characters

5. Detect this pointer (C++ member function)

A function is likely a member function if RCX shows these patterns:

• mov rax, [rcx] as the first or second instruction -- vtable load (definitive)
• Multiple field accesses like [rcx+0x10], [rcx+0x354], [rcx+0x3F3] -- reading struct fields
• this is passed through to callees: mov rcx, rbx where RBX was saved from the original RCX
• The function appears in a vtable (rtti vtable results include this address)

If RCX is a this pointer, the effective parameter count starts from RDX (arg2 becomes the "first visible" parameter).

6. Check for stack parameters (5th+ arguments)

If the function accesses [rsp+0x28] or higher offsets early in the function (before allocating locals), these are stack-passed arguments:

mov rax, [rsp+0x28]    ; 5th argument (after 0x20 shadow space)
mov rax, [rsp+0x30]    ; 6th argument
mov rax, [rsp+0x38]    ; 7th argument

Important: After sub rsp, N, the stack offsets shift. Stack parameters are at [rsp+N+0x28], [rsp+N+0x30], etc. Only check for stack params in instructions BEFORE the sub rsp or adjust offsets accordingly.

7. Detect return type

Scan the function's epilogue (instructions before ret):

• xor eax, eax then ret -- returns 0 (bool false or int 0)
• mov eax, 1 then ret -- returns true or success
• mov eax, [rbx+0x??] then ret -- returns an int32 field (getter)
• mov rax, rbx then ret -- returns a pointer
• lea rax, [rbx+0x??] then ret -- returns pointer to sub-object
• movss xmm0, [rbx+0x??] then ret -- returns float
• No eax/rax/xmm0 assignment before ret -- likely void return

Check multiple return paths (the function may have early returns with different patterns).

8. Cross-reference with call sites (optional, for validation)

Find callers to see what arguments they actually pass:

probe.exe "xref scan <address> <module> --type CALL --limit 5"

For each caller, disassemble the setup before the call:

probe.exe "disasm <call_site - 0x20> 12"

The argument setup at call sites confirms the parameter count and types:

• If callers always set RDX before calling, the function has at least 2 parameters
• If callers pass lea rdx, [rip+...] (string), the second parameter is const char*
• If callers pass xmm1 instead of RDX, the second parameter is float

9. Present the function signature

Build a C-style function signature:

Function: sub_7FF612345678 (client.dll + 0x5678)
Size: ~0x1A0 bytes

Detected Signature:
  int __fastcall sub_7FF612345678(void* this, int damage, const char* source, bool notify)

Parameters:
  RCX  this      void*         vtable at [rcx], field access [rcx+0x354], [rcx+0x3F3]
  RDX  damage    int32         compared with immediate, used in arithmetic
  R8   source    const char*   lea r8, [rip+...] -> "player" at call sites
  R9   notify    bool          test r9b, r9b -> conditional branch

Return:
  EAX  int32     xor eax,eax before early ret; mov eax,[rbx+0x354] before normal ret

Stack frame: 0x40 bytes (sub rsp, 0x40)
Saved registers: RBX, RDI (non-volatile)

If using the GUI bridge, label the function with the detected signature:

curl -s -X POST http://localhost:9996 -H "Content-Type: application/json" \
  -d '{"action":"store","store":"label","method":"setLabel","args":["0x<address>","int sub_XXXX(void* this, int arg2, char* arg3, bool arg4)"]}'

If the function is very small (< 5 instructions)

Small functions have obvious signatures:

• Getter: mov eax, [rcx+0x354]; ret -- int GetField(void* this) -- 1 param (this)
• Setter: mov [rcx+0x354], edx; ret -- void SetField(void* this, int value) -- 2 params
• Boolean getter: movzx eax, byte [rcx+0x103]; ret -- bool GetFlag(void* this) -- 1 param
• Thunk: jmp <other> -- follow the jump and analyze the real function

If register usage is ambiguous

Sometimes a register is both read and written in complex ways. Fallback strategies:

1. Check .pdata for unwind info -- it tells you the parameter count for SEH:

   probe.exe "call info <address>"
   

2. Count shadow space stores -- the number of mov [rsp+0x??], reg instructions in the prologue (where offset is 0x08, 0x10, 0x18, 0x20) directly indicates the parameter count.

3. Check callers -- the most reliable way to confirm parameter count is to see what callers actually pass.

Tips

• Windows x64 ABI always reserves 32 bytes of shadow space (0x20) even for functions with fewer than 4 parameters
• Leaf functions (no calls) may skip the prologue entirely -- no sub rsp, no register saves
• Variadic functions (printf-style) store all 4 register args to shadow space unconditionally
• If XMM0 is used AND RCX is used for different purposes, the function has a mix of float and int parameters
• The this pointer convention means C++ methods have one fewer "visible" parameter than register analysis suggests
• Compiler optimizations may pass parameters in unexpected registers -- callers are the ground truth