find-cphysbox-use-patchcaller

name

find-CPhysBox_Use_PatchCaller

description

Find and identify the CPhysBox_Use_PatchCaller patch in CS2 binary using IDA Pro MCP. This patch changes the third argument of the sub_XXXXXXXX call inside CPhysBox_Use from inputdata->pCaller to this (the CPhysBox entity pointer), so the caller is the physbox itself. Use this skill when reverse engineering CS2 server.dll or server.so to locate and patch the caller argument. Trigger: CPhysBox_Use patch caller, CPhysBox_Use_PatchCaller, physbox use caller patch

CPhysBox_Use_PatchCaller Patch Workflow

Overview

Locate the instruction inside CPhysBox_Use that loads inputdata->pCaller (the third argument to the inner call) and generate a patch that replaces it with this (the CPhysBox entity pointer).

The target code pattern in pseudocode (before patch):

__int64 __fastcall CPhysBox_Use(__int64 pthis, __int64 inputdata)
{
  __int64 result;
  result = CBaseEntity_Use(pthis, inputdata);
  if ( *(_BYTE *)(pthis + 2093) )
    return sub_XXXXXXXX((int)pthis + 2176, *(_QWORD *)inputdata, *(_QWORD *)(inputdata + 8));
                                                                   // ^^^ patch target: pCaller
  return result;
}

After patch:

    return sub_XXXXXXXX((int)pthis + 2176, *(_QWORD *)inputdata, pthis);
                                                                  // ^^^ now uses this

Windows

The target instruction:

mov     r8, [rbx+8]          ; r8 = inputdata->pCaller

Patched to:

mov     r8, rdi              ; r8 = this (pthis)
nop

• Original bytes: 4C 8B 43 08
• Patch bytes: 49 89 F8 90

Linux

The target instruction:

mov     rdx, [r12+8]         ; rdx = inputdata->pCaller

Patched to:

mov     rdx, rbx             ; rdx = this (pthis)
nop
nop

• Original bytes: 49 8B 54 24 08
• Patch bytes: 48 89 DA 90 90

Prerequisites

• CPhysBox_Use must already be identified. Use SKILL /get-func-from-yaml with func_name=CPhysBox_Use to load its address. If YAML does not exist, run SKILL /find-CPhysBox_Use first.

Location Steps

1. Get CPhysBox_Use Function Address

ALWAYS Use SKILL /get-func-from-yaml with func_name=CPhysBox_Use.

If the skill returns an error, stop and report to user.

2. Decompile and Locate the Caller Argument

Decompile the function:

mcp__ida-pro-mcp__decompile(addr="<func_va>")

In the decompiled output, search for the inner call pattern. The key indicators are:

• A call to CBaseEntity_Use(pthis, inputdata) at the beginning
• A byte check: if ( *(_BYTE *)(pthis + <offset>) )
• Inside the if-block: a call with three arguments where the third is *(_QWORD *)(inputdata + 8) — this is inputdata->pCaller

Note the address annotation on the line that loads inputdata + 8.

3. Disassemble Around the Caller Load

Disassemble the function to find the exact instruction that loads inputdata->pCaller:

mcp__ida-pro-mcp__disasm(addr="<func_va>", max_instructions=30)

Windows pattern:

mov     r8, [rbx+8]          ; load inputdata->pCaller into r8 (3rd arg)
lea     rcx, [rdi+880h]      ; 1st arg
mov     rdx, [rbx]            ; 2nd arg = inputdata->pActivator
call    sub_XXXXXXXX

Linux pattern:

mov     rdx, [r12+8]         ; load inputdata->pCaller into rdx (3rd arg in SysV ABI)
...
call    sub_XXXXXXXX

Record:

• patch_va: Address of the mov r8, [rbx+8] (Windows) or mov rdx, [r12+8] (Linux) instruction

4. Determine Patch Bytes

Read the original bytes of the instruction:

mcp__ida-pro-mcp__get_bytes(regions={"addr": "<patch_va>", "size": 5})

Windows (mov r8, [rbx+8] — 4 bytes: 4C 8B 43 08):

• Patch bytes: 49 89 F8 90 (mov r8, rdi + nop)

Linux (mov rdx, [r12+8] — 5 bytes: 49 8B 54 24 08):

• Patch bytes: 48 89 DA 90 90 (mov rdx, rbx + 2x nop)

5. Generate Patch Signature

ALWAYS Use SKILL /generate-signature-for-patch to generate and validate the signature.

Required context for the skill:

• func_name: CPhysBox_Use
• func_va: From step 1
• patch_va: Address of the mov instruction from step 3
• original_instruction: mov r8, [rbx+8] (Windows) or mov rdx, [r12+8] (Linux)
• patched_instruction: mov r8, rdi; nop (Windows) or mov rdx, rbx; nop; nop (Linux)
• description: Patch CPhysBox_Use to pass this instead of inputdata->pCaller as the third argument

6. Write YAML Output

ALWAYS Use SKILL /write-patch-as-yaml to persist the results.

Required parameters:

• patch_name: CPhysBox_Use_PatchCaller
• patch_sig: The validated signature from step 5
• patch_bytes: The computed patch bytes from step 4
• patch_sig_disp: From step 5 result (omit if 0)

Output YAML Files

• CPhysBox_Use_PatchCaller.windows.yaml
• CPhysBox_Use_PatchCaller.linux.yaml