Find CNetworkGameServerBaseGetChallengeType (Agent fallback)

Locate the CNetworkGameServerBase_GetChallengeType virtual function in CS2 engine2.dll / libengine2.so using IDA Pro MCP tools. This is the Agent fallback for the find-CNetworkGameServerBase_GetChallengeType preprocessor: it runs only when the preprocessor script returned failure. It reproduces the LLM_DECOMPILE step by hand — collecting the virtual-call reference to GetChallengeType inside its caller CNetworkGameServerBase_ReplyChallenge.

The output is a virtual dispatch (found_vcall): the YAML records the vtable slot and a vfunc_sig that pins the call instruction. There is no concrete implementation address — do not resolve or emit func_va.

Realworld Function Reference

Read the platform-relevant reference YAML before searching in IDA. It provides concrete disassembly, decompiler output, and the annotated call site. Treat its addresses and offsets as reference-build values only; verify every result against the current binary.

• Windows: ida_preprocessor_scripts/references/engine/CNetworkGameServerBase_ReplyChallenge.windows.yaml
• Linux: ida_preprocessor_scripts/references/engine/CNetworkGameServerBase_ReplyChallenge.linux.yaml

Background — where GetChallengeType is called

CNetworkGameServerBase_ReplyChallenge(this, netadr, a3) builds an S2C_CHALLENGE reply. Very early — right after it initializes a bitbuf on the stack (a call of the form sub_XXXX(&buf, storage, 512, 0xFFFFFFFF)) — it makes a virtual call on the server object itself to obtain the challenge / authentication type:

// Linux reference (offset 0x2B0)
v5 = (*(__int64 (__fastcall **)(__int64, _DWORD *))(*(_QWORD *)a1 + 688LL))(a1, a2);
// Windows reference (offset 0x288)
v6 = (*(__int64 (__fastcall **)(__int64, netadr_t *))(*(_QWORD *)a1 + 648LL))(a1, a2);

The returned value is the auth/challenge type. It is written into the bitbuf and later logged as the %u auth %d field of "Sending S2C_CHALLENGE [%u auth %d] to %s\n", and it drives the if (type != 3) / if (type == 3) branch. That semantic role is the anchor — not the raw offset, which changes across updates.

Step 0. Skip if already produced

If CNetworkGameServerBase_GetChallengeType.<platform>.yaml already exists next to the binary and parses to a non-empty mapping, skip — nothing to do. /get-func-from-yaml also reports existence.

Step 1. Load and decompile the caller

ALWAYS Use SKILL /get-func-from-yaml with func_name=CNetworkGameServerBase_ReplyChallenge to obtain its func_va.

If the skill returns an error, STOP and report to user (this fallback cannot run without the predecessor).

Decompile it:

mcp__ida-pro-mcp__decompile addr="<CNetworkGameServerBase_ReplyChallenge.func_va>"

Confirm the this register (first argument — rcx on Windows, rdi on Linux).

Step 2. Locate the GetChallengeType vcall

Find the first virtual call dispatched on this in ReplyChallenge:

mov     rax, [this]            ; load the object's vtable
...
call    qword ptr [rax+OFF]    ; OFF = vfunc_offset  (ref: 0x2B0 Linux / 0x288 Windows)

Confirm the semantic fingerprint:

1. It is the vcall on the server object (this), not on a client / netchan pointer.
2. It appears right after the bitbuf-init call sub_XXXX(&buf, storage, 512, 0xFFFFFFFF).
3. Its return value flows into the "Sending S2C_CHALLENGE [%u auth %d] to %s\n" log (the %d/%u auth field) and gates the != 3 branch.

Then:

• vfunc_offset = OFF (the displacement in the call qword ptr [rax+OFF] instruction)
• vfunc_index = vfunc_offset / 8 (ref: 86 Linux 0x2B0, 81 Windows 0x288)

Step 3. Generate the vfunc signature

ALWAYS Use SKILL /generate-signature-for-vfuncoffset on the call qword ptr [rax+OFF] instruction to obtain vfunc_sig (the offset bytes are fixed in the signature; vfunc_sig_disp is 0).

Step 4. Write the YAML

ALWAYS Use SKILL /write-vfunc-as-yaml with:

• func_name: CNetworkGameServerBase_GetChallengeType
• vtable_name: CNetworkGameServerBase
• vfunc_offset: the resolved offset (hex, e.g. 0x2B0 / 0x288)
• vfunc_index: vfunc_offset / 8
• vfunc_sig: from Step 3
• func_addr: None (no concrete implementation address for a virtual dispatch)
• func_sig: None

Failure handling

• If the predecessor CNetworkGameServerBase_ReplyChallenge YAML is missing → STOP and report to user.
• If the vcall cannot be located → STOP and report exactly what could not be found, so the user can extend the reference.

Output YAML filenames

• Windows (engine2.dll): CNetworkGameServerBase_GetChallengeType.windows.yaml
• Linux (libengine2.so): CNetworkGameServerBase_GetChallengeType.linux.yaml