Fix SteamInternal_CreateInterface throw

[Detanup01]

Fix SteamInternal_CreateInterface throw when called by NOT Client related interface version

[otavepto]

I would recommend returning a nullptr with some debug message if the interface name doesn't start with "SteamClient" instead of falling back to the generic interface getter.

Thanks to the weird design of Ghidra I can't export the decompilation in a portable format, but I'll try to explain how steam haneles it.

This is how the original function looks like in steamclient (same on Linux/Windows)

CreateInterface()

undefined8 CreateInterface(char *param_1,undefined4 *param_2)

{
  SteamItf_t **ppSVar1;
  int iVar2;
  undefined8 uVar3;
  SteamItf_t *pSVar4;
  
  pSVar4 = PTR_steamclient_itf_list;
  if (PTR_steamclient_itf_list != (SteamItf_t *)0x0) {
    do {
      iVar2 = strcmp(pSVar4->itf_ver,param_1);
      if (iVar2 == 0) {
        if (param_2 != (undefined4 *)0x0) {
          *param_2 = 0;
        }
                    /* WARNING: Could not recover jumptable at 0x0214c51c. Too many branches */
                    /* WARNING: Treating indirect jump as call */
        uVar3 = (*(code *)pSVar4->factory)();
        return uVar3;
      }
      ppSVar1 = &pSVar4->prev;
      pSVar4 = *ppSVar1;
    } while (*ppSVar1 != (SteamItf_t *)0x0);
  }
  if (param_2 != (undefined4 *)0x0) {
    *param_2 = 1;
  }
  return 0;
}

As you can see, it loops over a linked list and attempts to do a strcmp just like the emu, what we care about is this global pointer PTR_steamclient_itf_list

Based on reversing this structure, this is how I defined it in Ghidra

pic

If you search for all refs to this global pointer and look for the places where it is written, you'll find only 1 place where it is modified.

pic

If we follow this function, this is how it looks like after cleanup

code to add a node

void add_steamclient_itf(SteamItf_t *itf_node,void *factory,char *ver_name)
{
  SteamItf_t *pSVar1;
  
  itf_node->itf_ver = ver_name;
  itf_node->factory = factory;
  pSVar1 = itf_node;
  itf_node->prev = PTR_steamclient_itf_list;
  PTR_steamclient_itf_list = pSVar1;
  return;
}

So, just the usual function which adds a node to the linked list.
And, if you find all refs to this node appending function, you'll land at this code

code to init linked list

void _INIT_230(void)

{
  FUN_029c4f20(&DAT_02b974a0);
  FUN_00df1180(FUN_029c5700,&DAT_02b974a0,&PTR_LOOP_02b50500);
  _DAT_02b97478 = 0xffffffff;
  _DAT_02b97480 = 0xffffffff;
  _DAT_02b97460 = 0xffffffffffffffff;
  _DAT_02b97468 = 0x1ffffffff;
  FUN_01220730(&DAT_02b97490,0,0);
  _DAT_02b97470 = DAT_02b97490;
  FUN_00df1180(FUN_0121dec0,&DAT_02b97460,&PTR_LOOP_02b50500);
  add_steamclient_itf(&DAT_02b97440,FUN_0121aed0,"SteamClient006");
  add_steamclient_itf(&DAT_02b97420,FUN_0121aee0,"SteamClient007");
  add_steamclient_itf(&DAT_02b97400,FUN_0121aef0,"SteamClient008");
  add_steamclient_itf(&DAT_02b973e0,FUN_0121af00,"SteamClient009");
  add_steamclient_itf(&DAT_02b973c0,FUN_0121af10,"SteamClient010");
  add_steamclient_itf(&DAT_02b973a0,FUN_0121af20,"SteamClient011");
  add_steamclient_itf(&DAT_02b97380,FUN_0121af30,"SteamClient012");
  add_steamclient_itf(&DAT_02b97360,FUN_0121af40,"SteamClient013");
  add_steamclient_itf(&DAT_02b97340,FUN_0121af50,"SteamClient014");
  add_steamclient_itf(&DAT_02b97320,FUN_0121af60,"SteamClient015");
  add_steamclient_itf(&DAT_02b97300,FUN_0121af70,"SteamClient016");
  add_steamclient_itf(&DAT_02b972e0,FUN_0121af80,"SteamClient017");
  add_steamclient_itf(&DAT_02b972c0,FUN_0121af90,"SteamClient018");
  add_steamclient_itf(&DAT_02b972a0,FUN_0121afa0,"SteamClient019");
  add_steamclient_itf(&DAT_02b97280,FUN_0121afb0,"SteamClient020");
  add_steamclient_itf(&DAT_02b97260,steamclient_factory_SteamClient021,"SteamClient021");
  add_steamclient_itf(&DAT_02b97240,steamclient_factory_SteamClient022,"SteamClient022");
  return;
}

The weird name _INIT_xxx is GCC's constructor-style .so library entry function, so just a static function defined somewhere and called.

The other refs to this node appending function register some internal interfaces

code to init linked list (2)

add_steamclient_itf(&DAT_02b9d220,FUN_01555ca0,"CLIENTENGINE_INTERFACE_VERSION005");
add_steamclient_itf(&DAT_02b9d200,FUN_01551ac0,"IVALIDATE001");
add_steamclient_itf(&DAT_02bb7420,FUN_01d93900,"p2pvoicesingleton002");
add_steamclient_itf(&DAT_02bb7bb0,FUN_01d9b1b0,"p2pvoice002");

So the eventual conclusion is that real steam will return nullptr in case the interface name didn't match any of these.

I could be wrong or missing some important detail, so take my words with a grain of salt :/
A real test against steam would be the definitive answer.

Edit: tested returning null and the game falls back to calling SteamInternal_FindOrCreateUserInterface as it should have in the first place :/

[Detanup01]

Will change tomorrow (or if you already have stuff feel free to push in this branch)