Originally published June 7, 2007 on OpenRCE.

This topic comes up occasionally, so it's worth a quick investigation. Your IDA directory has a subdirectory called 'ids' that contains more directories, which in turn contain .IDS files. .IDS files do two things: they define a mapping between ordinal numbers and symbol names (which may be mangled, and may contain the number of function arguments and their types), and secondly they allow (optional) comments for those functions.

The IDSUtil Package from Hex-Rays' website (only available to customers) provides tools to create .IDT files from statically-linked libraries and then to convert those into .IDS files. .IDT files are flat text files whose syntax is described in the readme.txt inside of the IDSUTIL package.

The 'ar2idt' tool produces an .IDT file from a .LIB. Its command-line syntax is "ar2idt [filename].[lib/obj/o/etc.]" to produce [filename].IDT. This tool supports several different object-file formats, as different compiler vendors use different ones.

Here's a sample from an .IDT file:

0 Name=MSGS.DLL
1 Name=??0CBaseMtm@@IAE@AAVCRegisteredMtmDll@@AAVCMsvSession@@@Z
2 Name=??0CBaseServerMtm@@IAE@AAVCRegisteredMtmDll@@PAVCMsvServerEntry@@@Z
3 Name=??0CMsgActive@@IAE@H@Z
4 Name=??0CMsvDefaultServices@@QAE@XZ
5 Name=??0CMsvEntrySelection@@QAE@XZ
313 Name=??0CMsvFindOperation@@IAE@AAVCMsvSession@@ABVTDesC16@@IAAVTRequestStatus@@@Z
314 Name=??0CMsvFindResultSelection@@QAE@XZ
6 Name=??0CMsvOperation@@QAE@AAVCMsvSession@@HAAVTRequestStatus@@@Z

After you have an .IDT file, the zipids.exe tool is used to turn an .IDT file into an .IDS file. Its command-line is simply "zipids [filename].IDT" to create [filename].IDS.

A SymbianOS Example

While reverse engineering a SymbianOS worm in February 2006, I noticed that IDA wouldn't convert some by-ordinal imports from SymbianOS DLLs into their real names:

.idata:00405678 ;
.idata:00405678 ; Imports from PBKENG[101f4cce].DLL
.idata:00405678 ;
.idata:00405678 IMPORT __imp_PBKENG_18; DATA XREF: .text:off_404568
.idata:0040567C IMPORT __imp_PBKENG_21; DATA XREF: .text:off_4045A8
.idata:00405680 IMPORT __imp_PBKENG_43; DATA XREF: .text:off_404518
.idata:00405684 IMPORT __imp_PBKENG_72; DATA XREF: .text:off_404588
.idata:00405688 IMPORT __imp_PBKENG_73; DATA XREF: .text:off_404578
.idata:0040568C IMPORT __imp_PBKENG_101 ; DATA XREF: .text:off_404528
.idata:00405690 IMPORT __imp_PBKENG_110 ; DATA XREF: .text:off_404538
.idata:00405694 IMPORT __imp_PBKENG_173 ; DATA XREF: .text:off_404508
.idata:00405698 IMPORT __imp_PBKENG_180 ; DATA XREF: .text:off_404548
.idata:0040569C IMPORT __imp_PBKENG_185 ; DATA XREF: .text:off_404558
.idata:004056A0 IMPORT __imp_PBKENG_254 ; DATA XREF: .text:off_404598

I installed the SymbianOS SDK and then came up with a convoluted series of scripts wrapped around the GNU tool suite that would extract the function names and their ordinals from the relevant .LIB, and then create an IDC script that would rename any import-by-ordinal to its real name. A friend chuckled at this Rube Goldberg-esque contraption and suggested that I use the IDSUTIL package instead.

It couldn't be easier: just type "ar2idt pbkeng.lib && zipids pbkeng.idt" to produce an .IDS file for the pbkeng.lib static library. Now inside of IDA, go to File->Load File->IDS File, and select the .IDS file that was created. Alternatively, you can put this in the %IDA%\ids\epoc6\arm directory to have IDA load it automatically (after a restart). Here are the results of applying it:

.idata:00405678 ;
.idata:00405678 ; Imports from PBKENG[101f4cce].DLL
.idata:00405678 ;
.idata:00405678 ; CPbkContactItem::CardFields(void)const
.idata:00405678 IMPORT CardFields__C15CPbkContactItem
.idata:00405678 ; DATA XREF: .text:off_404568
.idata:0040567C ; CPbkContactEngine::CloseContactL(long)
.idata:0040567C IMPORT CloseContactL__17CPbkContactEnginel
.idata:0040567C ; DATA XREF: .text:off_4045A8
.idata:00405680 ; CPbkContactEngine::CreateContactIteratorLC(int)
.idata:00405680 IMPORT CreateContactIteratorLC__17CPbkContactEnginei
.idata:00405680 ; DATA XREF: .text:off_404518
.idata:00405684 ; CPbkFieldInfo::FieldId(void)const
.idata:00405684 IMPORT FieldId__C13CPbkFieldInfo
.idata:00405684 ; DATA XREF: .text:off_404588

MFC Example

Let's see how to convert the MFC .DEF file into an .IDS file. First, here's a snippet from the .DEF file:

; This is a part of the Microsoft Foundation Classes C++ library.
; Copyright (C) 1992-1998 Microsoft Corporation
; All rights reserved.

LIBRARY MFC42 

EXPORTS
DllGetClassObject @ 1 PRIVATE
DllCanUnloadNow @ 2 PRIVATE
DllRegisterServer @ 3 PRIVATE
DllUnregisterServer @ 4 PRIVATE
?classCCachedDataPathProperty@CCachedDataPathProperty@@2UCRuntimeClass@@B @ 5 DATA
?classCDataPathProperty@CDataPathProperty@@2UCRuntimeClass@@B @ 6 DATA
; MFC 4.2(final release)
??0_AFX_CHECKLIST_STATE@@QAE@XZ @ 256 NONAME

We can see that lines starting with a ";" are comments, any line containing the string " @ " is an actual export declaration, and everything else is part of the DEF file structure. We only want the export declarations. Let's run a quick sed/awk script on the .DEF file:

sed -e '/^ *;/d' MFC42.def | sed -n -e '/ @ /p' | gawk '{ print $3 " Name="$1 }' > MFC42.idt && zipids MFC42.idt

The first part of that command erases any comment-lines (those that begin with any number of spaces and then a semi-colon); the second part accepts any line that contains the string " @ "; and the third part converts the results into the .IDT file format.

To complete the job, we need to manually add a line that says "0 Name=MFC42.dll" to the top of the file. Also, be sure to name the .IDT file the same as the DLL/LIB base name, e.g. mfc42.idt. As before, we then run zipids on it to produce an .IDS file, which can be loaded into IDA and/or put into the %IDA%\ids directory to have it loaded automatically when appropriate.

Before applying the .IDS file:

.idata:4BB710DC extrn __imp_MFC42_6467:dword ; DATA XREF: MFC42_6467

Afterwards:

.idata:4BB710DC ; public: __thiscall AFX_MAINTAIN_STATE2::AFX_MAINTAIN_STATE2(class AFX_MODULE_STATE *)
.idata:4BB710DC extrn ??0AFX_MAINTAIN_STATE2@@QAE@PAVAFX_MODULE_STATE@@@Z:dword