Eric Eric - 11 days ago 6
C# Question

How to programatically read native DLL imports in C#?

How can I programatically analyze a native DLL to read its imports?




[EDIT: my original question looked like the following, along with a huge chunk of defective code. Please see answers below for more correct code.]

The C# code located at this link is intended to print the imports of a native DLL.

I find that when I run the sample code with the original example's target, MSCOREE.DLL, it prints all the imports fine. But when I use other dlls like GDI32.DLL or WSOCK32.DLL the imports do not get printed. What's missing from this code that would let it print all the imports as, for example, DUMPBIN.EXE does?

Answer

There is one very big problem in the code (namely the definition of THUNK_DATA) and various other smaller problems mostly concerning end-of-table detection (using IsBadReadPtr instead of NULL checks, and also not adding base address as needed).

Here is a fixed version that produces the same output as dumpbin at least for wsock32:

using System;
using System.Runtime.InteropServices;
using System.Security;

namespace PETest2
{
    [StructLayout(LayoutKind.Explicit)]
    public unsafe struct IMAGE_IMPORT_BY_NAME
    {
        [FieldOffset(0)]
        public ushort Hint;
        [FieldOffset(2)]
        public fixed char Name[1];
    }

    [StructLayout(LayoutKind.Explicit)]
    public struct IMAGE_IMPORT_DESCRIPTOR
    {
        #region union
        /// <summary>
        /// CSharp doesnt really support unions, but they can be emulated by a field offset 0
        /// </summary>

        [FieldOffset(0)]
        public uint Characteristics;            // 0 for terminating null import descriptor
        [FieldOffset(0)]
        public uint OriginalFirstThunk;         // RVA to original unbound IAT (PIMAGE_THUNK_DATA)
        #endregion

        [FieldOffset(4)]
        public uint TimeDateStamp;
        [FieldOffset(8)]
        public uint ForwarderChain;
        [FieldOffset(12)]
        public uint Name;
        [FieldOffset(16)]
        public uint FirstThunk;
    }

    [StructLayout(LayoutKind.Explicit)]
    public struct THUNK_DATA
    {
        [FieldOffset(0)]
        public uint ForwarderString;      // PBYTE 
        [FieldOffset(0)]
        public uint Function;             // PDWORD
        [FieldOffset(0)]
        public uint Ordinal;
        [FieldOffset(0)]
        public uint AddressOfData;        // PIMAGE_IMPORT_BY_NAME
    }

    public unsafe class Interop
    {
        #region Public Constants
        public static readonly ushort IMAGE_DIRECTORY_ENTRY_IMPORT = 1;
        #endregion
        #region Private Constants
        #region CallingConvention CALLING_CONVENTION
        /// <summary>
        ///     Specifies the calling convention.
        /// </summary>
        /// <remarks>
        ///     Specifies <see cref="CallingConvention.Winapi" /> for Windows to 
        ///     indicate that the default should be used.
        /// </remarks>
        private const CallingConvention CALLING_CONVENTION = CallingConvention.Winapi;
        #endregion CallingConvention CALLING_CONVENTION
        #region IMPORT DLL FUNCTIONS
        private const string KERNEL_DLL = "kernel32";
        private const string DBGHELP_DLL = "Dbghelp";
        #endregion
        #endregion Private Constants

        [DllImport(KERNEL_DLL, CallingConvention = CALLING_CONVENTION, EntryPoint = "GetModuleHandleA"), SuppressUnmanagedCodeSecurity]
        public static extern void* GetModuleHandleA(/*IN*/ char* lpModuleName);

        [DllImport(KERNEL_DLL, CallingConvention = CALLING_CONVENTION, EntryPoint = "GetModuleHandleW"), SuppressUnmanagedCodeSecurity]
        public static extern void* GetModuleHandleW(/*IN*/ char* lpModuleName);

        [DllImport(KERNEL_DLL, CallingConvention = CALLING_CONVENTION, EntryPoint = "IsBadReadPtr"), SuppressUnmanagedCodeSecurity]
        public static extern bool IsBadReadPtr(void* lpBase, uint ucb);

        [DllImport(DBGHELP_DLL, CallingConvention = CALLING_CONVENTION, EntryPoint = "ImageDirectoryEntryToData"), SuppressUnmanagedCodeSecurity]
        public static extern void* ImageDirectoryEntryToData(void* Base, bool MappedAsImage, ushort DirectoryEntry, out uint Size);


    }


    static class Foo
    {
        // From winbase.h in the Win32 platform SDK.
        //
        const uint DONT_RESOLVE_DLL_REFERENCES = 0x00000001;
        const uint LOAD_IGNORE_CODE_AUTHZ_LEVEL = 0x00000010;

        [DllImport("kernel32.dll"), SuppressUnmanagedCodeSecurity]
        static extern uint LoadLibraryEx(string fileName, uint notUsedMustBeZero, uint flags);

        public static void Main()
        {
            //var path = @"c:\windows\system32\mscoree.dll";
            //var path = @"c:\windows\system32\gdi32.dll";
            var path = @"c:\windows\system32\wsock32.dll";
            var hLib = LoadLibraryEx(path, 0,
                                     DONT_RESOLVE_DLL_REFERENCES | LOAD_IGNORE_CODE_AUTHZ_LEVEL);
            TestImports(hLib, true);

        }


        // using mscoree.dll as an example as it doesnt export any thing
        // so nothing shows up if you use your own module.
        // and the only none delayload in mscoree.dll is the Kernel32.dll
        private static void TestImports(uint hLib, bool mappedAsImage)
        {
            unsafe
            {
                //fixed (char* pszModule = "mscoree.dll")
                {
                    //void* hMod = Interop.GetModuleHandleW(pszModule);
                    void* hMod = (void*)hLib;

                    uint size = 0;
                    uint BaseAddress = (uint)hMod;

                    if (hMod != null)
                    {
                        Console.WriteLine("Got handle");

                        IMAGE_IMPORT_DESCRIPTOR* pIID = (IMAGE_IMPORT_DESCRIPTOR*)Interop.ImageDirectoryEntryToData((void*)hMod, mappedAsImage, Interop.IMAGE_DIRECTORY_ENTRY_IMPORT, out size);
                        if (pIID != null)
                        {
                            Console.WriteLine("Got Image Import Descriptor");
                            while (pIID->OriginalFirstThunk != 0)
                            {
                                try
                                {
                                    char* szName = (char*)(BaseAddress + pIID->Name);
                                    string name = Marshal.PtrToStringAnsi((IntPtr)szName);
                                    Console.WriteLine("pIID->Name = {0} BaseAddress - {1}", name, (uint)BaseAddress);

                                    THUNK_DATA* pThunkOrg = (THUNK_DATA*)(BaseAddress + pIID->OriginalFirstThunk);

                                    while (pThunkOrg->AddressOfData != 0)
                                    {
                                        char* szImportName;
                                        uint Ord;

                                        if ((pThunkOrg->Ordinal & 0x80000000) > 0)
                                        {
                                            Ord = pThunkOrg->Ordinal & 0xffff;
                                            Console.WriteLine("imports ({0}).Ordinal{1} - Address: {2}", name, Ord, pThunkOrg->Function);
                                        }
                                        else
                                        {
                                            IMAGE_IMPORT_BY_NAME* pIBN = (IMAGE_IMPORT_BY_NAME*)(BaseAddress + pThunkOrg->AddressOfData);

                                            if (!Interop.IsBadReadPtr((void*)pIBN, (uint)sizeof(IMAGE_IMPORT_BY_NAME)))
                                            {
                                                Ord = pIBN->Hint;
                                                szImportName = (char*)pIBN->Name;
                                                string sImportName = Marshal.PtrToStringAnsi((IntPtr)szImportName); // yes i know i am a lazy ass
                                                Console.WriteLine("imports ({0}).{1}@{2} - Address: {3}", name, sImportName, Ord, pThunkOrg->Function);
                                            }
                                            else
                                            {
                                                Console.WriteLine("Bad ReadPtr Detected or EOF on Imports");
                                                break;
                                            }
                                        }

                                        pThunkOrg++;
                                    }
                                }
                                catch (AccessViolationException e)
                                {
                                    Console.WriteLine("An Access violation occured\n" +
                                                      "this seems to suggest the end of the imports section\n");
                                    Console.WriteLine(e);
                                }

                                pIID++;
                            }

                        }

                    }
                }
            }

            Console.WriteLine("Press Any Key To Continue......");
            Console.ReadKey();
        }
    }
}
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