Delphi Inline ASM GetProcAddress Api Replacement

program GetProcAddress;

uses

  Windows;

const

  MAX_API_STRING_LENGTH = 150;

type

  PSEHStruct = ^TSEHStruct;

  TSEHStruct = record

    saveEsp: Cardinal;

    saveEbp: Cardinal;

    gotoEip: Cardinal;

  end;

var

  SEH: TSEHStruct;

function SEHHandler(p1, p2, p3, p4: Pointer): Cardinal; cdecl;

begin

  with PContext(p3)^ do

  begin

    Esp := SEH.saveEsp;

    Ebp := SEH.saveEbp;

    Eip := SEH.gotoEip;

  end;

  Result := 0;  // ExceptionContinueExecution

end;

function ASMGetProcAddress(DllBase: LongWord; ApiName: PChar): Pointer; pascal;

{ directive "pascal" to force stack frame }

asm

        { save used registers }

        PUSH    ESI

        PUSH    EDI

        PUSH    ECX

        PUSH    EBX

        PUSH    EDX

        { using [EDX] saves some space over [0] }

        XOR     EDX, EDX

        { set up SEH frame }

        PUSH    OFFSET SEHHandler

        PUSH    DWORD PTR FS:[EDX]

        MOV     SEH.saveEsp, ESP

        MOV     SEH.saveEbp, EBP

        MOV     SEH.gotoEip, OFFSET @@NotFound

        MOV     FS:[EDX], ESP

        { string length the target api }

        MOV     EDI, ApiName

        MOV     ECX, MAX_API_STRING_LENGTH

        XOR     AL, AL

        REPNZ   SCASB

        MOV     ECX, EDI

        SUB     ECX, ApiName

        { get and check PE file header }

        MOV     EDX, DllBase

        CMP     [EDX].TImageDosHeader.e_magic, IMAGE_DOS_SIGNATURE

        JNZ     @@NotFound

        ADD     EDX, [EDX].TImageDosHeader._lfanew

        CMP     [EDX].TImageNtHeaders.Signature, IMAGE_NT_SIGNATURE

        JNZ     @@NotFound

        { get and check export directory }

        MOV     EDX, [EDX].TImageNtHeaders.OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_EXPORT].TImageDataDirectory.VirtualAddress

        OR      EDX, EDX

        JZ      @@NotFound

        ADD     EDX, DllBase

        { scan names }

        MOV     EBX, [EDX].TImageExportDirectory.AddressOfNames

        ADD     EBX, DllBase

        XOR     EAX, EAX

@@Loop:

        MOV     EDI, [EBX]

        ADD     EDI, DllBase

        MOV     ESI, ApiName

        PUSH    ECX

        REPZ    CMPSB

        POP     ECX

        JE      @@Found

        ADD     EBX, 4

        INC     EAX

        CMP     EAX, [EDX].TImageExportDirectory.NumberOfNames

        JB      @@Loop

        JMP     @@NotFound

@@Found:

        { name -> ordinal }

        SHL     EAX, 1

        ADD     EAX, [EDX].TImageExportDirectory.AddressOfNameOrdinals

        ADD     EAX, DllBase

        MOV     AX, [EAX]

        AND     EAX, $0000FFFF

        { ordinal -> function }

        SHL     EAX, 2

        ADD     EAX, [EDX].TImageExportDirectory.AddressOfFunctions

        ADD     EAX, DllBase

        MOV     EAX, [EAX]

        ADD     EAX, DllBase

        JMP     @@Exit

@@NotFound:

        { return 0 on error }

        XOR     EAX, EAX

@@Exit:

        { clean up SEH frame }

        XOR     EDX, EDX

        POP     DWORD PTR FS:[EDX]

        ADD     ESP, 4

        { restore used registers }

        POP     EDX

        POP     EBX

        POP     ECX

        POP     EDI

        POP     ESI

end;

var

  xMessageBoxA : function(hWnd: HWND; lpText, lpCaption: PAnsiChar; uType: UINT): Integer; stdcall;

begin

  xMessageBoxA := ASMGetProcAddress(LoadLibrary('user32.dll'),'MessageBoxA');

  if Assigned(xMessageBoxA) then

    xMessageBoxA(0,'Assigned!','',0);

end.