How are PE Base Relocations build up?

Question

I'm currently having trouble understanding how PE Base Relocations are build up.

I understand there can be more then one relocation, I understand also why and how this is done, but I just don't understand it programmatically:

Which of the following is true (IMAGE_BASE_RELOCATION in WinNT.h)?

// Base relocation #1
DWORD   VirtualAddress;
DWORD   SizeOfBlock; // size of current relocation
WORD    TypeOffset[1];
// Base relocation #2
DWORD   VirtualAddress;
DWORD   SizeOfBlock; // size of current relocation
WORD    TypeOffset[1];
// Base relocation #3
DWORD   VirtualAddress;
DWORD   SizeOfBlock; // size of current relocation
WORD    TypeOffset[1];

Or

DWORD   VirtualAddress;
DWORD   SizeOfBlock; // size of all relocations
WORD    TypeOffset[1]; // relocation #1
WORD    TypeOffset[1]; // relocation #2
WORD    TypeOffset[1]; // relocation #3

Or are both incorrect? How must I loop through all base relocations programmatically?

Currently I have this code, seems to be incorrect somewhere:

DWORD baseRelocationSize = imageNtHeaders->OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_BASERELOC].Size;
            unsigned int baseRelocationCount = baseRelocationSize / sizeof(IMAGE_BASE_RELOCATION);
            DWORD baseDelta = (DWORD_PTR)moduleBase - (DWORD_PTR)imageNtHeaders->OptionalHeader.ImageBase;

            IMAGE_BASE_RELOCATION* baseRelocation = (IMAGE_BASE_RELOCATION*)((DWORD_PTR)moduleBase + (DWORD_PTR)imageNtHeaders->OptionalHeader.DataDirectory[IMAGE_DIRECTORY_ENTRY_BASERELOC].VirtualAddress);

            for(unsigned int i = 0; i != baseRelocationCount; ++i)
            {
                unsigned int entryCount = (baseRelocation->SizeOfBlock - sizeof(IMAGE_BASE_RELOCATION)) / sizeof(WORD);

                for(unsigned int j = 0; j != entryCount; ++j)
                {
                    WORD* entry = (WORD*)((DWORD_PTR)baseRelocation + (DWORD_PTR)sizeof(IMAGE_BASE_RELOCATION));
                    if((*entry >> 12) & IMAGE_REL_BASED_HIGHLOW)
                    {
                        DWORD* pdw = (PDWORD)((DWORD_PTR)moduleBase + (DWORD_PTR)baseRelocation->VirtualAddress + ((*entry) & 0xfff));
                        (*pdw) += baseDelta;
                    }

                    entry++;
                }

                baseRelocation += baseRelocation->SizeOfBlock;
            } 

Answer 1

Neither options you indicated entirely correct/true.

This excellent tutorial on How to inject code in a PE file shows that the actual IMAGE_BASE_RELOCATION structure is:

typedef struct _IMAGE_BASE_RELOCATION {
  DWORD   VirtualAddress;
  DWORD   SizeOfBlock;
} IMAGE_BASE_RELOCATION, *PIMAGE_BASE_RELOCATION;

Section 5.2 of this Microsoft Portable Executable and Common Object File Format Specification describe the structure. The SizeOfBlock-8 actually indicates how many WORD TypeOffset follow after the VirtualAddress and SizeOfBlock.

I think you would also be interested in Table 7 of the tutorial, which shows the structure of the blocks from the relocation table. I'll copy-paste the table here for quick-reference.

Answer 2

from some code.. aldo check out reactos :)

BOOL FixRelocs(void *base, void *rBase, IMAGE_NT_HEADERS *ntHd, IMAGE_BASE_RELOCATION *reloc,
               unsigned int size) {
    unsigned long ImageBase = ntHd->OptionalHeader.ImageBase;
    unsigned int nBytes = 0;
    unsigned long delta = MakeDelta(unsigned long, rBase, ImageBase);
    unsigned long *locBase;
unsigned int numRelocs;
unsigned short *locData;
unsigned int i;

while(1) {
  locBase =
     (unsigned long *)GetPtrFromRVA((DWORD)(reloc->VirtualAddress), ntHd, (PBYTE)base);
  numRelocs = (reloc->SizeOfBlock - sizeof(IMAGE_BASE_RELOCATION)) / sizeof(WORD);

  if(nBytes >= size) break;

  locData = MakePtr(unsigned short *, reloc, sizeof(IMAGE_BASE_RELOCATION));
  for(i = 0; i < numRelocs; i++) {       
     if(((*locData >> 12) == IMAGE_REL_BASED_HIGHLOW))
         *MakePtr(unsigned long *, locBase, (*locData & 0x0FFF)) += delta;
     locData++;
  }

  nBytes += reloc->SizeOfBlock;
  reloc = (IMAGE_BASE_RELOCATION *)locData;
   }

   return TRUE;

}