2

I have following function:

void __thiscall ParsePackage_v5(package_t *package, stream_t *stream)
{
  (*(*package->stream + 8))(package->stream, package, 4);
}
.text:0069F5A0 ; void __thiscall ParsePackage_v5(package_t *package, _DWORD stream)
.text:0069F5A0 ParsePackage_5  proc near               ; CODE XREF: ParsePackageHeader+FB↓p
.text:0069F5A0
.text:0069F5A0                 push    ebp
.text:0069F5A1                 mov     ebp, esp
.text:0069F5A3                 sub     esp, 134h
.text:0069F5A9                 push    ebx
.text:0069F5AA                 push    esi
.text:0069F5AB                 push    edi
.text:0069F5AC                 mov     esi, ecx
.text:0069F5AE                 mov     ecx, [esi+4]
.text:0069F5B1                 mov     eax, [ecx]
.text:0069F5B3                 push    4
.text:0069F5B5                 push    esi
.text:0069F5B6                 call    dword ptr [eax+8]

stream_t defines as:

00000000 stream_t        struc ; (sizeof=0x14, align=0x4, mappedto_136)
00000000 unknown         db ?
00000001                 db ? ; undefined
00000002                 db ? ; undefined
00000003                 db ? ; undefined
00000004 hFile           dd ?                    ; offset
00000008 ReadBytes       db ?
00000009                 db ? ; undefined
0000000A                 db ? ; undefined
0000000B                 db ? ; undefined
0000000C field_C         db ?
0000000D                 db ? ; undefined
0000000E                 db ? ; undefined
0000000F                 db ? ; undefined
00000010 field_10        db ?
00000011                 db ? ; undefined
00000012                 db ? ; undefined
00000013                 db ? ; undefined
00000014 stream_t        ends

(*(*package->stream + 8)) supposed to be converted into package->stream->ReadByte but it wont. How can i do this properly and declare function pointer as field type of ReadByte?

1 Answer 1

1

The first problem in your example is that the fields are defined as bytes, not dwords. Press D twice on those fields to turn them into dwords.

The second problem is that the fields don't have proper function pointer types applied to them. Highlight the fields and press Y to set the type. Enter something like void (__thiscall *ReadBytes)(stream_t *this, package_t *package, size_t size) to create a function pointer type.

5
  • Big thanks! Unfortunatelly, now i have (*(package->stream->unknown + 8))(package->stream, package, 4); instead of package->stream->ReadBytes(package->stream, package, 4); What's the problem? Nov 4, 2022 at 23:30
  • Ultimately you're going to need two structs. On the line ending in -AE, the code loads the stream_t pointer from the package_t object. On the next line, it loads *stream_t into eax, and the call to [eax+8] finally takes place on line -B6. This is a virtual function call. Your definition of stream_t is incorrect; the dword at +0 needs to be a pointer to a VTable struct that contains the function pointers. Nov 5, 2022 at 0:24
  • But how can I define vtable structure? Same as stream_t but each field is byte size? Then how (*(package->stream->unknown + 8)) knows that there are function and it needs to be invoked? Nov 5, 2022 at 0:36
  • No, it's a separate structure. I can't tell you what the structure looks like because all I have is a few lines of assembly, but you're going to want a struct called something like stream_t_vtbl which contains dword-sized members whose types are function pointers. The first element of stream_t needs to be a pointer to that structure. As for what the rest of stream_t looks like, I have no idea; the code snippet doesn't show any fields other than its VTable pointer at +0. Nov 5, 2022 at 1:31
  • I figured it out. Big thanks for helping! You saved my nerves :) Nov 5, 2022 at 3:15

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