Binary Patching - modifing existing instructions and keeping machine code alignment

Question

I'd like to edit the first instruction and change it to jmp 100h (give or take a few bytes) the mov edi,edi takes 2 bytes and the jmp 100h takes 5 bytes (correct me if im wrong)

I edit the machine code to jmp 100h and add a nop to round it to 6 bytes.

.text:08048DD5                 mov     edi, edi
.text:08048DD7                 mov     edi, edi
.text:08048DD9                 mov     edi, edi
.text:08048DDB                 mov     edi, edi
.text:08048DDD                 mov     edi, edi
.text:08048DDF                 mov     edi, edi
.text:08048DE1                 mov     edi, edi
.text:08048DE3                 mov     edi, edi
.text:08048DE5                 add     [ebp+var_C], 1  ; Add
.text:08048DE9                 mov     eax, offset format ; "Message %d: %s"
.text:08048DEE                 lea     edx, [ebp+s]    ; Load Effective Address
.text:08048DF4                 mov     [esp+8], edx

the result looks like:

.text:08048DD5                 jmp     loc_8048D41
.text:08048DD5 ; ---------------------------------------------------------------------------
.text:08048DDA                 db 90h
.text:08048DDB                 db 89h, 0FFh
.text:08048DDD                 db 89h, 0FFh
.text:08048DDF                 db 89h, 0FFh
.text:08048DE1                 db 89h, 0FFh
.text:08048DE3                 db 89h, 0FFh
.text:08048DE5 ; ---------------------------------------------------------------------------
.text:08048DE5                 add     [ebp+var_C], 1
.text:08048DE9                 mov     eax, offset aMessageDS ; "Message %d: %s"
.text:08048DEE                 lea     edx, [ebp+s]
.text:08048DF4                 mov     [esp+8], edx
.text:08048DF8                 mov     edx, [ebp+var_C]

what exactly goes wrong here? How do i keep the rest of the code intact?

Answer 1

Looks like it works, but the disassembler isn't interpreting the data as instructions. This is probably because of the preceding jmp instruction - the disassembler sees that the code right after it will never be reached, so it assumes it's not actually code (and thus interprets it as straight data).