Write a jump command to a x86-64 binary file

Question

I'm debugging a Mac OS X 64bit app with GDB. I see that jumping over a chunk of code solves all my problems.

But:

How can I patch the executable file to implement the jump? I want the app to automatically jump to a defined point in the code without the debugger.

This is what I want to do:

At address 0x1000027a9 (given by the debugger) jump to address 0x100003b6e. I'm trying very hard to do it via HexEdit, but with no success. I read anywhere about jmp to absolute addresses opcodes (FF seems the right opcode, but it's a call, not a jump...) but nothing works. Bad access, segfault.

How can I do that?

Answer 1

What you want is not a call, but a jmp, and you want a direct jmp. Direct jumps usually use an addressing relative to the next instruction's address (see my answer to SO question: How encode a relative short jmp in x86). Relative to the end of the jump instruction is another way to look at it.

So, you are at 0x1000027a9 and want to jump to 0x100003b6e.

0x100003b6e - 0x1000027a9 = 0x000013C5 = 5061d, so that definitively doesn't fit in a short jump (rel8 in Intel documentation), but you need jmp rel32. It would fit in rel16 too, but that's not supported in x86-64 (in 64-bit mode).

So, you want a jmp rel32. This is encoded relative to the next instruction after jmp, and as the length of the instruction is 5 bytes (E9 xx xx xx xx), rel32 will be 0x000013C0. As x86 is a little-endian architecture, it is encoded as E9 C0 13 00 00.

To confirm this, I assembled a small test executable with NASM and disassembled it with ndisasm (note I left first 0x10000000 bytes out, but as the jump is relative, it doesn't change anything in the encoding):

000027A8  90                nop
000027A9  E9C0130000        jmp dword 0x3b6e ; this is the instruction you need.
000027AE  90                nop