How to jump to memory location in intel syntax for both x64 and x32

Question

How would I jump to a known memory address in intel assembly syntax (both x32 and x64).

I think I have the 64 bit syntax down. For example, if in x64 I wanted to jmp to the code at 0x75767 and I was located at 0000, I would do:

0000: FF 25 01 00 00 00         jmp QWORD PTR [rip+0x75761]

Is that ^ correct? I thought I could dissemble those bytes that into x32 instruction using objdump objdump.exe -D -Mintel,i386 -b binary -m i386 test.bin which results in:

jmp    DWORD PTR 0x75761

Then just use clang++.exe -masm=intel -m32 -c test.o to convert this instruction to x32 bytes but it says:

error: invalid operand for instruction
jmp DWORD PTR 0x75761
^

I want to avoid writing into any registers.

Is my x64 jmp instruction correct?

How would I accomplish something similar in x32? Let's say in x32 I need to jmp to 0x400107 and I'm at 0x400000

I'm messing around with tweaking running process memory on Windows. Forgive me if my question has inaccuracies, I'm learning.

Answer 1

It's unclear whether you need assembly or machine code. If you want to jump to an absolute address then in 64 bit mode use an embedded pointer addressed rip relative:

jmp [rip+foo]
foo: .quad target_address

Machine code: ff 25 00 00 00 00 xx xx xx xx xx xx xx xx (with the last 8 bytes being the target absolute address).

In 32 bit code you can use the push+ret trick if you are not worried about branch prediction:

push offset foo
ret

Machine code: 68 xx xx xx xx c3

If you can calculate the relative address you can of course just use a normal jmp which is e9 xx xx xx xx with the last 4 bytes being the distance to jump (counted from the byte following the instruction, where execution would normally continue).