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How to Preserve 64-bit Register Values When Using movl in x86_64 ROP Constraints

Posted by Marvee Amasi
4:24 pm
04/02/2025

In x86 64 machine code, when the `movl` instruction is used with a register destination, it zeroes the higher order bytes of the 64 bit register. How can I use `movl` while preserving the original value of the full 64-bit register?

For example, if I “`movq %rsp, %rax“` and then call “`movl %eax, %edx“` is there a way to use `movl` or perform some other operations after `movl` so that `%rdx` retains the original value of `%rsp`?

I cant use `movq` bcus in my current situation ***a ROP attack**, only `movl` instructions are available in my gadget farm actually. I also have access to `andb`, `orb`, `cmpb`, and `testb` instructions