# Stack Alignment A small issue you may get when pwning on 64-bit systems is that your exploit works perfectly locally but fails remotely - or even fails when you try to use the provided LIBC version rather than your local one. This arises due to something called **stack alignment**. Essentially the [x86-64 ABI (application binary interface) guarantees 16-byte alignment on a `call` instruction](https://stackoverflow.com/questions/54393105/libcs-system-when-the-stack-pointer-is-not-16-padded-causes-segmentation-faul). LIBC takes advantage of this and uses [SSE data transfer instructions](https://docs.oracle.com/cd/E26502_01/html/E28388/eojde.html) to optimise execution; `system` in particular utilises instructions such as `movaps`. That means that if the stack is not 16-byte aligned - that is, RSP is not a multiple of 16 - the ROP chain will fail on `system`. The fix is simple - in your ROP chain, before the call to `system`, place a singular `ret` gadget: ```python ret = elf.address + 0x2439 [...] rop.raw(POP_RDI) rop.raw(0x4) # first parameter rop.raw(ret) # align the stack rop.raw(system) ``` This works because it will cause RSP to be popped an additional time, pushing it forward by 8 bytes and aligning it. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://ir0nstone.gitbook.io/notes/binexp/stack/return-oriented-programming/stack-alignment.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.