Improper hooks when RSP instructions are in the prologue

[BritishPiper]

Polyhook fails to properly translate prologues that directly modify rsp (e.g. mov rsp, [0xDEADBEEF]). This is a very uncommon thing, but I've encountered a DRM using this for whatever reason. The current translation by x64Detour::generateTranslationRoutine is:

lea rsp, rsp-0x80
push rax
push r15
mov r15, orig_rip + 0xDEADBEEF
mov rax, [r15]
mov rsp, rax
pop r15
pop rax
push rax
mov rax, return_addr
xchg [rsp], rax
ret 80

I don't know if this is an actual problem you'd want to solve, but a solution would be to not push/pop and instead use another scratch register as your stack pointer (usually rbp). Then restore it later with something like "mov rbp, [rbp - where_you_saved_rbp_in_the_stack]".

Also, shadow space spoiling prevention would need to go (lea rsp, [rsp - 0x80] and ret 80), but I don't see a need for it if you choose to never mess with rsp in the first place.

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On a side note, a minor thing I couldn't understand is why use r15 when you could just mov rax, [rax]. There are definitely situations where that scratch register is needed, but not for simple cases (like cmp [0xDEADBEEF], 0). Probably not important though.