x86_64 any and varargs crash with a misaligned stack pointer

[tilkinsc]

Hello, I wanted to bring up for y'all that you need to make sure that you stack pointer stays 16 byte aligned as per what System V x*6_64 ABI mandates. This means you need to be on the % 16th byte when you enter/re-enter C3 code. Possible signs you have a misaligned stack pointer is when things start to crash with any or varargs for seemingly no reason. The crash site may be at when you index the vararg for any.

A common scenario for this is when you are trampolining to C3 from assembly:

extern kernel_stack_top

section .text
align 16

global trampoline
trampoline:
    mov rsp, [kernel_stack_top]       ; this sets a new stack up without a call frame
    mov rsp, [kernel_stack_top + 8]   ; when call kmain, it will push an unaligned stack frame, so adjust for it
    call kmain                        ; pushes RIP (8 bytes) causing misalignment if not accounted for
    cli
halt:
    hlt
    jmp halt

[srkkov]

Thought I'd just say that and rsp, -16 will align the stack if you're ever unsure.