the actual problem is that calling a varargs function results in corrupting the caller's stack so in this case, it's not callback and indirect call stuff, it's that i need to fix the varargs passthroughs in mingw and/or ensure gcc is doing this in the same style as windows. one of the few places where calling conventions is more complicated _between_ modules, as windows was always supposed to support dlls and exes written in separate environments.
va_list is always black magic. it's often hard to track down an actual concrete definition.
going back to the source
among the most embarrassing thing i've ever done in my life is ask david edelsohn a question i hadn't thoroughly researched first.
ok stumbled on a raymond chen article:
https://devblogs.microsoft.com/oldnewthing/20180817-00/?p=99515
and now i understand... it's not va_list precisely, it's that using va_list causes the function to spill the registers. in reality, we're supposed to provide space for potential arg saves for the callee, _then_ the function subtracts from r1 (which raymond chen also mentions). this mirrors what i see in the prolog of sprintf
@prozacchiwawa I’m sure he didn’t hold it against you.