Re: [PATCH v3] selftests/x86: Fix sysret_rip assertion failure on FRED systems

["H. Peter Anvin" <hpa@zytor.com>]

On 2026-04-02 06:21, Andy Lutomirski wrote:
> 
> I don't really agree.  For quite a few years now, we've tried to make the exit path uniform, and we have this logic in syscall_64:
> 
>         /* SYSRET requires RCX == RIP and R11 == EFLAGS */
>         if (unlikely(regs->cx != regs->ip || regs->r11 != regs->flags))
>                 return false;  <-- fall back to IRET
> 
> and this is not just an aesthetic thing -- it allows us to have deliver signals and implement things like sigreturn without needing to track extra flag bits that mean "well, actually, we're in the syscall *code* but we're not returning from a syscall any more".  We had that a long time ago, and it was extremely difficult to understand and maintain.
> 
> So, on current kernels and kernels going back, I dunno, 10 years (I didn't try to dig out the git history, but I did write much of this code...), the semantics have been that we return to usermode in a state that matches pt_regs as precisely as we can arrange.  For the one case where we have a very longstanding divergence between entry and exit regs, we have orig_ax.
> 
> So it would be at least a fairly large maintainability regression to make the non-FRED SYSCALL behavior modify rcx and/or r11 on exit.
> 
> Now we have FRED.  Sure, it would be nice to remember the entry RCX and R11, but if we want to avoid the footgun where the effect of SYSCALL is different on FRED and non-FRED hardware, then we need the context after entry completes to have regs->rcx == regs->rip and regs->rcx == regs->flags (or perhaps RCX and R11 differently poisoned, but that seems a bit silly).
> 
> If we really want to have the option to fish the original rcx and r11 out from somewhere or perhaps to have extra-bonus-efficient many-parameter syscalls (I'm not sure why), then we could add orig_rcx and orig_r11.  Or we could invent a time machine and fix SYSCALL when it first came out.
> 

I certainly see what you're saying. I still don't like the idea of clobbering
registers "just because" for this reason and more...

	-hpa