Re: [RFC UKL 07/10] x86/signal: Adjust signal handler register values and return frame

From: Andy Lutomirski
Date: Tue Oct 04 2022 - 13:36:01 EST

On Mon, Oct 3, 2022, at 3:21 PM, Ali Raza wrote:
> For a UKL thread, returning to a signal handler is not done with iret or
> sysret. This means we need to adjust the way the return stack frame is
> handled for these threads. When constructing the signal frame, we leave
> the previous frame in place because we will return to it from the signal
> handler. We also leave space for pushing eflags and the return address.
> UKL threads will only use the __KERNEL_DS value in the ss register and 0xC3
> in the cs register.

This is unclear. Are you taking about returning from the kernel fault code *to* the signal handler or are you talking about returning *from* the user signal hander to the user code that was running when the signal happened?

In any case, I don't see what this has to do with iret or sysret. Surely UKL can use a sigreturn() just like regular Linux.

The part where a UKL thread has permission to return to a CPL0 context should be a separate patch.