Re: [PATCH 3/4] x86/ftrace: make ftrace_int3_handler() not to skip fops invocation

From: Linus Torvalds
Date: Mon Apr 29 2019 - 18:12:24 EST

On Mon, Apr 29, 2019 at 11:57 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >
> > Otherwise you could never trust the whole sti shadow thing - and it very much is part of the architecture.
> Is this documented somewhere?

Btw, if you really don't trust the sti shadow despite it going all the
way back to the 8086, then you could instead make the irqoff code do

push %gs:bp_call_return
push %gs:bp_call_target

which just keeps interrupts explicitly disabled over the whole use of
the percpu data.

The actual "ret" instruction doesn't matter, it's not going to change
in this model (where the code isn't dynamically generated or changed).
So I claim that it will still be protected by the sti shadow, but when
written that way it doesn't actually matter, and you could reschedule
immediately after the sti (getting an interrupt there might make the
stack frame look odd, but it doesn't really affect anything else)