Re: [REGRESSION] x86/entry: TIF_SINGLESTEP handling is still broken

[Andy Lutomirski]

On Sun, Jan 31, 2021 at 3:39 PM Kyle Huey <me@xxxxxxxxxxxx> wrote:
>
> On Sun, Jan 31, 2021 at 3:36 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > > The odd system call tracing part I have no idea who depends on it
> > > (apparently "rr", which I assume is some replay thing), and I suspect
> > > our semantics for it has been basically random historical one, and
> > > it's apparently what changed.
> > >
> > > That's the one that we _really_ should have a test-case for, along
> > > with some documentation and code comment what the actual semantics
> > > need to be so that we don't break it again.
> >
> > This rr thing may be tangled up with the nonsense semantics of SYSRET.  I’ll muck around with Kyle’s test and try to figure out what broke.
> >
> > I’m guessing the issue is that we are correctly setting TF in the EFLAGS image, but IRET helpfully only traps after the first user insn executes, which isn’t what the tracer is expects.
>
> The state of TF shouldn't really matter here. There should be no user
> space code execution in the example I gave. This behavior all happens
> in the kernel and not on the silicon.
>

I admit that PTRACE_SINGLESTEP seems like an odd way to spell "advance
to the end of the syscall", but you're right, it should work.