Re: [PATCH v2 2/2] x86: Rewrite ret_from_fork() in C

[Josh Poimboeuf]

On Wed, Jul 19, 2023 at 10:50:50PM +0200, Peter Zijlstra wrote:
> > The below cures things; Josh, did I miss anything?
> > 
> > ---
> >  arch/x86/entry/entry_64.S | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> > index 91f6818884fa..cfe7882ea9ae 100644
> > --- a/arch/x86/entry/entry_64.S
> > +++ b/arch/x86/entry/entry_64.S
> > @@ -285,7 +285,14 @@ SYM_FUNC_END(__switch_to_asm)
> >   */
> >  .pushsection .text, "ax"
> >  SYM_CODE_START(ret_from_fork_asm)
> > -	UNWIND_HINT_REGS
> > +	/*
> > +	 * This is the start of the kernel stack; even through there's a regs
> > +	 * set at the top, there is no real exception frame and one cannot
> > +	 * unwind further. This is the end.
> > +	 *
> > +	 * This ensures stack unwinds of kernel threads hit a known good state.
> > +	 */
> > +	UNWIND_HINT_END_OF_STACK

The comments may be a bit superfluous (to me at least) but the patch
looks fine.

> So unwind_orc.c:unwind_next_frame() will terminate on this hint *or* on
> user_mode(state->regs).
> 
> AFAICT way things are set up in copy_thread(), user_mode() will not be
> true -- after all there is no usermode, the kthread would first have to
> exec() something to create a usermode.
> 
> Yet I'm wondering if perhaps we should spoof the regs to make
> user_mode() true and auto-terminate without this explicit hint.

I'm not sure that would be worth the trouble / cleverness.  The hint is
straightforward IMO.

> Josh, do you remember the rationale for all this?

For what exactly :-)

-- 
Josh