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

Josh Poimboeuf
2023-07-19T23:31:18Z

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