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

Peter Zijlstra
2023-07-19T20:51:11Z

On Wed, Jul 19, 2023 at 10:15:38PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 19, 2023 at 10:02:22PM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 19, 2023 at 05:21:11PM +0200, Petr Mladek wrote:
> >
> > > This patch broke livepatching. Kthreads never have a reliable stack.
> > > It works when I revert it.
> >
> > > > +SYM_CODE_START(ret_from_fork_asm)
> > > > + UNWIND_HINT_REGS
> >
> > It works again when I change the above hint to UNWIND_HINT_END_OF_STACK,
> > so yeah. Doing this makes objtool unhappy with something else though,
> > so I'll go prod at things with something sharp...
>
>
> 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

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.

Josh, do you remember the rationale for all this?