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?
