On Wed, Jul 19, 2023 at 04:31:11PM -0700, Josh Poimboeuf wrote: > 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. Right, well, it took me a minute to figure out how it was all supposed to work, I figured I'd stick a comment on it. The bit I missed is that if you reach the return-to-user part, you will actually have user_mode() true on the regset. > > 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. I tried, it doesn't work, clearly I missed something.
