Re: [PATCH 1/2] x86/stacktrace: do not fail when regs on stack for ORC

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Thu, Nov 30, 2017 at 01:57:10PM -0600, Josh Poimboeuf wrote:
> On Thu, Nov 30, 2017 at 09:03:24AM +0100, Jiri Slaby wrote:
> > save_stack_trace_reliable now returns "non reliable" when there are
> > kernel pt_regs on stack. This means an interrupt or exception happened.
> > Somewhere down the route. It is a problem for frame pointer unwinder,
> > because the frame might now have been set up yet when the irq happened,
> > so it might fail to unwind from the interrupted function.
> > 
> > With ORC, this is not a problem, as ORC has out-of-band data. We can
> > find ORC data even for the IP in interrupted function and always unwind
> > one level up.
> > 
> > So introduce `unwind_regs_reliable' which decides if this is an issue
> > for the currently selected unwinder at all and change the code
> > accordingly.
> 
> Thanks.  I'm thinking there a few ways we can simplify things.  (Most of
> these are actually issues with the existing code.)
> 
> - Currently we check to make sure that there's no frame *after* the user
>   space regs.  I think there's no way that could ever happen and the
>   check is overkill.
> 
> - We should probably remove the STACKTRACE_DUMP_ONCE() warnings.  There
>   are some known places where a stack trace will fail, particularly with
>   generated code.  I wish we had a DEBUG_WARN_ON() macro which used
>   pr_debug(), but oh well.  At least the livepatch code has some helpful
>   pr_warn()s, those are probably good enough.
    ^^^^^^^
    meant to say pr_debug()s.

Also adding the live patching mailing list as an FYI.

> 
> - The unwind->error checks are superfluous.  The only errors we need to
>   check for are (a) whether the FP unwinder encountered a kernel irq and
>   b) whether we reached the final user regs frame.  So I think
>   unwind->error can be removed altogether.
> 
> So with those changes in mind, how about something like this (plus
> comments)?
> 
> 	for (unwind_start(&state, task, NULL, NULL); !unwind_done(&state);
> 	     unwind_next_frame(&state)) {
> 
> 		regs = unwind_get_entry_regs(&state);
> 		if (regs) {
> 			if (user_mode(regs))
> 				goto success;
> 
> 			if (IS_ENABLED(CONFIG_FRAME_POINTER))
> 				return -EINVAL;
> 		}
> 
> 		addr = unwind_get_return_address(&state);
> 		if (!addr)
> 			return -EINVAL;
> 
> 		if (save_stack_address(trace, addr, false))
> 			return -EINVAL;
> 	}
> 
> 	return -EINVAL;
> 
> success:
> 	if (trace->nr_entries < trace->max_entries)
> 		trace->entries[trace->nr_entries++] = ULONG_MAX;
> 
> 	return 0;
> 
> After these changes I believe we can enable
> CONFIG_HAVE_RELIABLE_STACKTRACE for ORC.
> 
> Also, when you post the next version, please cc the live patching
> mailing list, since this is directly relevant to livepatch.
> 
> Thanks!

-- 
Josh