Re: [RFC PATCH v1 12/16] unwind_user/backchain: Introduce back chain user space unwinding

[Josh Poimboeuf]

On Thu, Jul 17, 2025 at 02:20:12PM +0200, Jens Remus wrote:
> >> +done_backchain:
> >>  	state->topmost = false;
> >>  	return 0;
> > 
> > This feels very grafted on, is there not some way to make it more
> > generic, i.e., to just work with CONFIG_HAVE_UNWIND_USER_FP?
> 
> I agree.  It could probably be made to compute the cfa_off and ra.offset
> or ra.regnum.  Let me explore that, provided there would be any acceptance
> for unwind user backchain at all. Note that Power is using backchain as
> well, so they may want to build on that as well.
> 
> > Also, if distros aren't even compiling with -mbackchain, I wonder if we
> > can just not do this altogether :-)
> 
> My original intent was to use unwind user's for_each_user_frame() to
> replace the exiting stack tracing logic in arch_stack_walk_user_common()
> in arch/s390/kernel/stacktrace.c, which currently supports backchain.
> Given that for_each_user_frame() was made private in the latest unwind
> user series version hinders me.  The use was also low, because the
> currentl arch_stack_walk_user_common() implementation does not support
> page faults, so that the attempt to use unwind user sframe would always
> fail and fallback to unwind user backchain.  My hope was that somebody
> with more Kernel skills could give me a few hints at how it could be
> made to support deferred unwind. :-)

I believe stack_trace_save_user() is only used by ftrace, and that will
no longer be needed once ftrace starts using unwind_user.

Maybe Heiko knows if that backchain user stacktrace code has any users?

If distros aren't building with -mbackchain, maybe backchain support can
be considered obsoleted by sframe, and we can get away with not
implementing it.

-- 
Josh