On Thu, Jul 17, 2025 at 02:20:12PM +0200, Jens Remus wrote:

quoted

quoted

+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.

quoted

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