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
