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

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Message-ID: <ddwondzj74rr3fgvsdnkch7trrcwltasb236hvvx5tnywf2lhu@vo47rcoyu2nc>
Date: Thu, 17 Jul 2025 22:19:54 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	bpf@...r.kernel.org, x86@...nel.org, Steven Rostedt <rostedt@...nel.org>, 
	Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Ilya Leoshkevich <iii@...ux.ibm.com>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Andrii Nakryiko <andrii@...nel.org>, 
	Indu Bhagat <indu.bhagat@...cle.com>, "Jose E. Marchesi" <jemarch@....org>, 
	Beau Belgrave <beaub@...ux.microsoft.com>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Jens Axboe <axboe@...nel.dk>, 
	Florian Weimer <fweimer@...hat.com>, Sam James <sam@...too.org>
Subject: Re: [RFC PATCH v1 12/16] unwind_user/backchain: Introduce back chain
 user space unwinding

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