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

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Message-ID: <a4dd5okskro2h45zmqgg3etj6uwici2hoop2uaf6iqrlaej7yh@xlduwjqke4ec>
Date: Wed, 16 Jul 2025 19:06:02 -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 10, 2025 at 06:35:18PM +0200, Jens Remus wrote:
> @@ -66,12 +73,20 @@ static int unwind_user_next(struct unwind_user_state *state)
>  		/* sframe expects the frame to be local storage */
>  		frame = &_frame;
>  		if (sframe_find(state->ip, frame, topmost)) {
> -			if (!IS_ENABLED(CONFIG_HAVE_UNWIND_USER_FP))
> -				goto done;
> -			frame = &fp_frame;
> +			if (IS_ENABLED(CONFIG_HAVE_UNWIND_USER_FP)) {
> +				frame = &fp_frame;
> +			} else if (IS_ENABLED(CONFIG_HAVE_UNWIND_USER_BACKCHAIN)) {
> +				if (unwind_user_backchain_next(state))
> +					goto done;
> +				goto done_backchain;
> +			}
>  		}
>  	} else if (fp_state(state)) {
>  		frame = &fp_frame;
> +	} else if (backchain_state(state)) {
> +		if (unwind_user_backchain_next(state))
> +			goto done;
> +		goto done_backchain;
>  	} else {
>  		goto done;
>  	}
> @@ -153,6 +168,7 @@ static int unwind_user_next(struct unwind_user_state *state)
>  
>  	arch_unwind_user_next(state);
>  
> +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?

Also, if distros aren't even compiling with -mbackchain, I wonder if we
can just not do this altogether :-)

-- 
Josh