linux-kernel - Re: [RFC PATCH v2 14/15] unwind_user/backchain: Introduce back chain user space unwinding

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Message-ID: <iidpbjmxnjf3zu4fa3atiubgb365yonv4gymaj76l6jvuxy67s@2y5o4txs4vhr>
Date: Sun, 7 Dec 2025 07:10:44 -0800
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, 
	linux-s390@...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>, 
	Arnaldo Carvalho de Melo <acme@...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>, Florian Weimer <fweimer@...hat.com>, Kees Cook <kees@...nel.org>, 
	Carlos O'Donell <codonell@...hat.com>, Sam James <sam@...too.org>, Dylan Hatch <dylanbhatch@...gle.com>
Subject: Re: [RFC PATCH v2 14/15] unwind_user/backchain: Introduce back chain
 user space unwinding

On Fri, Dec 05, 2025 at 06:14:45PM +0100, Jens Remus wrote:
> @@ -159,6 +165,10 @@ static int unwind_user_next(struct unwind_user_state *state)
>  			if (!unwind_user_next_fp(state))
>  				return 0;
>  			continue;
> +		case UNWIND_USER_TYPE_BACKCHAIN:
> +			if (!unwind_user_next_backchain(state))
> +				return 0;
> +			continue;		/* Try next method. */
>  		default:
>  			WARN_ONCE(1, "Undefined unwind bit %d", bit);
>  			break;
> @@ -187,6 +197,8 @@ static int unwind_user_start(struct unwind_user_state *state)
>  		state->available_types |= UNWIND_USER_TYPE_SFRAME;
>  	if (IS_ENABLED(CONFIG_HAVE_UNWIND_USER_FP))
>  		state->available_types |= UNWIND_USER_TYPE_FP;
> +	if (IS_ENABLED(CONFIG_HAVE_UNWIND_USER_BACKCHAIN))
> +		state->available_types |= UNWIND_USER_TYPE_BACKCHAIN;

Any reason not to just use the existing CONFIG_HAVE_UNWIND_USER_FP hook
here rather than create the new BACKCHAIN one?

-- 
Josh