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

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Message-ID: <11b2438c-ef1f-423f-96c9-3005a75ec008@linux.ibm.com>
Date: Mon, 8 Dec 2025 16:38:16 +0100
From: Jens Remus <jremus@...ux.ibm.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
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

Hello Josh,

thank you for your feedback!

On 12/7/2025 4:10 PM, Josh Poimboeuf wrote:
> 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?

At first I thought this would not be a good idea, as my unwind user
backchain implementation relies on being standalone without using
unwind_user_next_common().  Mainly because s390 back chain unwinding
does not have fixed CFA, FP, and RA offsets/locations.  But then I gave
it a try and it does not look that bad actually.

I'll send a RFC v3 soon.

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@...ibm.com

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