linux-kernel - Re: [RFC PATCH v1 08/16] unwind_user: Enable archs that save RA/FP in other registers

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Message-ID: <qk5sfunxgef6qtnfqorszybubhai7kqg5h473dx5wcmvlphq5a@om7pohqhho6a>
Date: Thu, 17 Jul 2025 21:52:24 -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 08/16] unwind_user: Enable archs that save RA/FP
 in other registers

On Thu, Jul 17, 2025 at 02:07:05PM +0200, Jens Remus wrote:
> On 17.07.2025 04:50, Josh Poimboeuf wrote:
> > So the following is wrong:
> > 
> > 	case UNWIND_USER_LOC_STACK:
> > 		if (!frame->fp.frame_off)
> > 			goto done;
> > 		if (unwind_get_user_long(fp, cfa + frame->fp.frame_off, state))
> > 			goto done;
> > 		break;
> > 
> > Instead of having !fp.frame_off stopping the unwind, it should just
> > break out of the switch statement and keep going.
> 
> If frame->fp.loc is UNWIND_USER_LOC_STACK then frame->fp.frame_off must
> have a value != 0.  At least if we keep the original semantic.
> 
> We can omit this check, if we assume all producers of frame behave
> correctly.  For instance user unwind sframe would not produce that
> (see below).  Could it somehow be made a debug-config-only check?

Ah... the !frame->fp.frame_off check for the UNWIND_USER_LOC_STACK case
completely threw me for a loop.  I was confusing that with
UNWIND_USER_LOC_NONE.  So never mind.

And yes, I think that check has no use and can be removed.

-- 
Josh