linux-kernel - Re: [PATCH v3 07/19] unwind: Add user space unwinding API

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Message-ID: <20241211174830.3l3kxbjg6ctyxnqq@jpoimboe>
Date: Wed, 11 Dec 2024 09:48:30 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>, linux-trace-kernel@...r.kerne.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v3 07/19] unwind: Add user space unwinding API

On Wed, Dec 11, 2024 at 03:53:26PM +0100, Jens Remus wrote:
> On 09.12.2024 21:54, Josh Poimboeuf wrote:
> > if (cfa <= state->sp)
> > 	goto the_end;
> 
> Assuming the x86 definition of the CFA (CFA == SP at call site) this
> translates into:
> 
> if (sp <= state->sp)
> 	goto the_end;
> 
> That won't work for architectures that pass the return address in a
> register instead of on the stack, such as s390. At least in the
> topmost frame the unwound SP may be unchanged. For instance when in
> the function prologue or when in a leaf function.
> 
> One of my patches for s390 support introduces a state->first flag,
> indicating whether it is the topmost user space frame. Using that
> your check could be extended to:
> 
> if ((state->first && sp < state->sp) || (!state->first && sp <= state->sp))
> 	goto the_end;
> 
> Which could be simplified to:
> 
> if (sp <= state->sp - state->first)
> 	goto the_end;

Since my patches are x86-only, how about I leave the "sp <= state->sp"
check and then you add something like that in your patches on top?

> Btw. neither would work for architectures with an upwards-growing
> stack, such as hppa. Not sure if that needs to be considered.

I don't think that's needed until if/when sframe becomes supported for
such an arch.

-- 
Josh