linux-kernel - Re: [PATCH v12 00/14] unwind_user: x86: Deferred unwinding infrastructure

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Message-ID: <202507020751.CB3EFC2D@keescook>
Date: Wed, 2 Jul 2025 07:56:06 -0700
From: Kees Cook <kees@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	bpf@...r.kernel.org, x86@...nel.org,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	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>,
	Jens Remus <jremus@...ux.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>, Florian Weimer <fweimer@...hat.com>
Subject: Re: [PATCH v12 00/14] unwind_user: x86: Deferred unwinding
 infrastructure

On Tue, Jul 01, 2025 at 07:26:19PM -0400, Steven Rostedt wrote:
> On Tue, 1 Jul 2025 15:49:23 -0700 Kees Cook <kees@...nel.org> wrote:
> > Okay, I've read the cover letter and this wiki page, but I am dense: why
> > does the _kernel_ want to do this? Shouldn't it only be userspace that
> > cares about userspace unwinding? I don't use perf, ftrace, and ebpf
> > enough to make this obvious to me, I guess. ;)
> [...]
> Anyway, yeah, it's something that has a ton of interest, as it's the way
> for tools like perf to give nice graphs of where user space bottlenecks
> exist.

Right! Yeah, I know it's very wanted -- I wasn't saying "don't this in
the kernel", but quite literally, "*I* am missing something about why
this is so important." :) And thank you, now I see: the sampling-based
profiling of userspace must happen via the kernel.

-- 
Kees Cook