linux-kernel - Re: [PATCH v4 32/39] perf: Remove get_perf

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Message-ID: <20250124220045.hduc3az4neoutebq@jpoimboe>
Date: Fri, 24 Jan 2025 14:00:45 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.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.kernel.org,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Weinan Liu <wnliu@...gle.com>
Subject: Re: [PATCH v4 32/39] perf: Remove get_perf_callchain() 'crosstask'
 argument

On Fri, Jan 24, 2025 at 10:13:23AM -0800, Andrii Nakryiko wrote:
> On Tue, Jan 21, 2025 at 6:32 PM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> > @@ -430,10 +429,8 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
> >         if (task && user && !user_mode(regs))
> >                 goto err_fault;
> >
> > -       /* get_perf_callchain does not support crosstask user stack walking
> > -        * but returns an empty stack instead of NULL.
> > -        */
> > -       if (crosstask && user) {
> > +       /* get_perf_callchain() does not support crosstask stack walking */
> > +       if (crosstask) {
> 
> crosstask stack trace is supported for kernel stack traces (see
> get_callchain_entry_for_task() call), so this is breaking that case

Oh I see, thanks.

BTW, that seems dubious, does it do anything to ensure the task isn't
running?   Otherwise the unwind is going to be a wild ride.

-- 
Josh