linux-kernel - Re: [PATCH v16 4/4] perf tools: Merge deferred user callchains

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Message-ID: <aRZQ4PMG0zmoF-rQ@google.com>
Date: Thu, 13 Nov 2025 13:42:56 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <rostedt@...nel.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>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...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>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Florian Weimer <fweimer@...hat.com>, Sam James <sam@...too.org>,
	Kees Cook <kees@...nel.org>, Carlos O'Donell <codonell@...hat.com>
Subject: Re: [PATCH v16 4/4] perf tools: Merge deferred user callchains

Hello,

Sorry for the delay.  And I'm happy that the kernel part is merge to the
tip tree! :)

On Fri, Oct 24, 2025 at 03:02:03PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 02, 2025 at 01:49:38PM -0400, Steven Rostedt wrote:
> > On Mon, 08 Sep 2025 13:53:23 -0400
> > Steven Rostedt <rostedt@...nel.org> wrote:
> > 
> > > +static int evlist__deliver_deferred_samples(struct evlist *evlist,
> > > +					    const struct perf_tool *tool,
> > > +					    union  perf_event *event,
> > > +					    struct perf_sample *sample,
> > > +					    struct machine *machine)
> > > +{
> > > +	struct deferred_event *de, *tmp;
> > > +	struct evsel *evsel;
> > > +	int ret = 0;
> > > +
> > > +	if (!tool->merge_deferred_callchains) {
> > > +		evsel = evlist__id2evsel(evlist, sample->id);
> > > +		return tool->callchain_deferred(tool, event, sample,
> > > +						evsel, machine);
> > > +	}
> > > +
> > > +	list_for_each_entry_safe(de, tmp, &evlist->deferred_samples, list) {
> > > +		struct perf_sample orig_sample;
> > 
> > orig_sample is not initialized and can then contain junk.

Yep.

> > 
> > > +
> > > +		ret = evlist__parse_sample(evlist, de->event, &orig_sample);

But here you call evlist__parse_sample() and evsel__parse_sample() which
should initialize the sample properly.


> > > +		if (ret < 0) {
> > > +			pr_err("failed to parse original sample\n");
> > > +			break;
> > > +		}
> > > +
> > > +		if (sample->tid != orig_sample.tid)
> > > +			continue;
> > > +
> > > +		if (event->callchain_deferred.cookie == orig_sample.deferred_cookie)
> > > +			sample__merge_deferred_callchain(&orig_sample, sample);
> > 
> > The sample__merge_deferred_callchain() initializes both
> > orig_sample.deferred_callchain and the callchain. But now that it's not
> > being called, it can cause the below free to happen with junk as the
> > callchain. This needs:
> > 
> > 		else
> > 			orig_sample.deferred_callchain = false;
> 
> Ah, so I saw crashes from here and just deleted both free()s and got on
> with things ;-)

I don't understand how it can have the garbage.  But having the else
part would be safer.

Thanks,
Namhyung

> > > +
> > > +		evsel = evlist__id2evsel(evlist, orig_sample.id);
> > > +		ret = evlist__deliver_sample(evlist, tool, de->event,
> > > +					     &orig_sample, evsel,> machine); +
> > > +		if (orig_sample.deferred_callchain)
> > > +			free(orig_sample.callchain);
> > > +
> > > +		list_del(&de->list);
> > > +		free(de);
> > > +
> > > +		if (ret)
> > > +			break;
> > > +	}
> > > +	return ret;
> > > +}
> > 
> > -- Steve