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

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Message-ID: <20251024130203.GC3245006@noisy.programming.kicks-ass.net>
Date: Fri, 24 Oct 2025 15:02:03 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: 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>,
	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>,
	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

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

> > +
> > +		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