linux-kernel - Re: UBSAN: Undefined behaviour in arch/x86/events/intel/p6.c:116:29

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Date:   Wed, 4 Dec 2019 16:06:56 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Meelis Roos <mroos@...ux.ee>, LKML <linux-kernel@...r.kernel.org>,
        x86@...nel.org, Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: UBSAN: Undefined behaviour in arch/x86/events/intel/p6.c:116:29

On Wed, Dec 04, 2019 at 01:15:40PM +0100, Jiri Olsa wrote:
> On Tue, Dec 03, 2019 at 03:39:49PM +0200, Meelis Roos wrote:
> > > Does something like so fix it?
> > 
> > Unfortunately not (tested on top of todays git):
> 
> hi,
> which p6 model are you seeing this on?
> how do you trigger that?

Triggers on any p6 model. I hacked up perf and used "qemu-system-x86_64
-cpu pentium2".

The below seems to cure things.

---
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 9a89d98c55bd..f17417644665 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -1642,9 +1643,12 @@ static struct attribute_group x86_pmu_format_group __ro_after_init = {
 
 ssize_t events_sysfs_show(struct device *dev, struct device_attribute *attr, char *page)
 {
-	struct perf_pmu_events_attr *pmu_attr = \
+	struct perf_pmu_events_attr *pmu_attr =
 		container_of(attr, struct perf_pmu_events_attr, attr);
-	u64 config = x86_pmu.event_map(pmu_attr->id);
+	u64 config = 0;
+
+	if (pmu_attr->id < x86_pmu.max_events)
+		x86_pmu.event_map(pmu_attr->id);
 
 	/* string trumps id */
 	if (pmu_attr->event_str)
@@ -1713,6 +1717,9 @@ is_visible(struct kobject *kobj, struct attribute *attr, int idx)
 {
 	struct perf_pmu_events_attr *pmu_attr;
 
+	if (idx >= x86_pmu.max_events)
+		return 0;
+
 	pmu_attr = container_of(attr, struct perf_pmu_events_attr, attr.attr);
 	/* str trumps id */
 	return pmu_attr->event_str || x86_pmu.event_map(idx) ? attr->mode : 0;