[PATCH] perf test: Add test case for event group throttling with inactive events

[Yunseong Kim <ysk@kzalloc.com>]

A recent UBSAN shift-out-of-bounds report was identified when throttling
event groups that included inactive (PERF_EVENT_STATE_OFF) events.
This occurred because pmu->start()/stop() could be called on these events,
potentially leaving event->hw.idx at -1. This leads to undefined behavior
when PMU code later uses this negative index as a shift exponent.

The issue need to ensuring perf_event_throttle() and
perf_event_unthrottle() skip inactive events entirely.

Introduce a new perf test suite, "event group throttle", to verify this
fix and prevent regressions.

The test sets up a scenario designed to trigger frequent throttling:
1. A parent event (leader) is created with sample_period = 1.
2. A child event is created in the same group but initialized with
   disabled = 1 (inactive).

A process opens these events and runs in a tight loop. The frequent
sampling of the leader causes the entire group, including the inactive
child event, to be rapidly throttled and unthrottled by the kernel.

The test monitors /dev/kmsg during execution, looking for "UBSAN",
"Invalid PMEV" "WARNING:", or "BUG:" messages.

To ensure robustness and avoid false positives from unrelated prior kernel
messages, the test opens /dev/kmsg and uses lseek(SEEK_END) to skip all
existing log entries before starting the test loop. If /dev/kmsg cannot be
accessed or seeked (e.g., lack of CAP_SYSLOG), the test handles it
appropriately by skipping or failing.

Related Reproducer by Mark Rutland
Link: https://lore.kernel.org/lkml/aIEePonPatjNrJVk@J2N7QTR9R3/

Related Kernel Fix
Link: https://lore.kernel.org/lkml/20250812012722.127646-1-ysk@kzalloc.com/
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Yunseong Kim <ysk@kzalloc.com>
---
 tools/perf/tests/Build                  |   1 +
 tools/perf/tests/builtin-test.c         |   1 +
 tools/perf/tests/event_group_throttle.c | 132 ++++++++++++++++++++++++
 tools/perf/tests/tests.h                |   1 +
 4 files changed, 135 insertions(+)
 create mode 100644 tools/perf/tests/event_group_throttle.c

diff --git a/tools/perf/tests/Build b/tools/perf/tests/Build
index 3e8394be15ae..e22e2f285500 100644
--- a/tools/perf/tests/Build
+++ b/tools/perf/tests/Build
@@ -70,6 +70,7 @@ perf-test-y += util.o
 perf-test-y += hwmon_pmu.o
 perf-test-y += tool_pmu.o
 perf-test-y += subcmd-help.o
+perf-test-y += event_group_throttle.o
 
 ifeq ($(SRCARCH),$(filter $(SRCARCH),x86 arm arm64 powerpc))
 perf-test-$(CONFIG_DWARF_UNWIND) += dwarf-unwind.o
diff --git a/tools/perf/tests/builtin-test.c b/tools/perf/tests/builtin-test.c
index 85142dfb3e01..d302bf9d1535 100644
--- a/tools/perf/tests/builtin-test.c
+++ b/tools/perf/tests/builtin-test.c
@@ -141,6 +141,7 @@ static struct test_suite *generic_tests[] = {
 	&suite__symbols,
 	&suite__util,
 	&suite__subcmd_help,
+	&suite__event_group_throttle,
 	NULL,
 };
 
diff --git a/tools/perf/tests/event_group_throttle.c b/tools/perf/tests/event_group_throttle.c
new file mode 100644
index 000000000000..7d5191d7e812
--- /dev/null
+++ b/tools/perf/tests/event_group_throttle.c
@@ -0,0 +1,132 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <stdlib.h>
+#include <unistd.h>
+#include <time.h>
+#include <sys/wait.h>
+#include <signal.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <string.h>
+#include <stdio.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <errno.h>
+#include <linux/perf_event.h>
+#include "perf-sys.h"
+#include "tests.h"
+#include "debug.h"
+
+static struct perf_event_attr attr_parent = {
+	.type = PERF_TYPE_HARDWARE,
+	.size = sizeof(attr_parent),
+	.config = PERF_COUNT_HW_CPU_CYCLES,
+	.sample_period = 1,
+	.exclude_kernel = 1,
+};
+
+static struct perf_event_attr attr_child = {
+	.type = PERF_TYPE_HARDWARE,
+	.size = sizeof(attr_child),
+	.config = PERF_COUNT_HW_CPU_CYCLES,
+	.exclude_kernel = 1,
+	.disabled = 1,
+};
+
+static pid_t run_event_group_throttle(void)
+{
+	pid_t pid = fork();
+
+	if (pid == 0) {
+		int parent, child;
+
+		parent = sys_perf_event_open(&attr_parent, 0, -1, -1, 0);
+		if (parent < 0) {
+			pr_debug("Unable to create event: %d\n", parent);
+			exit(-1);
+		}
+
+		child = sys_perf_event_open(&attr_child, 0, -1, parent, 0);
+		if (child < 0) {
+			pr_debug("Unable to create event: %d\n", child);
+			exit(-1);
+		}
+
+		for (;;)
+			asm("" ::: "memory");
+
+		_exit(0);
+	}
+	return pid;
+}
+
+static bool is_kmsg_err(int fd)
+{
+	char buf[1024];
+	ssize_t len;
+
+	while ((len = read(fd, buf, sizeof(buf) - 1)) > 0) {
+		buf[len] = '\0';
+
+		if (strstr(buf, "UBSAN") || strstr(buf, "WARNING:") ||
+		    strstr(buf, "BUG:") || strstr(buf, "Invalid PMEV")) {
+			pr_debug("Kernel log error detected: %s", buf);
+			return true;
+		}
+	}
+
+	if (len < 0 && errno != EAGAIN) {
+		pr_debug("Error reading /dev/kmsg: %s\n", strerror(errno));
+		return true;
+	}
+
+	return false;
+}
+
+static int test__event_group_throttle(struct test_suite *test __maybe_unused,
+				int subtest __maybe_unused)
+{
+	time_t start;
+	pid_t pid;
+	int fd;
+
+	fd = open("/dev/kmsg", O_RDONLY | O_NONBLOCK);
+	if (fd < 0) {
+		/*
+		 * If /dev/kmsg cannot be opened (e.g. permission denied), skip the test
+		 * as we cannot verify the absence of kernel errors.
+		 */
+		pr_debug("Failed to open /dev/kmsg: %s. Skipping test.\n", strerror(errno));
+		return TEST_SKIP;
+	}
+
+	/*
+	 * Seek to the end to ignore past events (like EFI boot warnings).
+	 * This typically requires CAP_SYSLOG.
+	 */
+	if (lseek(fd, 0, SEEK_END) < 0) {
+		pr_debug("Failed to seek to end of /dev/kmsg: %s\n", strerror(errno));
+		return TEST_FAIL;
+	}
+
+	start = time(NULL);
+	do {
+		pr_debug("Starting event group throttling...\n");
+		pid = run_event_group_throttle();
+
+		sleep(8);
+
+		pr_debug("event group throttler(PID=%d)\n", pid);
+		kill(pid, SIGKILL);
+		waitpid(pid, NULL, 0);
+
+		/* Check for errors during the run */
+		if (is_kmsg_err(fd)) {
+			close(fd);
+			return TEST_FAIL;
+		}
+	} while (time(NULL) - start < 10);
+
+	return TEST_OK;
+}
+
+DEFINE_SUITE("event group throttle", event_group_throttle);
diff --git a/tools/perf/tests/tests.h b/tools/perf/tests/tests.h
index 97e62db8764a..031856a710b2 100644
--- a/tools/perf/tests/tests.h
+++ b/tools/perf/tests/tests.h
@@ -179,6 +179,7 @@ DECLARE_SUITE(event_groups);
 DECLARE_SUITE(symbols);
 DECLARE_SUITE(util);
 DECLARE_SUITE(subcmd_help);
+DECLARE_SUITE(event_group_throttle);
 
 /*
  * PowerPC and S390 do not support creation of instruction breakpoints using the
-- 
2.50.0