From fdf4eb632683bfc2840acebe62716cb468d43e10 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@kernel.org>
Date: Sun, 26 Apr 2026 17:51:07 +0200
Subject: [PATCH] selftests/rseq: Validate legacy behavior

The RSEQ legacy mode behavior requires that the ID fields in the rseq
region are unconditionally updated on every context switch and before
signal delivery even if not required by the ABI specification.

To ensure that this behavior is preserved for legacy users in the future,
add a test which validates that with a sleep() and a signal sent to self.

Provide a run script which prevents GLIBC from registering a RSEQ region,
so that the test can register it's own legacy sized region.

Fixes: 566d8015f7ee ("rseq: Avoid CPU/MM CID updates when no event pending")
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://patch.msgid.link/20260428224427.764705536%40kernel.org
Cc: stable@vger.kernel.org
---
 tools/testing/selftests/rseq/Makefile         |  5 +-
 tools/testing/selftests/rseq/legacy_check.c   | 65 +++++++++++++++++++
 .../selftests/rseq/run_legacy_check.sh        |  4 ++
 3 files changed, 72 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/rseq/legacy_check.c
 create mode 100755 tools/testing/selftests/rseq/run_legacy_check.sh

diff --git a/tools/testing/selftests/rseq/Makefile b/tools/testing/selftests/rseq/Makefile
index 0d1947c0d623..0293a2f17f51 100644
--- a/tools/testing/selftests/rseq/Makefile
+++ b/tools/testing/selftests/rseq/Makefile
@@ -22,9 +22,10 @@ TEST_GEN_PROGS_EXTENDED = librseq.so \
 	param_test_compare_twice \
 	param_test_mm_cid \
 	param_test_mm_cid_compare_twice \
-	syscall_errors_test
+	syscall_errors_test \
+	legacy_check
 
-TEST_PROGS = run_param_test.sh run_syscall_errors_test.sh
+TEST_PROGS = run_param_test.sh run_syscall_errors_test.sh run_legacy_check.sh
 
 TEST_FILES := settings
 
diff --git a/tools/testing/selftests/rseq/legacy_check.c b/tools/testing/selftests/rseq/legacy_check.c
new file mode 100644
index 000000000000..3f7de4e28303
--- /dev/null
+++ b/tools/testing/selftests/rseq/legacy_check.c
@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
+#include <errno.h>
+#include <signal.h>
+#include <stdint.h>
+#include <unistd.h>
+
+#include "rseq.h"
+
+#include "../kselftest_harness.h"
+
+FIXTURE(legacy)
+{
+};
+
+static int cpu_id_in_sigfn = -1;
+
+static void sigfn(int sig)
+{
+	struct rseq_abi *rs = rseq_get_abi();
+
+	cpu_id_in_sigfn = rs->cpu_id_start;
+}
+
+FIXTURE_SETUP(legacy)
+{
+	int res = __rseq_register_current_thread(true, true);
+
+	switch (res) {
+	case -ENOSYS:
+		SKIP(return, "RSEQ not enabled\n");
+	case -EBUSY:
+		SKIP(return, "GLIBC owns RSEQ. Disable GLIBC RSEQ registration\n");
+	default:
+		ASSERT_EQ(res, 0);
+	}
+
+	ASSERT_NE(signal(SIGUSR1, sigfn), SIG_ERR);
+}
+
+FIXTURE_TEARDOWN(legacy)
+{
+}
+
+TEST_F(legacy, legacy_test)
+{
+	struct rseq_abi *rs = rseq_get_abi();
+
+	ASSERT_NE(rs, NULL);
+
+	/* Overwrite rs::cpu_id_start */
+	rs->cpu_id_start = -1;
+	sleep(1);
+	ASSERT_NE(rs->cpu_id_start, -1);
+
+	rs->cpu_id_start = -1;
+	ASSERT_EQ(raise(SIGUSR1), 0);
+	ASSERT_NE(rs->cpu_id_start, -1);
+	ASSERT_NE(cpu_id_in_sigfn, -1);
+}
+
+TEST_HARNESS_MAIN
diff --git a/tools/testing/selftests/rseq/run_legacy_check.sh b/tools/testing/selftests/rseq/run_legacy_check.sh
new file mode 100755
index 000000000000..5577b46ea092
--- /dev/null
+++ b/tools/testing/selftests/rseq/run_legacy_check.sh
@@ -0,0 +1,4 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+GLIBC_TUNABLES="${GLIBC_TUNABLES:-}:glibc.pthread.rseq=0" ./legacy_check