selftests/bpf: Reject graph kptr use after RCU unlockgrafted

Add a sleepable verifier test that loads a graph-node local kptr in an
explicit RCU read-side critical section, then passes its node to
bpf_rbtree_remove() after the section ends.

Before the verifier fix, the stale NON_OWN_REF flag makes the node look like
a live borrowed reference and the program is accepted. After the fix, the
pointer is demoted without NON_OWN_REF and the graph kfunc argument is
rejected.

Also exercise a graph kptr loaded while a spin lock provides implicit RCU
protection. The pointer must be invalidated when the lock is released, which
guards the required ordering between non-owning-reference invalidation and
RCU demotion.

Update the existing fault-protected load test state description. The
post-unlock pointer no longer carries NON_OWN_REF, but remains readable
because the load is rewritten to use BPF_PROBE_MEM.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904084325.52250-7-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 60 insertions, 0 deletions

diff --git a/samples/kdb/Makefile b/samples/kdb/Makefile
new file mode 100644
index 000000000..947cb8522
--- /dev/null
+++ b/samples/kdb/Makefile
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+obj-$(CONFIG_SAMPLE_KDB) += kdb_hello.o
diff --git a/samples/kdb/kdb_hello.c b/samples/kdb/kdb_hello.c
new file mode 100644
index 000000000..82736e5a5
--- /dev/null
+++ b/samples/kdb/kdb_hello.c
@@ -0,0 +1,58 @@
+/*
+ * Created by: Jason Wessel <jason.wessel@windriver.com>
+ *
+ * Copyright (c) 2010 Wind River Systems, Inc.  All Rights Reserved.
+ *
+ * This file is licensed under the terms of the GNU General Public
+ * License version 2. This program is licensed "as is" without any
+ * warranty of any kind, whether express or implied.
+ */
+
+#include <linux/module.h>
+#include <linux/kdb.h>
+
+/*
+ * All kdb shell command call backs receive argc and argv, where
+ * argv[0] is the command the end user typed
+ */
+static int kdb_hello_cmd(int argc, const char **argv)
+{
+	if (argc > 1)
+		return KDB_ARGCOUNT;
+
+	if (argc)
+		kdb_printf("Hello %s.\n", argv[1]);
+	else
+		kdb_printf("Hello world!\n");
+
+	return 0;
+}
+
+static kdbtab_t hello_cmd = {
+	.name = "hello",
+	.func = kdb_hello_cmd,
+	.usage = "[string]",
+	.help = "Say Hello World or Hello [string]",
+};
+
+static int __init kdb_hello_cmd_init(void)
+{
+	/*
+	 * Registration of a dynamically added kdb command is done with
+	 * kdb_register().
+	 */
+	kdb_register(&hello_cmd);
+	return 0;
+}
+
+static void __exit kdb_hello_cmd_exit(void)
+{
+	kdb_unregister(&hello_cmd);
+}
+
+module_init(kdb_hello_cmd_init);
+module_exit(kdb_hello_cmd_exit);
+
+MODULE_AUTHOR("WindRiver");
+MODULE_DESCRIPTION("KDB example to add a hello command");
+MODULE_LICENSE("GPL");