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>

1 files changed, 22 insertions, 0 deletions

diff --git a/drivers/md/dm-vdo/string-utils.c b/drivers/md/dm-vdo/string-utils.c
new file mode 100644
index 000000000..71e44b468
--- /dev/null
+++ b/drivers/md/dm-vdo/string-utils.c
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright 2023 Red Hat
+ */
+
+#include "string-utils.h"
+
+char *vdo_append_to_buffer(char *buffer, char *buf_end, const char *fmt, ...)
+{
+	va_list args;
+	size_t n;
+
+	va_start(args, fmt);
+	n = vsnprintf(buffer, buf_end - buffer, fmt, args);
+	if (n >= (size_t) (buf_end - buffer))
+		buffer = buf_end;
+	else
+		buffer += n;
+	va_end(args);
+
+	return buffer;
+}