[PATCH bpf v3 2/2] selftests/bpf: add test for nullable PTR_TO_BUF access

[Qi Tang <tpluszz77@gmail.com>]

Add iter_buf_null_fail.c with two tests:
  - iter_buf_null_deref: verifier must reject direct dereference of
    ctx->key (PTR_TO_BUF | PTR_MAYBE_NULL) without a null check
  - iter_buf_null_check_ok: verifier must accept dereference after
    an explicit null check

Signed-off-by: Qi Tang <tpluszz77@gmail.com>
---
 .../selftests/bpf/progs/iter_buf_null_fail.c  | 51 +++++++++++++++++++
 1 file changed, 51 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/iter_buf_null_fail.c

diff --git a/tools/testing/selftests/bpf/progs/iter_buf_null_fail.c b/tools/testing/selftests/bpf/progs/iter_buf_null_fail.c
new file mode 100644
index 000000000000..963a4cfc33f3
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/iter_buf_null_fail.c
@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Qi Tang */
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, __u64);
+} hashmap SEC(".maps");
+
+/*
+ * Verify that the verifier rejects direct access to nullable PTR_TO_BUF
+ * (ctx->key) without a null check.  On the iterator stop callback,
+ * ctx->key is NULL, so unconditional access would be a NULL deref.
+ */
+SEC("iter/bpf_map_elem")
+__failure __msg("invalid mem access")
+int iter_buf_null_deref(struct bpf_iter__bpf_map_elem *ctx)
+{
+	/* ctx->key is PTR_TO_BUF | PTR_MAYBE_NULL | MEM_RDONLY.
+	 * Direct access without null check must be rejected.
+	 */
+	volatile __u32 v = *(__u32 *)ctx->key;
+
+	(void)v;
+	return 0;
+}
+
+/*
+ * Verify that access after a null check is still accepted.
+ */
+SEC("iter/bpf_map_elem")
+__success
+int iter_buf_null_check_ok(struct bpf_iter__bpf_map_elem *ctx)
+{
+	__u32 *key = ctx->key;
+
+	if (!key)
+		return 0;
+
+	volatile __u32 v = *key;
+
+	(void)v;
+	return 0;
+}
-- 
2.43.0