[PATCH bpf-next v3 2/2] selftests/bpf: add verification for BPF_PROG_QUERY attr size boundaries

[Yuyang Huang <yuyanghuang@google.com>]

Add a new selftest to verify that the BPF syscall (specifically
BPF_PROG_QUERY) correctly handles different user-declared attribute sizes.

Specifically, verify that:
- For cgroup queries, a query with a size that covers 'prog_cnt' but is
  smaller than 'revision' (OLD_QUERY_SIZE) succeeds, but does not write
  to 'revision' (verifying backward compatibility).
- A query with full size (FULL_QUERY_SIZE) succeeds and writes both
  'prog_cnt' and 'revision'.

Fixes: 120933984460 ("bpf: Implement mprog API on top of existing cgroup progs")
Cc: Maciej Żenczykowski <maze@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
---
 .../selftests/bpf/prog_tests/bpf_attr_size.c  | 69 +++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/bpf_attr_size.c

diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_attr_size.c b/tools/testing/selftests/bpf/prog_tests/bpf_attr_size.c
new file mode 100644
index 000000000000..32159dc64da8
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_attr_size.c
@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Google LLC */
+#include <linux/bpf.h>
+#include <unistd.h>
+#include <sys/syscall.h>
+#include <test_progs.h>
+#include <cgroup_helpers.h>
+#include "cgroup_skb_direct_packet_access.skel.h"
+
+#define OLD_QUERY_SIZE		offsetofend(union bpf_attr, query.prog_cnt)
+#define FULL_QUERY_SIZE		offsetofend(union bpf_attr, query.revision)
+
+static void test_query_size_boundaries(void)
+{
+	struct cgroup_skb_direct_packet_access *skel;
+	struct bpf_link *link = NULL;
+	union bpf_attr attr;
+	int cg_fd = -1;
+	int err;
+
+	skel = cgroup_skb_direct_packet_access__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_load"))
+		return;
+
+	cg_fd = test__join_cgroup("/attr_size_cg");
+	if (!ASSERT_GE(cg_fd, 0, "join_cgroup"))
+		goto cleanup;
+
+	link = bpf_program__attach_cgroup(skel->progs.direct_packet_access,
+					  cg_fd);
+	if (!ASSERT_OK_PTR(link, "cg_attach"))
+		goto cleanup;
+
+	memset(&attr, 0, sizeof(attr));
+	attr.query.target_fd = cg_fd;
+	attr.query.attach_type = BPF_CGROUP_INET_INGRESS;
+	attr.query.revision = 0xdeadbeefdeadbeefULL;
+
+	err = syscall(__NR_bpf, BPF_PROG_QUERY, &attr, OLD_QUERY_SIZE);
+	if (ASSERT_OK(err, "query_old_size")) {
+		ASSERT_EQ(attr.query.prog_cnt, 1, "prog_cnt_written_old");
+		ASSERT_EQ(attr.query.revision, 0xdeadbeefdeadbeefULL,
+			  "revision_not_written_old");
+	}
+
+	memset(&attr, 0, sizeof(attr));
+	attr.query.target_fd = cg_fd;
+	attr.query.attach_type = BPF_CGROUP_INET_INGRESS;
+
+	err = syscall(__NR_bpf, BPF_PROG_QUERY, &attr, FULL_QUERY_SIZE);
+	if (!ASSERT_OK(err, "query_full_size"))
+		goto cleanup;
+
+	ASSERT_EQ(attr.query.prog_cnt, 1, "prog_cnt_written");
+	ASSERT_GT(attr.query.revision, 0, "revision_written");
+
+cleanup:
+	if (link)
+		bpf_link__destroy(link);
+	if (cg_fd >= 0)
+		close(cg_fd);
+	cgroup_skb_direct_packet_access__destroy(skel);
+}
+
+void test_bpf_attr_size(void)
+{
+	if (test__start_subtest("query_size_boundaries"))
+		test_query_size_boundaries();
+}
-- 
2.54.0.823.g6e5bcc1fc9-goog