[PATCH bpf-next v6 2/2] selftests/bpf: Cover attach type checks in link update

[Sanghyun Park <sanghyun.park.cnu@gmail.com>]

Pin down the two contracts changed by the companion patch. LINK_UPDATE
validates a replacement program against the link's attach type, while
CGROUP_SKB capability checks apply at LINK_CREATE and not LINK_UPDATE.

Cover incompatible UDP4/UDP6 and LSM attach flavors, plus a CGROUP_SKB
link update after dropping CAP_NET_ADMIN and CAP_SYS_ADMIN.

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Sanghyun Park <sanghyun.park.cnu@gmail.com>
---
v6:
  - Remove the unused msg_src_ip6 store from sendmsg6().
  - Add Reviewed-by tag.
v5: https://lore.kernel.org/r/20260901014713.3502900-4-sanghyun.park.cnu@gmail.com
  - Isolate attach and update coverage in separate subtests with their own setup.
  - Inline the one-use LSM program FD and use a descriptive link FD.
  - Explain the create-versus-update contracts covered by the tests.
v4: https://lore.kernel.org/r/20260831023918.1857658-2-sanghyun.park.cnu@gmail.com
---
 .../testing/selftests/bpf/prog_tests/cgroup_link.c | 81 +++++++++++++++++++++-
 .../testing/selftests/bpf/prog_tests/lsm_cgroup.c  | 12 ++++
 tools/testing/selftests/bpf/progs/lsm_cgroup.c     |  6 ++
 .../testing/selftests/bpf/progs/test_cgroup_link.c | 13 +++-
 4 files changed, 110 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/cgroup_link.c b/tools/testing/selftests/bpf/prog_tests/cgroup_link.c
index 15093a69510e..3b43c34917cc 100644
--- a/tools/testing/selftests/bpf/prog_tests/cgroup_link.c
+++ b/tools/testing/selftests/bpf/prog_tests/cgroup_link.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 
 #include <test_progs.h>
+#include "cap_helpers.h"
 #include "cgroup_helpers.h"
 #include "testing_helpers.h"
 #include "test_cgroup_link.skel.h"
@@ -24,7 +25,77 @@ int ping_and_check(int exp_calls, int exp_alt_calls)
 	return 0;
 }
 
-void serial_test_cgroup_link(void)
+static void test_cgroup_link_update(void)
+{
+	const __u64 caps = (1ULL << CAP_NET_ADMIN) | (1ULL << CAP_SYS_ADMIN);
+	struct bpf_link *link = NULL;
+	__u64 saved_caps = 0;
+	int cg_fd = -1, err;
+
+	skel = test_cgroup_link__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_load"))
+		return;
+
+	err = setup_cgroup_environment();
+	if (!ASSERT_OK(err, "cg_init"))
+		goto cleanup;
+
+	cg_fd = create_and_get_cgroup("/cgroup_link_update");
+	if (!ASSERT_GE(cg_fd, 0, "cg_create"))
+		goto cleanup;
+
+	err = join_cgroup("/cgroup_link_update");
+	if (!ASSERT_OK(err, "cg_join"))
+		goto cleanup;
+
+	link = bpf_program__attach_cgroup(skel->progs.sendmsg4, cg_fd);
+	if (!ASSERT_OK_PTR(link, "attach_sendmsg4"))
+		goto cleanup;
+
+	err = bpf_link__update_program(link, skel->progs.sendmsg6);
+	ASSERT_EQ(err, -EINVAL, "reject_sendmsg6_update");
+	bpf_link__destroy(link);
+	link = NULL;
+
+	err = cap_disable_effective(caps, &saved_caps);
+	if (!ASSERT_OK(err, "drop_caps_for_attach"))
+		goto cleanup;
+
+	link = bpf_program__attach_cgroup(skel->progs.egress, cg_fd);
+	if (!ASSERT_ERR_PTR(link, "attach_without_net_admin"))
+		goto cleanup;
+	err = libbpf_get_error(link);
+	link = NULL;
+	if (!ASSERT_EQ(err, -EPERM, "attach_err"))
+		goto cleanup;
+
+	err = cap_enable_effective(saved_caps & caps, NULL);
+	if (!ASSERT_OK(err, "restore_caps_for_attach"))
+		goto cleanup;
+
+	link = bpf_program__attach_cgroup(skel->progs.egress, cg_fd);
+	if (!ASSERT_OK_PTR(link, "attach_egress"))
+		goto cleanup;
+
+	err = cap_disable_effective(caps, &saved_caps);
+	if (!ASSERT_OK(err, "drop_caps"))
+		goto cleanup;
+
+	err = bpf_link__update_program(link, skel->progs.egress_alt);
+	ASSERT_OK(err, "update_without_net_admin");
+
+cleanup:
+	err = cap_enable_effective(saved_caps & caps, NULL);
+	ASSERT_OK(err, "restore_caps");
+	bpf_link__destroy(link);
+	if (cg_fd >= 0)
+		close(cg_fd);
+	cleanup_cgroup_environment();
+	test_cgroup_link__destroy(skel);
+	skel = NULL;
+}
+
+static void test_cgroup_link_attach(void)
 {
 	struct {
 		const char *path;
@@ -253,3 +324,11 @@ void serial_test_cgroup_link(void)
 	}
 	cleanup_cgroup_environment();
 }
+
+void serial_test_cgroup_link(void)
+{
+	if (test__start_subtest("attach"))
+		test_cgroup_link_attach();
+	if (test__start_subtest("update"))
+		test_cgroup_link_update();
+}
diff --git a/tools/testing/selftests/bpf/prog_tests/lsm_cgroup.c b/tools/testing/selftests/bpf/prog_tests/lsm_cgroup.c
index 41e867467f6c..0dd44dbb9c95 100644
--- a/tools/testing/selftests/bpf/prog_tests/lsm_cgroup.c
+++ b/tools/testing/selftests/bpf/prog_tests/lsm_cgroup.c
@@ -74,6 +74,7 @@ static void test_lsm_cgroup_functional(void)
 	int bind_prog_fd = -1;
 	int bind_link_fd = -1;
 	int clone_prog_fd = -1;
+	int socket_link_fd = -1;
 	int err, fd, prio;
 	socklen_t socklen;
 
@@ -156,6 +157,17 @@ static void test_lsm_cgroup_functional(void)
 	ASSERT_EQ(query_prog_cnt(cgroup_fd, "bpf_lsm_socket_bind"), 1, "prog count");
 	ASSERT_EQ(query_prog_cnt(cgroup_fd, NULL), 4, "total prog count");
 
+	socket_link_fd =
+		bpf_link_create(bpf_program__fd(skel->progs.socket_first),
+				cgroup_fd, BPF_LSM_CGROUP, NULL);
+	if (!ASSERT_GE(socket_link_fd, 0, "link create socket_first"))
+		goto detach_cgroup;
+	err = bpf_link_update(socket_link_fd,
+			      bpf_program__fd(skel->progs.socket_create_lsm),
+			      NULL);
+	ASSERT_EQ(err, -EINVAL, "reject lsm_mac link update");
+	close(socket_link_fd);
+
 	/* Attach another instance of bind program to another cgroup.
 	 * This should trigger the reuse of the trampoline shim (two
 	 * programs attaching to the same btf_id).
diff --git a/tools/testing/selftests/bpf/progs/lsm_cgroup.c b/tools/testing/selftests/bpf/progs/lsm_cgroup.c
index 3bfa479104be..30727945cecc 100644
--- a/tools/testing/selftests/bpf/progs/lsm_cgroup.c
+++ b/tools/testing/selftests/bpf/progs/lsm_cgroup.c
@@ -213,6 +213,12 @@ int BPF_PROG(socket_first, int family, int type, int protocol, int kern)
 	return 0;
 }
 
+SEC("lsm/socket_create")
+int BPF_PROG(socket_create_lsm, int family, int type, int protocol, int kern)
+{
+	return 0;
+}
+
 SEC("lsm_cgroup/socket_create")
 int BPF_PROG(socket_second, int family, int type, int protocol, int kern)
 {
diff --git a/tools/testing/selftests/bpf/progs/test_cgroup_link.c b/tools/testing/selftests/bpf/progs/test_cgroup_link.c
index 4faba88e45a5..107429638f54 100644
--- a/tools/testing/selftests/bpf/progs/test_cgroup_link.c
+++ b/tools/testing/selftests/bpf/progs/test_cgroup_link.c
@@ -20,5 +20,16 @@ int egress_alt(struct __sk_buff *skb)
 	return 1;
 }
 
-char _license[] SEC("license") = "GPL";
+SEC("cgroup/sendmsg4")
+int sendmsg4(struct bpf_sock_addr *ctx)
+{
+	return 1;
+}
 
+SEC("cgroup/sendmsg6")
+int sendmsg6(struct bpf_sock_addr *ctx)
+{
+	return 1;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.48.1