[PATCH bpf v2 2/2] selftests/bpf: Exercise negative optlen in cgroup getsockopt hook

[Junseo Lim <zirajs7@gmail.com>]

Add a cgroup getsockopt selftest that sets ctx->optlen to -1. Use
TCP_ZEROCOPY_RECEIVE to exercise the kernel-buffer getsockopt hook.

The userspace-visible result is -EFAULT on both patched and unpatched
kernels, so the return value alone cannot distinguish the bug. The test
still exercises the kernel-buffer getsockopt path with a negative
ctx->optlen, which reproduces the hardened usercopy warning on unpatched
kernels.

Signed-off-by: Junseo Lim <zirajs7@gmail.com>
---
Adding the selftest to sockopt_test triggered the problematic path, but
detecting the broken behavior was not trivial because getsockopt()
returned -EFAULT on both patched and unpatched kernels. The selftest
mainly serves as a reproducer for the hardened usercopy warning.

 .../selftests/bpf/prog_tests/sockopt.c        | 22 +++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/sockopt.c b/tools/testing/selftests/bpf/prog_tests/sockopt.c
index eaac83a7f388..3834dcf0f9dd 100644
--- a/tools/testing/selftests/bpf/prog_tests/sockopt.c
+++ b/tools/testing/selftests/bpf/prog_tests/sockopt.c
@@ -1,4 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0
+#include <linux/tcp.h>
 #include <test_progs.h>
 #include <io_uring/mini_liburing.h>
 #include "cgroup_helpers.h"
@@ -283,6 +284,27 @@ static struct sockopt_test {
 		.error = EFAULT_GETSOCKOPT,
 		.io_uring_support = true,
 	},
+	{
+		.descr = "getsockopt: deny negative ctx->optlen in TCP_ZEROCOPY_RECEIVE",
+		.insns = {
+			/* ctx->optlen = -1 */
+			BPF_MOV64_IMM(BPF_REG_0, -1),
+			BPF_STX_MEM(BPF_W, BPF_REG_1, BPF_REG_0,
+				    offsetof(struct bpf_sockopt, optlen)),
+
+			/* return 1 */
+			BPF_MOV64_IMM(BPF_REG_0, 1),
+			BPF_EXIT_INSN(),
+		},
+		.attach_type = BPF_CGROUP_GETSOCKOPT,
+		.expected_attach_type = BPF_CGROUP_GETSOCKOPT,
+
+		.get_level = IPPROTO_TCP,
+		.get_optname = TCP_ZEROCOPY_RECEIVE,
+		.get_optlen = sizeof(struct tcp_zerocopy_receive),
+
+		.error = EFAULT_GETSOCKOPT,
+	},
 	{
 		.descr = "getsockopt: ignore >PAGE_SIZE optlen",
 		.insns = {
-- 
2.55.0