[PATCH bpf v1 2/2] selftests/bpf: Add test for SOCK_OPS_GET_SK with same src/dst register

Jiayuan Chen
2026-04-04T14:10:48Z

Add a selftest that verifies SOCK_OPS_GET_SK() correctly returns NULL
when dst_reg == src_reg and is_fullsock == 0.

The BPF program reads ctx->is_fullsock, then loads ctx->sk using the
same register for source and destination. When is_fullsock == 0, sk
must be NULL. The test triggers this path via a TCP handshake (which
causes TCP_NEW_SYN_RECV state) and checks:
- null_seen == 1: the is_fullsock==0 path was hit with correct NULL sk
- bug_detected == 0: sk was never non-NULL when is_fullsock==0

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
.../bpf/prog_tests/sock_ops_get_sk.c | 62 +++++++++++++++++++
.../selftests/bpf/progs/sock_ops_get_sk.c | 44 +++++++++++++
2 files changed, 106 insertions(+)
create mode 100644 tools/testing/selftests/bpf/prog_tests/sock_ops_get_sk.c
create mode 100644 tools/testing/selftests/bpf/progs/sock_ops_get_sk.c

diff --git a/tools/testing/selftests/bpf/prog_tests/sock_ops_get_sk.c b/tools/testing/selftests/bpf/prog_tests/sock_ops_get_sk.c
new file mode 100644
index 0000000000000..9eaf97786c1d9
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/sock_ops_get_sk.c
@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include "cgroup_helpers.h"
+#include "network_helpers.h"
+#include "sock_ops_get_sk.skel.h"
+
+/*
+ * Test that reading ctx->sk with dst_reg == src_reg in a sock_ops program
+ * correctly returns NULL when is_fullsock == 0 (TCP_NEW_SYN_RECV state).
+ *
+ * The bug was in SOCK_OPS_GET_SK() macro which failed to zero the destination
+ * register when it was the same as the source register and the socket was not
+ * a full socket. This left the ctx pointer in the register, which then passed
+ * the NULL check and could be used as a socket pointer, causing OOB access.
+ */
+void test_sock_ops_get_sk(void)
+{
+ struct sock_ops_get_sk *skel;
+ int cgroup_fd, server_fd, client_fd;
+ int prog_fd, err;
+
+ cgroup_fd = test__join_cgroup("/sock_ops_get_sk");
+ if (!ASSERT_GE(cgroup_fd, 0, "join_cgroup"))
+ return;
+
+ skel = sock_ops_get_sk__open_and_load();
+ if (!ASSERT_OK_PTR(skel, "skel_open_load"))
+ goto close_cgroup;
+
+ prog_fd = bpf_program__fd(skel->progs.sock_ops_get_sk_same_reg);
+ err = bpf_prog_attach(prog_fd, cgroup_fd, BPF_CGROUP_SOCK_OPS, 0);
+ if (!ASSERT_OK(err, "prog_attach"))
+ goto destroy_skel;
+
+ server_fd = start_server(AF_INET, SOCK_STREAM, NULL, 0, 0);
+ if (!ASSERT_GE(server_fd, 0, "start_server"))
+ goto detach;
+
+ /* Trigger TCP handshake which causes TCP_NEW_SYN_RECV state where
+ * is_fullsock == 0. With the bug, ctx->sk loaded with same src/dst
+ * register would return a stale ctx pointer instead of NULL.
+ */
+ client_fd = connect_to_fd(server_fd, 0);
+ if (!ASSERT_GE(client_fd, 0, "connect_to_fd"))
+ goto close_server;
+
+ close(client_fd);
+
+ /* Verify that the is_fullsock == 0 path was hit and sk was NULL */
+ ASSERT_EQ(skel->bss->null_seen, 1, "null_seen");
+ ASSERT_EQ(skel->bss->bug_detected, 0, "bug_not_detected");
+
+close_server:
+ close(server_fd);
+detach:
+ bpf_prog_detach(cgroup_fd, BPF_CGROUP_SOCK_OPS);
+destroy_skel:
+ sock_ops_get_sk__destroy(skel);
+close_cgroup:
+ close(cgroup_fd);
+}
diff --git a/tools/testing/selftests/bpf/progs/sock_ops_get_sk.c b/tools/testing/selftests/bpf/progs/sock_ops_get_sk.c
new file mode 100644
index 0000000000000..4a75614b21eb5
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/sock_ops_get_sk.c
@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+/*
+ * Test that SOCK_OPS_GET_SK() correctly returns NULL when dst_reg == src_reg
+ * and is_fullsock == 0 (e.g., during TCP_NEW_SYN_RECV). Before the fix, the
+ * macro failed to zero the destination register, leaving a stale ctx pointer
+ * that bypassed the NULL check.
+ */
+
+int bug_detected;
+int null_seen;
+
+SEC("sockops")
+__naked void sock_ops_get_sk_same_reg(void)
+{
+ asm volatile (
+ "r7 = *(u32 *)(r1 + %[is_fullsock_off]);"
+ "r1 = *(u64 *)(r1 + %[sk_off]);"
+ "if r7 != 0 goto 2f;"
+ "if r1 == 0 goto 1f;"
+ "r1 = %[bug_detected] ll;"
+ "r2 = 1;"
+ "*(u32 *)(r1 + 0) = r2;"
+ "goto 2f;"
+ "1:"
+ "r1 = %[null_seen] ll;"
+ "r2 = 1;"
+ "*(u32 *)(r1 + 0) = r2;"
+ "2:"
+ "r0 = 1;"
+ "exit;"
+ :
+ : __imm_const(is_fullsock_off, offsetof(struct bpf_sock_ops, is_fullsock)),
+ __imm_const(sk_off, offsetof(struct bpf_sock_ops, sk)),
+ __imm_addr(bug_detected),
+ __imm_addr(null_seen)
+ : __clobber_all);
+}
+
+char _license[] SEC("license") = "GPL";
--
2.43.0