[PATCH bpf v2 2/2] selftests/bpf: Add tests for sock_ops ctx access with same src/dst register

[Jiayuan Chen <jiayuan.chen@linux.dev>]

Add selftests to verify SOCK_OPS_GET_SK() and SOCK_OPS_GET_FIELD()
correctly return NULL/zero when dst_reg == src_reg and is_fullsock == 0.

Three subtests are included:
 - get_sk: ctx->sk with same src/dst register (SOCK_OPS_GET_SK)
 - get_field: ctx->snd_cwnd with same src/dst register (SOCK_OPS_GET_FIELD)
 - get_sk_diff_reg: ctx->sk with different src/dst register (baseline)

Each BPF program uses inline asm (__naked) to force specific register
allocation, reads is_fullsock first, then loads the field using the same
(or different) register. The test triggers TCP_NEW_SYN_RECV via a TCP
handshake and checks that the result is NULL/zero when is_fullsock == 0.

Test:
	./test_progs -a sock_ops_get_sk -v
	test_sock_ops_get_sk:PASS:join_cgroup 0 nsec
	test_sock_ops_get_sk:PASS:skel_open_load 0 nsec
	run_sock_ops_test:PASS:prog_attach 0 nsec
	run_sock_ops_test:PASS:start_server 0 nsec
	run_sock_ops_test:PASS:connect_to_fd 0 nsec
	test_sock_ops_get_sk:PASS:null_seen 0 nsec
	test_sock_ops_get_sk:PASS:bug_not_detected 0 nsec
	#419/1   sock_ops_get_sk/get_sk:OK
	run_sock_ops_test:PASS:prog_attach 0 nsec
	run_sock_ops_test:PASS:start_server 0 nsec
	run_sock_ops_test:PASS:connect_to_fd 0 nsec
	test_sock_ops_get_sk:PASS:field_null_seen 0 nsec
	test_sock_ops_get_sk:PASS:field_bug_not_detected 0 nsec
	#419/2   sock_ops_get_sk/get_field:OK
	run_sock_ops_test:PASS:prog_attach 0 nsec
	run_sock_ops_test:PASS:start_server 0 nsec
	run_sock_ops_test:PASS:connect_to_fd 0 nsec
	test_sock_ops_get_sk:PASS:diff_reg_null_seen 0 nsec
	test_sock_ops_get_sk:PASS:diff_reg_bug_not_detected 0 nsec
	#419/3   sock_ops_get_sk/get_sk_diff_reg:OK
	#419     sock_ops_get_sk:OK
	Summary: 1/3 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 .../bpf/prog_tests/sock_ops_get_sk.c          |  77 ++++++++++++
 .../selftests/bpf/progs/sock_ops_get_sk.c     | 117 ++++++++++++++++++
 2 files changed, 194 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..e086f7abb197e
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/sock_ops_get_sk.c
@@ -0,0 +1,77 @@
+// 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"
+
+/* See progs/sock_ops_get_sk.c for the bug description. */
+static void run_sock_ops_test(struct sock_ops_get_sk *skel, int cgroup_fd,
+			      int prog_fd)
+{
+	int server_fd, client_fd, err;
+
+	err = bpf_prog_attach(prog_fd, cgroup_fd, BPF_CGROUP_SOCK_OPS, 0);
+	if (!ASSERT_OK(err, "prog_attach"))
+		return;
+
+	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 and is_locked_tcp_sock == 0.
+	 */
+	client_fd = connect_to_fd(server_fd, 0);
+	if (!ASSERT_GE(client_fd, 0, "connect_to_fd"))
+		goto close_server;
+
+	close(client_fd);
+
+close_server:
+	close(server_fd);
+detach:
+	bpf_prog_detach(cgroup_fd, BPF_CGROUP_SOCK_OPS);
+}
+
+void test_sock_ops_get_sk(void)
+{
+	struct sock_ops_get_sk *skel;
+	int cgroup_fd;
+
+	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;
+
+	/* Test SOCK_OPS_GET_SK with same src/dst register */
+	if (test__start_subtest("get_sk")) {
+		run_sock_ops_test(skel, cgroup_fd,
+				  bpf_program__fd(skel->progs.sock_ops_get_sk_same_reg));
+		ASSERT_EQ(skel->bss->null_seen, 1, "null_seen");
+		ASSERT_EQ(skel->bss->bug_detected, 0, "bug_not_detected");
+	}
+
+	/* Test SOCK_OPS_GET_FIELD with same src/dst register */
+	if (test__start_subtest("get_field")) {
+		run_sock_ops_test(skel, cgroup_fd,
+				  bpf_program__fd(skel->progs.sock_ops_get_field_same_reg));
+		ASSERT_EQ(skel->bss->field_null_seen, 1, "field_null_seen");
+		ASSERT_EQ(skel->bss->field_bug_detected, 0, "field_bug_not_detected");
+	}
+
+	/* Test SOCK_OPS_GET_SK with different src/dst register */
+	if (test__start_subtest("get_sk_diff_reg")) {
+		run_sock_ops_test(skel, cgroup_fd,
+				  bpf_program__fd(skel->progs.sock_ops_get_sk_diff_reg));
+		ASSERT_EQ(skel->bss->diff_reg_null_seen, 1, "diff_reg_null_seen");
+		ASSERT_EQ(skel->bss->diff_reg_bug_detected, 0, "diff_reg_bug_not_detected");
+	}
+
+	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..3a0689f8ce7ca
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/sock_ops_get_sk.c
@@ -0,0 +1,117 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+/*
+ * Test the SOCK_OPS_GET_SK() and SOCK_OPS_GET_FIELD() macros in
+ * sock_ops_convert_ctx_access() when dst_reg == src_reg.
+ *
+ * When dst_reg == src_reg, the macros borrow a temporary register to load
+ * is_fullsock / is_locked_tcp_sock, because dst_reg holds the ctx pointer
+ * and cannot be clobbered before ctx->sk / ctx->field is read. If
+ * is_fullsock == 0 (e.g., TCP_NEW_SYN_RECV with a request_sock), the macro
+ * must still zero dst_reg so the verifier's PTR_TO_SOCKET_OR_NULL /
+ * SCALAR_VALUE type is correct at runtime. A missing clear leaves a stale
+ * ctx pointer in dst_reg that passes NULL checks (GET_SK) or leaks a kernel
+ * address as a scalar (GET_FIELD).
+ *
+ * When dst_reg != src_reg, dst_reg itself is used to load is_fullsock, so
+ * the JEQ (dst_reg == 0) naturally leaves it zeroed on the !fullsock path.
+ */
+
+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);
+}
+
+/* SOCK_OPS_GET_FIELD: same-register, is_locked_tcp_sock == 0 path. */
+int field_bug_detected;
+int field_null_seen;
+
+SEC("sockops")
+__naked void sock_ops_get_field_same_reg(void)
+{
+	asm volatile (
+		"r7 = *(u32 *)(r1 + %[is_fullsock_off]);"
+		"r1 = *(u32 *)(r1 + %[snd_cwnd_off]);"
+		"if r7 != 0 goto 2f;"
+		"if r1 == 0 goto 1f;"
+		"r1 = %[field_bug_detected] ll;"
+		"r2 = 1;"
+		"*(u32 *)(r1 + 0) = r2;"
+		"goto 2f;"
+	"1:"
+		"r1 = %[field_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(snd_cwnd_off, offsetof(struct bpf_sock_ops, snd_cwnd)),
+		  __imm_addr(field_bug_detected),
+		  __imm_addr(field_null_seen)
+		: __clobber_all);
+}
+
+/* SOCK_OPS_GET_SK: different-register, is_fullsock == 0 path. */
+int diff_reg_bug_detected;
+int diff_reg_null_seen;
+
+SEC("sockops")
+__naked void sock_ops_get_sk_diff_reg(void)
+{
+	asm volatile (
+		"r7 = r1;"
+		"r6 = *(u32 *)(r7 + %[is_fullsock_off]);"
+		"r2 = *(u64 *)(r7 + %[sk_off]);"
+		"if r6 != 0 goto 2f;"
+		"if r2 == 0 goto 1f;"
+		"r1 = %[diff_reg_bug_detected] ll;"
+		"r3 = 1;"
+		"*(u32 *)(r1 + 0) = r3;"
+		"goto 2f;"
+	"1:"
+		"r1 = %[diff_reg_null_seen] ll;"
+		"r3 = 1;"
+		"*(u32 *)(r1 + 0) = r3;"
+	"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(diff_reg_bug_detected),
+		  __imm_addr(diff_reg_null_seen)
+		: __clobber_all);
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.43.0