selftests/bpf: Add tests for uprobe nop10 red zone clobbering

The uprobe nop5 optimization used to replace a 5-byte NOP with a 5-byte
CALL to a trampoline. The CALL pushes a return address onto the stack at
[rsp-8], clobbering whatever was stored there.

On x86-64, the red zone is the 128 bytes below rsp that user code may use
for temporary storage without adjusting rsp. Compilers can place USDT
argument operands there, generating specs like "8@-8(%rbp)" when rbp ==
rsp. With the CALL-based optimization, the return address overwrites that
argument before the BPF-side USDT argument fetch runs.

Add two tests for this case. The uprobe_syscall subtest stores known values
at -8(%rsp), -16(%rsp), and -24(%rsp), executes an optimized nop10 uprobe,
and verifies the red-zone data is still intact. The USDT subtest triggers a
probe in a function where the compiler places three USDT operands in the
red zone and verifies that all 10 optimized invocations deliver the expected
argument values to BPF.

On an unfixed kernel, the first hit goes through the INT3 path and later
hits use the optimized CALL path, so the red-zone checks fail after
optimization.

[ updates to use nop10 ]
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://patch.msgid.link/20260703114917.238144-13-jolsa@kernel.org

tools/testing/selftests/bpf/prog_tests/uprobe_syscall.c

@@ -357,6 +357,50 @@ __nocf_check __weak void usdt_test(void)
 	USDT(optimized_uprobe, usdt);
 }
 
+/*
+ * Assembly-level red zone clobbering test. Stores known values in the
+ * red zone (below RSP), executes a nop10 (uprobe site), and checks that
+ * the values survived. Returns 0 if intact, 1 if clobbered.
+ *
+ * The nop5 optimization used CALL (which pushes a return address to
+ * [rsp-8]), the value at -8(%rsp) was overwritten. The nop10 optimization
+ * should escape that by moving stackpointer below the redzone before
+ * doing the CALL.
+ *
+ * Align the code at 64 bytes, to make sure nop10 is not on page boundary.
+ */
+__attribute__((aligned(64)))
+__nocf_check __weak __naked unsigned long uprobe_red_zone_test(void)
+{
+	asm volatile (
+		"movabs $0x1111111111111111, %%rax\n"
+		"movq   %%rax, -8(%%rsp)\n"
+		"movabs $0x2222222222222222, %%rax\n"
+		"movq   %%rax, -16(%%rsp)\n"
+		"movabs $0x3333333333333333, %%rax\n"
+		"movq   %%rax, -24(%%rsp)\n"
+
+		".byte 0x66, 0x2e, 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00\n" /* nop10: uprobe site */
+
+		"movabs $0x1111111111111111, %%rax\n"
+		"cmpq   %%rax, -8(%%rsp)\n"
+		"jne    1f\n"
+		"movabs $0x2222222222222222, %%rax\n"
+		"cmpq   %%rax, -16(%%rsp)\n"
+		"jne    1f\n"
+		"movabs $0x3333333333333333, %%rax\n"
+		"cmpq   %%rax, -24(%%rsp)\n"
+		"jne    1f\n"
+
+		"xorl   %%eax, %%eax\n"
+		"retq\n"
+		"1:\n"
+		"movl   $1, %%eax\n"
+		"retq\n"
+		::: "rax", "memory"
+	);
+}
+
 static int find_uprobes_trampoline(void *tramp_addr)
 {
 	void *start, *end;
@@ -871,6 +915,37 @@ static void test_uprobe_race(void)
 #define __NR_uprobe 336
 #endif
 
+static void test_uprobe_red_zone(void)
+{
+	struct uprobe_syscall_executed *skel;
+	struct bpf_link *link;
+	void *nop10_addr;
+	size_t offset;
+	int i;
+
+	nop10_addr = find_nop10(uprobe_red_zone_test);
+	if (!ASSERT_NEQ(nop10_addr, NULL, "find_nop10"))
+		return;
+
+	skel = uprobe_syscall_executed__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+
+	offset = get_uprobe_offset(nop10_addr);
+	link = bpf_program__attach_uprobe_opts(skel->progs.test_uprobe,
+			0, "/proc/self/exe", offset, NULL);
+	if (!ASSERT_OK_PTR(link, "attach_uprobe"))
+		goto cleanup;
+
+	for (i = 0; i < 10; i++)
+		ASSERT_EQ(uprobe_red_zone_test(), 0, "red_zone_intact");
+
+	bpf_link__destroy(link);
+
+cleanup:
+	uprobe_syscall_executed__destroy(skel);
+}
+
 static void test_uprobe_error(void)
 {
 	long err = syscall(__NR_uprobe);
@@ -897,6 +972,8 @@ static void __test_uprobe_syscall(void)
 		test_uprobe_usdt();
 	if (test__start_subtest("uprobe_race"))
 		test_uprobe_race();
+	if (test__start_subtest("uprobe_red_zone"))
+		test_uprobe_red_zone();
 	if (test__start_subtest("uprobe_error"))
 		test_uprobe_error();
 	if (test__start_subtest("uprobe_regs_equal"))

tools/testing/selftests/bpf/prog_tests/usdt.c

@@ -250,6 +250,7 @@ static void subtest_basic_usdt(bool optimized)
 #ifdef __x86_64__
 extern void usdt_1(void);
 extern void usdt_2(void);
+extern void usdt_red_zone_trigger(void);
 
 static unsigned char nop1[1] = { 0x90 };
 static unsigned char nop1_nop10_combo[11] = { 0x90, 0x66, 0x2e, 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00 };
@@ -340,6 +341,52 @@ static void subtest_optimized_attach(void)
 cleanup:
 	test_usdt__destroy(skel);
 }
+
+/*
+ * Test that USDT arguments survive nop10 optimization in a function where
+ * the compiler places operands in the red zone.
+ *
+ * Signal handlers are prone to having the compiler place USDT argument
+ * operands in the red zone (below rsp).
+ *
+ * The nop5 optimization used CALL (which pushes a return address to
+ * [rsp-8]), the value at -8(%rsp) was overwritten. The nop10 optimization
+ * should escape that by moving stackpointer below the redzone before
+ * doing the CALL.
+ */
+static void subtest_optimized_red_zone(void)
+{
+	struct test_usdt *skel;
+	int i;
+
+	skel = test_usdt__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+
+	skel->bss->expected_arg[0] = 0xDEADBEEF;
+	skel->bss->expected_arg[1] = 0xCAFEBABE;
+	skel->bss->expected_arg[2] = 0xFEEDFACE;
+	skel->bss->expected_pid = getpid();
+
+	skel->links.usdt_check_arg = bpf_program__attach_usdt(
+		skel->progs.usdt_check_arg, 0, "/proc/self/exe",
+		"optimized_attach", "usdt_red_zone", NULL);
+	if (!ASSERT_OK_PTR(skel->links.usdt_check_arg, "attach_usdt_red_zone"))
+		goto cleanup;
+
+	for (i = 0; i < 10; i++)
+		usdt_red_zone_trigger();
+
+	ASSERT_EQ(skel->bss->arg_total, 10, "arg_total");
+	ASSERT_EQ(skel->bss->arg_bad, 0, "arg_bad");
+	ASSERT_EQ(skel->bss->arg_last[0], 0xDEADBEEF, "arg_last_1");
+	ASSERT_EQ(skel->bss->arg_last[1], 0xCAFEBABE, "arg_last_2");
+	ASSERT_EQ(skel->bss->arg_last[2], 0xFEEDFACE, "arg_last_3");
+
+cleanup:
+	test_usdt__destroy(skel);
+}
+
 #endif
 
 unsigned short test_usdt_100_semaphore SEC(".probes");
@@ -613,6 +660,8 @@ void test_usdt(void)
 		subtest_basic_usdt(true);
 	if (test__start_subtest("optimized_attach"))
 		subtest_optimized_attach();
+	if (test__start_subtest("optimized_red_zone"))
+		subtest_optimized_red_zone();
 #endif
 	if (test__start_subtest("multispec"))
 		subtest_multispec_usdt();

tools/testing/selftests/bpf/progs/test_usdt.c

@@ -149,5 +149,30 @@ int usdt_executed(struct pt_regs *ctx)
 		executed++;
 	return 0;
 }
+
+int arg_total;
+int arg_bad;
+long arg_last[3];
+long expected_arg[3];
+int expected_pid;
+
+SEC("usdt")
+int BPF_USDT(usdt_check_arg, long arg1, long arg2, long arg3)
+{
+	if (expected_pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	__sync_fetch_and_add(&arg_total, 1);
+	arg_last[0] = arg1;
+	arg_last[1] = arg2;
+	arg_last[2] = arg3;
+
+	if (arg1 != expected_arg[0] ||
+	    arg2 != expected_arg[1] ||
+	    arg3 != expected_arg[2])
+		__sync_fetch_and_add(&arg_bad, 1);
+
+	return 0;
+}
 #endif
 char _license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/usdt_2.c

@@ -13,4 +13,17 @@ void usdt_2(void)
 	USDT(optimized_attach, usdt_2);
 }
 
+static volatile unsigned long usdt_red_zone_arg1 = 0xDEADBEEF;
+static volatile unsigned long usdt_red_zone_arg2 = 0xCAFEBABE;
+static volatile unsigned long usdt_red_zone_arg3 = 0xFEEDFACE;
+
+void __attribute__((noinline)) usdt_red_zone_trigger(void)
+{
+	unsigned long a1 = usdt_red_zone_arg1;
+	unsigned long a2 = usdt_red_zone_arg2;
+	unsigned long a3 = usdt_red_zone_arg3;
+
+	USDT(optimized_attach, usdt_red_zone, a1, a2, a3);
+}
+
 #endif