[PATCH bpf-next v6 2/2] selftests/bpf: Add deep call stack selftests

[Emil Tsalapatis <emil@etsalapatis.com>]

Add tests that demonstrate the verifier support for deep call stacks
while still enforcing maximum stack size limits.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
---
 .../bpf/prog_tests/test_global_funcs.c        |  2 +
 .../bpf/progs/test_global_func_deep_stack.c   | 95 +++++++++++++++++++
 2 files changed, 97 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/test_global_func_deep_stack.c

diff --git a/tools/testing/selftests/bpf/prog_tests/test_global_funcs.c b/tools/testing/selftests/bpf/prog_tests/test_global_funcs.c
index e905cbaf6b3d..500446808908 100644
--- a/tools/testing/selftests/bpf/prog_tests/test_global_funcs.c
+++ b/tools/testing/selftests/bpf/prog_tests/test_global_funcs.c
@@ -18,6 +18,7 @@
 #include "test_global_func15.skel.h"
 #include "test_global_func16.skel.h"
 #include "test_global_func17.skel.h"
+#include "test_global_func_deep_stack.skel.h"
 #include "test_global_func_ctx_args.skel.h"
 
 #include "bpf/libbpf_internal.h"
@@ -155,6 +156,7 @@ void test_test_global_funcs(void)
 	RUN_TESTS(test_global_func15);
 	RUN_TESTS(test_global_func16);
 	RUN_TESTS(test_global_func17);
+	RUN_TESTS(test_global_func_deep_stack);
 	RUN_TESTS(test_global_func_ctx_args);
 
 	if (test__start_subtest("ctx_arg_rewrite"))
diff --git a/tools/testing/selftests/bpf/progs/test_global_func_deep_stack.c b/tools/testing/selftests/bpf/progs/test_global_func_deep_stack.c
new file mode 100644
index 000000000000..1b634b543b62
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_global_func_deep_stack.c
@@ -0,0 +1,95 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2026 Meta Platforms, Inc and affiliates. */
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+/*
+ * Macro tricks to tersely define for long non-recursive call chains. Add
+ * computation to the functions prevent tail recursion from reducing the
+ * stack size to 0.
+ */
+
+#define CAT(a, b) a ## b
+#define XCAT(a, b) CAT(a, b)
+
+#define F_0 \
+__attribute__((noinline))         \
+int f0(unsigned long a)           \
+{                                 \
+	volatile long b = a + 16; \
+	if (a == 0)               \
+		return 0;         \
+	return b;                 \
+}
+
+#define FN(n, prev) \
+__attribute__((noinline))                        \
+int XCAT(f, n)(unsigned long a)                  \
+{                                                \
+	volatile long b = XCAT(f, prev)(a - 1);  \
+	if (!b)                                  \
+		return 0;                        \
+	return b + 1;                            \
+}
+
+/* Call chain 33 levels deep. */
+#define F_1 F_0         FN(1, 0)
+#define F_2 F_1         FN(2, 1)
+#define F_3 F_2         FN(3, 2)
+#define F_4 F_3         FN(4, 3)
+#define F_5 F_4         FN(5, 4)
+#define F_6 F_5         FN(6, 5)
+#define F_7 F_6         FN(7, 6)
+#define F_8 F_7         FN(8, 7)
+#define F_9 F_8         FN(9, 8)
+#define F_10 F_9        FN(10, 9)
+#define F_11 F_10       FN(11, 10)
+#define F_12 F_11       FN(12, 11)
+#define F_13 F_12       FN(13, 12)
+#define F_14 F_13       FN(14, 13)
+#define F_15 F_14       FN(15, 14)
+#define F_16 F_15       FN(16, 15)
+#define F_17 F_16       FN(17, 16)
+#define F_18 F_17       FN(18, 17)
+#define F_19 F_18       FN(19, 18)
+#define F_20 F_19       FN(20, 19)
+#define F_21 F_20       FN(21, 20)
+#define F_22 F_21       FN(22, 21)
+#define F_23 F_22       FN(23, 22)
+#define F_24 F_23       FN(24, 23)
+#define F_25 F_24       FN(25, 24)
+#define F_26 F_25       FN(26, 25)
+#define F_27 F_26       FN(27, 26)
+#define F_28 F_27       FN(28, 27)
+#define F_29 F_28       FN(29, 28)
+#define F_30 F_29       FN(30, 29)
+#define F_31 F_30       FN(31, 30)
+#define F_32 F_31       FN(32, 31)
+
+#define CAT2(a, b) a ## b
+#define XCAT2(a, b) CAT2(a, b)
+
+#define F(n) XCAT2(F_, n)
+
+F(32)
+
+/* Ensure that even 32 levels deep, the function verifies. */
+SEC("syscall")
+__success
+int global_func_deep_stack_success(struct __sk_buff *skb)
+{
+	return f31(55);
+}
+
+/*
+ * Check we actually honor stack limits (33 * 16 = 528 > 512 = MAX_STACK_DEPTH).
+ * The stack depth is 16 because the verifier calls round_up_stack_depth() on
+ * the size.
+ */
+SEC("syscall")
+__failure __msg("combined stack size of 34 calls")
+int global_func_deep_stack_fail(struct __sk_buff *skb)
+{
+	return f32(123);
+}
-- 
2.49.0