selftests/bpf: Clobber a lot of registers in tailcall_bpf2bpf_hierarchy tests

Clobbering a lot of registers and stack slots helps exposing tail call
counter overwrite bugs in JITs.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20250813121016.163375-5-iii@linux.ibm.com

Author: iii-i

tools/testing/selftests/bpf/progs/bpf_test_utils.h

@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __BPF_TEST_UTILS_H__
+#define __BPF_TEST_UTILS_H__
+
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+/* Clobber as many native registers and stack slots as possible. */
+static __always_inline void clobber_regs_stack(void)
+{
+	char tmp_str[] = "123456789";
+	unsigned long tmp;
+
+	bpf_strtoul(tmp_str, sizeof(tmp_str), 0, &tmp);
+	__sink(tmp);
+}
+
+#endif

tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy1.c

@@ -2,6 +2,7 @@
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
 #include "bpf_legacy.h"
+#include "bpf_test_utils.h"
 
 struct {
 	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
@@ -24,6 +25,8 @@ int entry(struct __sk_buff *skb)
 {
 	int ret = 1;
 
+	clobber_regs_stack();
+
 	count++;
 	subprog_tail(skb);
 	subprog_tail(skb);

tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c

@@ -2,6 +2,7 @@
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
 #include "bpf_misc.h"
+#include "bpf_test_utils.h"
 
 int classifier_0(struct __sk_buff *skb);
 int classifier_1(struct __sk_buff *skb);
@@ -60,6 +61,8 @@ int tailcall_bpf2bpf_hierarchy_2(struct __sk_buff *skb)
 {
 	int ret = 0;
 
+	clobber_regs_stack();
+
 	subprog_tail0(skb);
 	subprog_tail1(skb);
 

tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy3.c

@@ -2,6 +2,7 @@
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
 #include "bpf_misc.h"
+#include "bpf_test_utils.h"
 
 int classifier_0(struct __sk_buff *skb);
 
@@ -53,6 +54,8 @@ int tailcall_bpf2bpf_hierarchy_3(struct __sk_buff *skb)
 {
 	int ret = 0;
 
+	clobber_regs_stack();
+
 	bpf_tail_call_static(skb, &jmp_table0, 0);
 
 	__sink(ret);

tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy_fentry.c

@@ -4,6 +4,7 @@
 #include "vmlinux.h"
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
+#include "bpf_test_utils.h"
 
 struct {
 	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
@@ -24,6 +25,8 @@ int subprog_tail(void *ctx)
 SEC("fentry/dummy")
 int BPF_PROG(fentry, struct sk_buff *skb)
 {
+	clobber_regs_stack();
+
 	count++;
 	subprog_tail(ctx);
 	subprog_tail(ctx);