selftests/bpf: Add test for bpf_list_{front,back}

This patch adds the "list_peek" test to use the new
bpf_list_{front,back} kfunc.

The test_{front,back}* tests ensure that the return value
is a non_own_ref node pointer and requires the spinlock to be held.

Suggested-by: Kumar Kartikeya Dwivedi <memxor@gmail.com> # check non_own_ref marking
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20250506015857.817950-9-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -7,6 +7,7 @@
 
 #include "linked_list.skel.h"
 #include "linked_list_fail.skel.h"
+#include "linked_list_peek.skel.h"
 
 static char log_buf[1024 * 1024];
 
@@ -805,3 +806,8 @@ void test_linked_list(void)
 	test_linked_list_success(LIST_IN_LIST, true);
 	test_linked_list_success(TEST_ALL, false);
 }
+
+void test_linked_list_peek(void)
+{
+	RUN_TESTS(linked_list_peek);
+}

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

@@ -0,0 +1,113 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+#include "bpf_experimental.h"
+
+struct node_data {
+	struct bpf_list_node l;
+	int key;
+};
+
+#define private(name) SEC(".data." #name) __hidden __attribute__((aligned(8)))
+private(A) struct bpf_spin_lock glock;
+private(A) struct bpf_list_head ghead __contains(node_data, l);
+
+#define list_entry(ptr, type, member) container_of(ptr, type, member)
+#define NR_NODES 16
+
+int zero = 0;
+
+SEC("syscall")
+__retval(0)
+long list_peek(void *ctx)
+{
+	struct bpf_list_node *l_n;
+	struct node_data *n;
+	int i, err = 0;
+
+	bpf_spin_lock(&glock);
+	l_n = bpf_list_front(&ghead);
+	bpf_spin_unlock(&glock);
+	if (l_n)
+		return __LINE__;
+
+	bpf_spin_lock(&glock);
+	l_n = bpf_list_back(&ghead);
+	bpf_spin_unlock(&glock);
+	if (l_n)
+		return __LINE__;
+
+	for (i = zero; i < NR_NODES && can_loop; i++) {
+		n = bpf_obj_new(typeof(*n));
+		if (!n)
+			return __LINE__;
+		n->key = i;
+		bpf_spin_lock(&glock);
+		bpf_list_push_back(&ghead, &n->l);
+		bpf_spin_unlock(&glock);
+	}
+
+	bpf_spin_lock(&glock);
+
+	l_n = bpf_list_front(&ghead);
+	if (!l_n) {
+		err = __LINE__;
+		goto done;
+	}
+
+	n = list_entry(l_n, struct node_data, l);
+	if (n->key != 0) {
+		err = __LINE__;
+		goto done;
+	}
+
+	l_n = bpf_list_back(&ghead);
+	if (!l_n) {
+		err = __LINE__;
+		goto done;
+	}
+
+	n = list_entry(l_n, struct node_data, l);
+	if (n->key != NR_NODES - 1) {
+		err = __LINE__;
+		goto done;
+	}
+
+done:
+	bpf_spin_unlock(&glock);
+	return err;
+}
+
+#define TEST_FB(op, dolock)					\
+SEC("syscall")							\
+__failure __msg(MSG)						\
+long test_##op##_spinlock_##dolock(void *ctx)			\
+{								\
+	struct bpf_list_node *l_n;				\
+	__u64 jiffies = 0;					\
+								\
+	if (dolock)						\
+		bpf_spin_lock(&glock);				\
+	l_n = bpf_list_##op(&ghead);				\
+	if (l_n)						\
+		jiffies = bpf_jiffies64();			\
+	if (dolock)						\
+		bpf_spin_unlock(&glock);			\
+								\
+	return !!jiffies;					\
+}
+
+#define MSG "call bpf_list_{{(front|back).+}}; R0{{(_w)?}}=ptr_or_null_node_data(id={{[0-9]+}},non_own_ref"
+TEST_FB(front, true)
+TEST_FB(back, true)
+#undef MSG
+
+#define MSG "bpf_spin_lock at off=0 must be held for bpf_list_head"
+TEST_FB(front, false)
+TEST_FB(back, false)
+#undef MSG
+
+char _license[] SEC("license") = "GPL";