selftests/bpf: Add overwrite mode test for BPF ring buffer

Add overwrite mode test for BPF ring buffer. The test creates a BPF ring
buffer in overwrite mode, then repeatedly reserves and commits records
to check if the ring buffer works as expected both before and after
overwriting occurs.

Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20251018035738.4039621-3-xukuohai@huaweicloud.com

tools/testing/selftests/bpf/Makefile

@@ -499,7 +499,8 @@ LINKED_SKELS := test_static_linked.skel.h linked_funcs.skel.h		\
 
 LSKELS := fexit_sleep.c trace_printk.c trace_vprintk.c map_ptr_kern.c 	\
 	core_kern.c core_kern_overflow.c test_ringbuf.c			\
-	test_ringbuf_n.c test_ringbuf_map_key.c test_ringbuf_write.c
+	test_ringbuf_n.c test_ringbuf_map_key.c test_ringbuf_write.c    \
+	test_ringbuf_overwrite.c
 
 LSKELS_SIGNED := fentry_test.c fexit_test.c atomics.c
 

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

@@ -17,6 +17,7 @@
 #include "test_ringbuf_n.lskel.h"
 #include "test_ringbuf_map_key.lskel.h"
 #include "test_ringbuf_write.lskel.h"
+#include "test_ringbuf_overwrite.lskel.h"
 
 #define EDONE 7777
 
@@ -497,6 +498,68 @@ static void ringbuf_map_key_subtest(void)
 	test_ringbuf_map_key_lskel__destroy(skel_map_key);
 }
 
+static void ringbuf_overwrite_mode_subtest(void)
+{
+	unsigned long size, len1, len2, len3, len4, len5;
+	unsigned long expect_avail_data, expect_prod_pos, expect_over_pos;
+	struct test_ringbuf_overwrite_lskel *skel;
+	int page_size = getpagesize();
+	int err;
+
+	skel = test_ringbuf_overwrite_lskel__open();
+	if (!ASSERT_OK_PTR(skel, "skel_open"))
+		return;
+
+	size = page_size;
+	len1 = page_size / 2;
+	len2 = page_size / 4;
+	len3 = size - len1 - len2 - BPF_RINGBUF_HDR_SZ * 3;
+	len4 = len3 - 8;
+	len5 = len3; /* retry with len3 */
+
+	skel->maps.ringbuf.max_entries = size;
+	skel->rodata->LEN1 = len1;
+	skel->rodata->LEN2 = len2;
+	skel->rodata->LEN3 = len3;
+	skel->rodata->LEN4 = len4;
+	skel->rodata->LEN5 = len5;
+
+	skel->bss->pid = getpid();
+
+	err = test_ringbuf_overwrite_lskel__load(skel);
+	if (!ASSERT_OK(err, "skel_load"))
+		goto cleanup;
+
+	err = test_ringbuf_overwrite_lskel__attach(skel);
+	if (!ASSERT_OK(err, "skel_attach"))
+		goto cleanup;
+
+	syscall(__NR_getpgid);
+
+	ASSERT_EQ(skel->bss->reserve1_fail, 0, "reserve 1");
+	ASSERT_EQ(skel->bss->reserve2_fail, 0, "reserve 2");
+	ASSERT_EQ(skel->bss->reserve3_fail, 1, "reserve 3");
+	ASSERT_EQ(skel->bss->reserve4_fail, 0, "reserve 4");
+	ASSERT_EQ(skel->bss->reserve5_fail, 0, "reserve 5");
+
+	ASSERT_EQ(skel->bss->ring_size, size, "check_ring_size");
+
+	expect_avail_data = len2 + len4 + len5 + 3 * BPF_RINGBUF_HDR_SZ;
+	ASSERT_EQ(skel->bss->avail_data, expect_avail_data, "check_avail_size");
+
+	ASSERT_EQ(skel->bss->cons_pos, 0, "check_cons_pos");
+
+	expect_prod_pos = len1 + len2 + len4 + len5 + 4 * BPF_RINGBUF_HDR_SZ;
+	ASSERT_EQ(skel->bss->prod_pos, expect_prod_pos, "check_prod_pos");
+
+	expect_over_pos = len1 + BPF_RINGBUF_HDR_SZ;
+	ASSERT_EQ(skel->bss->over_pos, expect_over_pos, "check_over_pos");
+
+	test_ringbuf_overwrite_lskel__detach(skel);
+cleanup:
+	test_ringbuf_overwrite_lskel__destroy(skel);
+}
+
 void test_ringbuf(void)
 {
 	if (test__start_subtest("ringbuf"))
@@ -507,4 +570,6 @@ void test_ringbuf(void)
 		ringbuf_map_key_subtest();
 	if (test__start_subtest("ringbuf_write"))
 		ringbuf_write_subtest();
+	if (test__start_subtest("ringbuf_overwrite_mode"))
+		ringbuf_overwrite_mode_subtest();
 }

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

@@ -0,0 +1,98 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2025. Huawei Technologies Co., Ltd */
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+struct {
+	__uint(type, BPF_MAP_TYPE_RINGBUF);
+	__uint(map_flags, BPF_F_RB_OVERWRITE);
+} ringbuf SEC(".maps");
+
+int pid;
+
+const volatile unsigned long LEN1;
+const volatile unsigned long LEN2;
+const volatile unsigned long LEN3;
+const volatile unsigned long LEN4;
+const volatile unsigned long LEN5;
+
+long reserve1_fail = 0;
+long reserve2_fail = 0;
+long reserve3_fail = 0;
+long reserve4_fail = 0;
+long reserve5_fail = 0;
+
+unsigned long avail_data = 0;
+unsigned long ring_size = 0;
+unsigned long cons_pos = 0;
+unsigned long prod_pos = 0;
+unsigned long over_pos = 0;
+
+SEC("fentry/" SYS_PREFIX "sys_getpgid")
+int test_overwrite_ringbuf(void *ctx)
+{
+	char *rec1, *rec2, *rec3, *rec4, *rec5;
+	int cur_pid = bpf_get_current_pid_tgid() >> 32;
+
+	if (cur_pid != pid)
+		return 0;
+
+	rec1 = bpf_ringbuf_reserve(&ringbuf, LEN1, 0);
+	if (!rec1) {
+		reserve1_fail = 1;
+		return 0;
+	}
+
+	rec2 = bpf_ringbuf_reserve(&ringbuf, LEN2, 0);
+	if (!rec2) {
+		bpf_ringbuf_discard(rec1, 0);
+		reserve2_fail = 1;
+		return 0;
+	}
+
+	rec3 = bpf_ringbuf_reserve(&ringbuf, LEN3, 0);
+	/* expect failure */
+	if (!rec3) {
+		reserve3_fail = 1;
+	} else {
+		bpf_ringbuf_discard(rec1, 0);
+		bpf_ringbuf_discard(rec2, 0);
+		bpf_ringbuf_discard(rec3, 0);
+		return 0;
+	}
+
+	rec4 = bpf_ringbuf_reserve(&ringbuf, LEN4, 0);
+	if (!rec4) {
+		reserve4_fail = 1;
+		bpf_ringbuf_discard(rec1, 0);
+		bpf_ringbuf_discard(rec2, 0);
+		return 0;
+	}
+
+	bpf_ringbuf_submit(rec1, 0);
+	bpf_ringbuf_submit(rec2, 0);
+	bpf_ringbuf_submit(rec4, 0);
+
+	rec5 = bpf_ringbuf_reserve(&ringbuf, LEN5, 0);
+	if (!rec5) {
+		reserve5_fail = 1;
+		return 0;
+	}
+
+	for (int i = 0; i < LEN3; i++)
+		rec5[i] = 0xdd;
+
+	bpf_ringbuf_submit(rec5, 0);
+
+	ring_size = bpf_ringbuf_query(&ringbuf, BPF_RB_RING_SIZE);
+	avail_data = bpf_ringbuf_query(&ringbuf, BPF_RB_AVAIL_DATA);
+	cons_pos = bpf_ringbuf_query(&ringbuf, BPF_RB_CONS_POS);
+	prod_pos = bpf_ringbuf_query(&ringbuf, BPF_RB_PROD_POS);
+	over_pos = bpf_ringbuf_query(&ringbuf, BPF_RB_OVERWRITE_POS);
+
+	return 0;
+}