selftests/bpf: Add more ring buffer test coverage

Add test coverage for reservations beyond the ring buffer size in order
to validate that bpf_ringbuf_reserve() rejects the request with NULL, all
other ring buffer tests keep passing as well:

  # ./vmtest.sh -- ./test_progs -t ringbuf
  [...]
  ./test_progs -t ringbuf
  [    1.165434] bpf_testmod: loading out-of-tree module taints kernel.
  [    1.165825] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
  [    1.284001] tsc: Refined TSC clocksource calibration: 3407.982 MHz
  [    1.286871] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x311fc34e357, max_idle_ns: 440795379773 ns
  [    1.289555] clocksource: Switched to clocksource tsc
  torvalds#274/1   ringbuf/ringbuf:OK
  torvalds#274/2   ringbuf/ringbuf_n:OK
  torvalds#274/3   ringbuf/ringbuf_map_key:OK
  torvalds#274/4   ringbuf/ringbuf_write:OK
  torvalds#274     ringbuf:OK
  torvalds#275     ringbuf_multi:OK
  [...]

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240621140828.18238-2-daniel@iogearbox.net

tools/testing/selftests/bpf/Makefile

@@ -457,7 +457,7 @@ LINKED_SKELS := test_static_linked.skel.h linked_funcs.skel.h		\
 LSKELS := fentry_test.c fexit_test.c fexit_sleep.c atomics.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_n.c test_ringbuf_map_key.c test_ringbuf_write.c
 
 # Generate both light skeleton and libbpf skeleton for these
 LSKELS_EXTRA := test_ksyms_module.c test_ksyms_weak.c kfunc_call_test.c \

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

@@ -12,9 +12,11 @@
 #include <sys/sysinfo.h>
 #include <linux/perf_event.h>
 #include <linux/ring_buffer.h>
+
 #include "test_ringbuf.lskel.h"
 #include "test_ringbuf_n.lskel.h"
 #include "test_ringbuf_map_key.lskel.h"
+#include "test_ringbuf_write.lskel.h"
 
 #define EDONE 7777
 
@@ -84,6 +86,49 @@ static void *poll_thread(void *input)
 	return (void *)(long)ring_buffer__poll(ringbuf, timeout);
 }
 
+static void ringbuf_write_subtest(void)
+{
+	struct test_ringbuf_write_lskel *skel;
+	int page_size = getpagesize();
+	size_t *mmap_ptr;
+	int err, rb_fd;
+
+	skel = test_ringbuf_write_lskel__open();
+	if (!ASSERT_OK_PTR(skel, "skel_load"))
+		return;
+
+	skel->maps.ringbuf.max_entries = 0x4000;
+
+	err = test_ringbuf_write_lskel__load(skel);
+	if (!ASSERT_OK(err, "ringbuf_write"))
+		goto cleanup;
+
+	rb_fd = skel->maps.ringbuf.map_fd;
+
+	mmap_ptr = mmap(NULL, page_size, PROT_READ | PROT_WRITE, MAP_SHARED, rb_fd, 0);
+	if (!ASSERT_OK_PTR(mmap_ptr, "rw_cons_pos"))
+		goto cleanup;
+	*mmap_ptr = 0x3000;
+	ASSERT_OK(munmap(mmap_ptr, page_size), "unmap_rw");
+
+	skel->bss->pid = getpid();
+
+	ringbuf = ring_buffer__new(rb_fd, process_sample, NULL, NULL);
+	if (!ASSERT_OK_PTR(ringbuf, "ringbuf_create"))
+		goto cleanup;
+
+	err = test_ringbuf_write_lskel__attach(skel);
+	if (!ASSERT_OK(err, "ringbuf_write"))
+		goto cleanup;
+
+	trigger_samples();
+	ASSERT_GE(skel->bss->discarded, 1, "discarded");
+	ASSERT_EQ(skel->bss->passed, 0, "passed");
+cleanup:
+	ring_buffer__free(ringbuf);
+	test_ringbuf_write_lskel__destroy(skel);
+}
+
 static void ringbuf_subtest(void)
 {
 	const size_t rec_sz = BPF_RINGBUF_HDR_SZ + sizeof(struct sample);
@@ -451,4 +496,6 @@ void test_ringbuf(void)
 		ringbuf_n_subtest();
 	if (test__start_subtest("ringbuf_map_key"))
 		ringbuf_map_key_subtest();
+	if (test__start_subtest("ringbuf_write"))
+		ringbuf_write_subtest();
 }

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

@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#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);
+} ringbuf SEC(".maps");
+
+long discarded, passed;
+int pid;
+
+SEC("fentry/" SYS_PREFIX "sys_getpgid")
+int test_ringbuf_write(void *ctx)
+{
+	int *foo, cur_pid = bpf_get_current_pid_tgid() >> 32;
+	void *sample1, *sample2;
+
+	if (cur_pid != pid)
+		return 0;
+
+	sample1 = bpf_ringbuf_reserve(&ringbuf, 0x3000, 0);
+	if (!sample1)
+		return 0;
+	/* first one can pass */
+	sample2 = bpf_ringbuf_reserve(&ringbuf, 0x3000, 0);
+	if (!sample2) {
+		bpf_ringbuf_discard(sample1, 0);
+		__sync_fetch_and_add(&discarded, 1);
+		return 0;
+	}
+	/* second one must not */
+	__sync_fetch_and_add(&passed, 1);
+	foo = sample2 + 4084;
+	*foo = 256;
+	bpf_ringbuf_discard(sample1, 0);
+	bpf_ringbuf_discard(sample2, 0);
+	return 0;
+}