bpf/selftests: coverage for bpf_map_ops errors

These tests expose the issue of being unable to properly check for errors
returned from inlined bpf map helpers that make calls to the bpf_map_ops
functions. At best, a check for zero or non-zero can be done but these
tests show it is not possible to check for a negative value or for a
specific error value.

Signed-off-by: JP Kobryn <[e-mail]>
Tested-by: Eduard Zingerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
+
+#include <errno.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#include "test_map_ops.skel.h"
+#include "test_progs.h"
+
+static void map_update(void)
+{
+	(void)syscall(__NR_getpid);
+}
+
+static void map_delete(void)
+{
+	(void)syscall(__NR_getppid);
+}
+
+static void map_push(void)
+{
+	(void)syscall(__NR_getuid);
+}
+
+static void map_pop(void)
+{
+	(void)syscall(__NR_geteuid);
+}
+
+static void map_peek(void)
+{
+	(void)syscall(__NR_getgid);
+}
+
+static void map_for_each_pass(void)
+{
+	(void)syscall(__NR_gettid);
+}
+
+static void map_for_each_fail(void)
+{
+	(void)syscall(__NR_getpgid);
+}
+
+static int setup(struct test_map_ops **skel)
+{
+	int err = 0;
+
+	if (!skel)
+		return -1;
+
+	*skel = test_map_ops__open();
+	if (!ASSERT_OK_PTR(*skel, "test_map_ops__open"))
+		return -1;
+
+	(*skel)->rodata->pid = getpid();
+
+	err = test_map_ops__load(*skel);
+	if (!ASSERT_OK(err, "test_map_ops__load"))
+		return err;
+
+	err = test_map_ops__attach(*skel);
+	if (!ASSERT_OK(err, "test_map_ops__attach"))
+		return err;
+
+	return err;
+}
+
+static void teardown(struct test_map_ops **skel)
+{
+	if (skel && *skel)
+		test_map_ops__destroy(*skel);
+}
+
+static void map_ops_update_delete_subtest(void)
+{
+	struct test_map_ops *skel;
+
+	if (setup(&skel))
+		goto teardown;
+
+	map_update();
+	ASSERT_OK(skel->bss->err, "map_update_initial");
+
+	map_update();
+	ASSERT_LT(skel->bss->err, 0, "map_update_existing");
+	ASSERT_EQ(skel->bss->err, -EEXIST, "map_update_existing");
+
+	map_delete();
+	ASSERT_OK(skel->bss->err, "map_delete_existing");
+
+	map_delete();
+	ASSERT_LT(skel->bss->err, 0, "map_delete_non_existing");
+	ASSERT_EQ(skel->bss->err, -ENOENT, "map_delete_non_existing");
+
+teardown:
+	teardown(&skel);
+}
+
+static void map_ops_push_peek_pop_subtest(void)
+{
+	struct test_map_ops *skel;
+
+	if (setup(&skel))
+		goto teardown;
+
+	map_push();
+	ASSERT_OK(skel->bss->err, "map_push_initial");
+
+	map_push();
+	ASSERT_LT(skel->bss->err, 0, "map_push_when_full");
+	ASSERT_EQ(skel->bss->err, -E2BIG, "map_push_when_full");
+
+	map_peek();
+	ASSERT_OK(skel->bss->err, "map_peek");
+
+	map_pop();
+	ASSERT_OK(skel->bss->err, "map_pop");
+
+	map_peek();
+	ASSERT_LT(skel->bss->err, 0, "map_peek_when_empty");
+	ASSERT_EQ(skel->bss->err, -ENOENT, "map_peek_when_empty");
+
+	map_pop();
+	ASSERT_LT(skel->bss->err, 0, "map_pop_when_empty");
+	ASSERT_EQ(skel->bss->err, -ENOENT, "map_pop_when_empty");
+
+teardown:
+	teardown(&skel);
+}
+
+static void map_ops_for_each_subtest(void)
+{
+	struct test_map_ops *skel;
+
+	if (setup(&skel))
+		goto teardown;
+
+	map_for_each_pass();
+	/* expect to iterate over 1 element */
+	ASSERT_EQ(skel->bss->err, 1, "map_for_each_no_flags");
+
+	map_for_each_fail();
+	ASSERT_LT(skel->bss->err, 0, "map_for_each_with_flags");
+	ASSERT_EQ(skel->bss->err, -EINVAL, "map_for_each_with_flags");
+
+teardown:
+	teardown(&skel);
+}
+
+void test_map_ops(void)
+{
+	if (test__start_subtest("map_ops_update_delete"))
+		map_ops_update_delete_subtest();
+
+	if (test__start_subtest("map_ops_push_peek_pop"))
+		map_ops_push_peek_pop_subtest();
+
+	if (test__start_subtest("map_ops_for_each"))
+		map_ops_for_each_subtest();
+}

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

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, int);
+} hash_map SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_STACK);
+	__uint(max_entries, 1);
+	__type(value, int);
+} stack_map SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, int);
+} array_map SEC(".maps");
+
+const volatile pid_t pid;
+long err = 0;
+
+static u64 callback(u64 map, u64 key, u64 val, u64 ctx, u64 flags)
+{
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_getpid")
+int map_update(void *ctx)
+{
+	const int key = 0;
+	const int val = 1;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	err = bpf_map_update_elem(&hash_map, &key, &val, BPF_NOEXIST);
+
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_getppid")
+int map_delete(void *ctx)
+{
+	const int key = 0;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	err = bpf_map_delete_elem(&hash_map, &key);
+
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_getuid")
+int map_push(void *ctx)
+{
+	const int val = 1;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	err = bpf_map_push_elem(&stack_map, &val, 0);
+
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_geteuid")
+int map_pop(void *ctx)
+{
+	int val;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	err = bpf_map_pop_elem(&stack_map, &val);
+
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_getgid")
+int map_peek(void *ctx)
+{
+	int val;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	err = bpf_map_peek_elem(&stack_map, &val);
+
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_gettid")
+int map_for_each_pass(void *ctx)
+{
+	const int key = 0;
+	const int val = 1;
+	const u64 flags = 0;
+	int callback_ctx;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	bpf_map_update_elem(&array_map, &key, &val, flags);
+
+	err = bpf_for_each_map_elem(&array_map, callback, &callback_ctx, flags);
+
+	return 0;
+}
+
+SEC("tp/syscalls/sys_enter_getpgid")
+int map_for_each_fail(void *ctx)
+{
+	const int key = 0;
+	const int val = 1;
+	const u64 flags = BPF_NOEXIST;
+	int callback_ctx;
+
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	bpf_map_update_elem(&array_map, &key, &val, flags);
+
+	/* calling for_each with non-zero flags will return error */
+	err = bpf_for_each_map_elem(&array_map, callback, &callback_ctx, flags);
+
+	return 0;
+}