selftests/bpf: Add test for prealloc_lru_pop bug

Add a regression test to check against invalid check_and_init_map_value
call inside prealloc_lru_pop.

The kptr should not be reset to NULL once we set it after deleting the
map element. Hence, we trigger a program that updates the element
causing its reuse, and checks whether the unref kptr is reset or not.
If it is, prealloc_lru_pop does an incorrect check_and_init_map_value
call and the test fails.
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220809213033.24147-4-memxor@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+
+#include "lru_bug.skel.h"
+
+void test_lru_bug(void)
+{
+	struct lru_bug *skel;
+	int ret;
+
+	skel = lru_bug__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "lru_bug__open_and_load"))
+		return;
+	ret = lru_bug__attach(skel);
+	if (!ASSERT_OK(ret, "lru_bug__attach"))
+		goto end;
+	usleep(1);
+	ASSERT_OK(skel->data->result, "prealloc_lru_pop doesn't call check_and_init_map_value");
+end:
+	lru_bug__destroy(skel);
+}

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

+// SPDX-License-Identifier: GPL-2.0
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_helpers.h>
+
+struct map_value {
+	struct task_struct __kptr *ptr;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_LRU_HASH);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, struct map_value);
+} lru_map SEC(".maps");
+
+int pid = 0;
+int result = 1;
+
+SEC("fentry/bpf_ktime_get_ns")
+int printk(void *ctx)
+{
+	struct map_value v = {};
+
+	if (pid == bpf_get_current_task_btf()->pid)
+		bpf_map_update_elem(&lru_map, &(int){0}, &v, 0);
+	return 0;
+}
+
+SEC("fentry/do_nanosleep")
+int nanosleep(void *ctx)
+{
+	struct map_value val = {}, *v;
+	struct task_struct *current;
+
+	bpf_map_update_elem(&lru_map, &(int){0}, &val, 0);
+	v = bpf_map_lookup_elem(&lru_map, &(int){0});
+	if (!v)
+		return 0;
+	bpf_map_delete_elem(&lru_map, &(int){0});
+	current = bpf_get_current_task_btf();
+	v->ptr = current;
+	pid = current->pid;
+	bpf_ktime_get_ns();
+	result = !v->ptr;
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";