tools/bpf: selftests: Add iter progs for bpf_map/task/task_file

The implementation is arbitrary, just to show how the bpf programs
can be written for bpf_map/task/task_file. They can be costomized
for specific needs.

For example, for bpf_map, the iterator prints out:
  $ cat /sys/fs/bpf/my_bpf_map
      id   refcnt  usercnt  locked_vm
       3        2        0         20
       6        2        0         20
       9        2        0         20
      12        2        0         20
      13        2        0         20
      16        2        0         20
      19        2        0         20
      %%% END %%%

For task, the iterator prints out:
  $ cat /sys/fs/bpf/my_task
    tgid      gid
       1        1
       2        2
    ....
    1944     1944
    1948     1948
    1949     1949
    1953     1953
    === END ===

For task/file, the iterator prints out:
  $ cat /sys/fs/bpf/my_task_file
    tgid      gid       fd      file
       1        1        0 ffffffff95c97600
       1        1        1 ffffffff95c97600
       1        1        2 ffffffff95c97600
    ....
    1895     1895      255 ffffffff95c8fe00
    1932     1932        0 ffffffff95c8fe00
    1932     1932        1 ffffffff95c8fe00
    1932     1932        2 ffffffff95c8fe00
    1932     1932        3 ffffffff95c185c0

This is able to print out all open files (fd and file->f_op), so user can compare
f_op against a particular kernel file operations to find what it is.
For example, from /proc/kallsyms, we can find
  ffffffff95c185c0 r eventfd_fops
so we will know tgid 1932 fd 3 is an eventfd file descriptor.
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200509175922.2477576-1-yhs@fb.com

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+SEC("iter/bpf_map")
+int dump_bpf_map(struct bpf_iter__bpf_map *ctx)
+{
+	struct seq_file *seq = ctx->meta->seq;
+	__u64 seq_num = ctx->meta->seq_num;
+	struct bpf_map *map = ctx->map;
+
+	if (map == (void *)0) {
+		BPF_SEQ_PRINTF(seq, "      %%%%%% END %%%%%%\n");
+		return 0;
+	}
+
+	if (seq_num == 0)
+		BPF_SEQ_PRINTF(seq, "      id   refcnt  usercnt  locked_vm\n");
+
+	BPF_SEQ_PRINTF(seq, "%8u %8ld %8ld %10lu\n", map->id, map->refcnt.counter,
+		       map->usercnt.counter,
+		       map->memory.user->locked_vm.counter);
+	return 0;
+}

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+SEC("iter/task")
+int dump_task(struct bpf_iter__task *ctx)
+{
+	struct seq_file *seq = ctx->meta->seq;
+	struct task_struct *task = ctx->task;
+
+	if (task == (void *)0) {
+		BPF_SEQ_PRINTF(seq, "    === END ===\n");
+		return 0;
+	}
+
+	if (ctx->meta->seq_num == 0)
+		BPF_SEQ_PRINTF(seq, "    tgid      gid\n");
+
+	BPF_SEQ_PRINTF(seq, "%8d %8d\n", task->tgid, task->pid);
+	return 0;
+}

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+SEC("iter/task_file")
+int dump_task_file(struct bpf_iter__task_file *ctx)
+{
+	struct seq_file *seq = ctx->meta->seq;
+	struct task_struct *task = ctx->task;
+	__u32 fd = ctx->fd;
+	struct file *file = ctx->file;
+
+	if (task == (void *)0 || file == (void *)0)
+		return 0;
+
+	if (ctx->meta->seq_num == 0)
+		BPF_SEQ_PRINTF(seq, "    tgid      gid       fd      file\n");
+
+	BPF_SEQ_PRINTF(seq, "%8d %8d %8d %lx\n", task->tgid, task->pid, fd,
+		       (long)file->f_op);
+	return 0;
+}