bpf: Create anonymous bpf iterator

A new bpf command BPF_ITER_CREATE is added.

The anonymous bpf iterator is seq_file based.
The seq_file private data are referenced by targets.
The bpf_iter infrastructure allocated additional space
at seq_file->private before the space used by targets
to store some meta data, e.g.,
  prog:       prog to run
  session_id: an unique id for each opened seq_file
  seq_num:    how many times bpf programs are queried in this session
  done_stop:  an internal state to decide whether bpf program
              should be called in seq_ops->stop() or not

The seq_num will start from 0 for valid objects.
The bpf program may see the same seq_num more than once if
 - seq_file buffer overflow happens and the same object
   is retried by bpf_seq_read(), or
 - the bpf program explicitly requests a retry of the
   same object

Since module is not supported for bpf_iter, all target
registeration happens at __init time, so there is no
need to change bpf_iter_unreg_target() as it is used
mostly in error path of the init function at which time
no bpf iterators have been created yet.

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/20200509175905.2475770-1-yhs@fb.com

tools/include/uapi/linux/bpf.h

@@ -116,6 +116,7 @@ enum bpf_cmd {
 	BPF_LINK_GET_FD_BY_ID,
 	BPF_LINK_GET_NEXT_ID,
 	BPF_ENABLE_STATS,
+	BPF_ITER_CREATE,
 };
 
 enum bpf_map_type {
@@ -614,6 +615,11 @@ union bpf_attr {
 		__u32		type;
 	} enable_stats;
 
+	struct { /* struct used by BPF_ITER_CREATE command */
+		__u32		link_fd;
+		__u32		flags;
+	} iter_create;
+
 } __attribute__((aligned(8)));
 
 /* The description below is an attempt at providing documentation to eBPF