bpf: Create file bpf iterator

To produce a file bpf iterator, the fd must be
corresponding to a link_fd assocciated with a
trace/iter program. When the pinned file is
opened, a seq_file will be generated.

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/20200509175906.2475893-1-yhs@fb.com

kernel/bpf/bpf_iter.c

@@ -39,6 +39,8 @@ static DEFINE_MUTEX(link_mutex);
 /* incremented on every opened seq_file */
 static atomic64_t session_id;
 
+static int prepare_seq_file(struct file *file, struct bpf_iter_link *link);
+
 /* bpf_seq_read, a customized and simpler version for bpf iterator.
  * no_llseek is assumed for this file.
  * The following are differences from seq_read():
@@ -162,6 +164,13 @@ done:
 	return copied;
 }
 
+static int iter_open(struct inode *inode, struct file *file)
+{
+	struct bpf_iter_link *link = inode->i_private;
+
+	return prepare_seq_file(file, link);
+}
+
 static int iter_release(struct inode *inode, struct file *file)
 {
 	struct bpf_iter_priv_data *iter_priv;
@@ -183,7 +192,8 @@ static int iter_release(struct inode *inode, struct file *file)
 	return seq_release_private(inode, file);
 }
 
-static const struct file_operations bpf_iter_fops = {
+const struct file_operations bpf_iter_fops = {
+	.open		= iter_open,
 	.llseek		= no_llseek,
 	.read		= bpf_seq_read,
 	.release	= iter_release,
@@ -310,6 +320,11 @@ static const struct bpf_link_ops bpf_iter_link_lops = {
 	.update_prog = bpf_iter_link_replace,
 };
 
+bool bpf_link_is_iter(struct bpf_link *link)
+{
+	return link->ops == &bpf_iter_link_lops;
+}
+
 int bpf_iter_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 {
 	struct bpf_link_primer link_primer;