bpf: Add a bpf_sock_from_file helper

While eBPF programs can check whether a file is a socket by file->f_op
== &socket_file_ops, they cannot convert the void private_data pointer
to a struct socket BTF pointer. In order to do this a new helper
wrapping sock_from_file is added.

This is useful to tracing programs but also other program types
inheriting this set of helpers such as iterators or LSM programs.

Signed-off-by: Florent Revest <revest@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: KP Singh <kpsingh@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20201204113609.1850150-2-revest@google.com
diff --git a/scripts/bpf_helpers_doc.py b/scripts/bpf_helpers_doc.py
index 8b829748..867ada2 100755
--- a/scripts/bpf_helpers_doc.py
+++ b/scripts/bpf_helpers_doc.py
@@ -437,6 +437,8 @@
             'struct path',
             'struct btf_ptr',
             'struct inode',
+            'struct socket',
+            'struct file',
     ]
     known_types = {
             '...',
@@ -482,6 +484,8 @@
             'struct path',
             'struct btf_ptr',
             'struct inode',
+            'struct socket',
+            'struct file',
     }
     mapped_types = {
             'u8': '__u8',