bpf: sk_msg program helper bpf_msg_push_data

This allows user to push data into a msg using sk_msg program types.
The format is as follows,

	bpf_msg_push_data(msg, offset, len, flags)

this will insert 'len' bytes at offset 'offset'. For example to
prepend 10 bytes at the front of the message the user can,

	bpf_msg_push_data(msg, 0, 10, 0);

This will invalidate data bounds so BPF user will have to then recheck
data bounds after calling this. After this the msg size will have been
updated and the user is free to write into the added bytes. We allow
any offset/len as long as it is within the (data, data_end) range.
However, a copy will be required if the ring is full and its possible
for the helper to fail with ENOMEM or EINVAL errors which need to be
handled by the BPF program.

This can be used similar to XDP metadata to pass data between sk_msg
layer and lower layers.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
index 84e18863f6a4..2a11e9d91dfa 100644
--- a/include/linux/skmsg.h
+++ b/include/linux/skmsg.h
@@ -207,6 +207,11 @@ static inline struct scatterlist *sk_msg_elem(struct sk_msg *msg, int which)
 	return &msg->sg.data[which];
 }
 
+static inline struct scatterlist sk_msg_elem_cpy(struct sk_msg *msg, int which)
+{
+	return msg->sg.data[which];
+}
+
 static inline struct page *sk_msg_page(struct sk_msg *msg, int which)
 {
 	return sg_page(sk_msg_elem(msg, which));