From: John Fastabend <john.fastabend@gmail.com>
Date: Mon, 29 Oct 2018 12:31:28 -0700
Subject: bpf: tcp_bpf_recvmsg should return EAGAIN when nonblocking and no
 data
Patch-mainline: v4.20-rc1
Git-commit: 27b31e68bc9fc25c519c7772fa23913687218d5f
References: bsc#1109837

We return 0 in the case of a nonblocking socket that has no data
available. However, this is incorrect and may confuse applications.
After this patch we do the correct thing and return the error
EAGAIN.

Quoting return codes from recvmsg manpage,

EAGAIN or EWOULDBLOCK
 The socket is marked nonblocking and the receive operation would
 block, or a receive timeout had been set and the timeout expired
 before data was received.

Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 net/ipv4/tcp_bpf.c |    1 +
 1 file changed, 1 insertion(+)

--- a/net/ipv4/tcp_bpf.c
+++ b/net/ipv4/tcp_bpf.c
@@ -145,6 +145,7 @@ msg_bytes_ready:
 			ret = err;
 			goto out;
 		}
+		copied = -EAGAIN;
 	}
 	ret = copied;
 out: