packet: compat support for sock_fprog

Socket option PACKET_FANOUT_DATA takes a struct sock_fprog as argument
if PACKET_FANOUT has mode PACKET_FANOUT_CBPF. This structure contains
a pointer into user memory. If userland is 32-bit and kernel is 64-bit
the two disagree about the layout of struct sock_fprog.

Add compat setsockopt support to convert a 32-bit compat_sock_fprog to
a 64-bit sock_fprog. This is analogous to compat_sock_fprog support for
SO_REUSEPORT added in commit 1957598840f4 ("soreuseport: add compat
case for setsockopt SO_ATTACH_REUSEPORT_CBPF").

Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/compat.h b/include/net/compat.h
index 48103cf..13de0cc 100644
--- a/include/net/compat.h
+++ b/include/net/compat.h
@@ -42,6 +42,7 @@
 
 int get_compat_msghdr(struct msghdr *, struct compat_msghdr __user *,
 		      struct sockaddr __user **, struct iovec **);
+struct sock_fprog __user *get_compat_bpf_fprog(char __user *optval);
 asmlinkage long compat_sys_sendmsg(int, struct compat_msghdr __user *,
 				   unsigned int);
 asmlinkage long compat_sys_sendmmsg(int, struct compat_mmsghdr __user *,