sctp: Allow struct sctp_event_subscribe to grow without breaking binaries

BugLink: http://bugs.launchpad.net/bugs/990544

[ Upstream commit acdd5985364f8dc511a0762fab2e683f29d9d692 ]

getsockopt(..., SCTP_EVENTS, ...) performs a length check and returns
an error if the user provides less bytes than the size of struct
sctp_event_subscribe.

Struct sctp_event_subscribe needs to be extended by an u8 for every
new event or notification type that is added.

This obviously makes getsockopt fail for binaries that are compiled
against an older versions of <net/sctp/user.h> which do not contain
all event types.

This patch changes getsockopt behaviour to no longer return an error
if not enough bytes are being provided by the user. Instead, it
returns as much of sctp_event_subscribe as fits into the provided buffer.

This leads to the new behavior that users see what they have been aware
of at compile time.

The setsockopt(..., SCTP_EVENTS, ...) API is already behaving like this.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index fa9b5c7739a..4434853a9fe 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -4009,9 +4009,10 @@ static int sctp_getsockopt_disable_fragments(struct sock *sk, int len,
 static int sctp_getsockopt_events(struct sock *sk, int len, char __user *optval,
                                   int __user *optlen)
 {
-        if (len < sizeof(struct sctp_event_subscribe))
+        if (len <= 0)
                 return -EINVAL;
-        len = sizeof(struct sctp_event_subscribe);
+        if (len > sizeof(struct sctp_event_subscribe))
+                len = sizeof(struct sctp_event_subscribe);
         if (put_user(len, optlen))
                 return -EFAULT;
         if (copy_to_user(optval, &sctp_sk(sk)->subscribe, len))