This patchset adds the ability to administratively close a socket without any action from the process owning the socket or the socket protocol.
It implements this by adding a new diag_destroy function pointer to struct proto. In-kernel callers can access this functionality directly by calling sk->sk_prot->diag_destroy(sk, err). It also exposes this functionality to userspace via a new SOCK_DESTROY operation in the NETLINK_SOCK_DIAG sockets. This allows a privileged userspace process, such as a connection manager or system administration tool, to close sockets belonging to other apps when the network they were established on has disconnected. It is needed on laptops and mobile hosts to ensure that network switches / disconnects do not result in applications being blocked for long periods of time (minutes) in read or connect calls on TCP sockets that will never succeed because the IP address they are bound to is no longer on the system. Closing the sockets causes these calls to fail fast and allows the apps to reconnect on another network. Userspace intervention is necessary because in many cases the kernel does not have enough information to know that a connection is now inoperable. The kernel can know if a packet can't be routed, but in general it won't know if a TCP connection is stuck because it is now routed to a network where its source address is no longer valid [5][6]. Many other operating systems offer similar functionality: - FreeBSD has had this since 5.4 in 2005 [2]. It is available to privileged userspace and there is a tool to use it [3]. - The FreeBSD commit description states that the idea came from OpenBSD. - iOS has been administratively closing app sockets since iOS 4 - see [4], which states that a socket "might get reclaimed by the kernel" and after that will return EBADF]. For many years Android kernels have supported this via an out-of-tree SIOCKILLADDR ioctl that is called on every RTM_DELADDR event, but this solution is cleaner, more robust and more flexible: the connection manager can iterate over all connections on the deleted IP address and close all of them. It can also be used to close all sockets opened by a given app process, for example if the user has restricted that app from using the network, if a secure network such as a VPN has connected and security policy requires all of an application's connections to be routed via the VPN, etc. - For many years Android kernels have supported an out-of-tree SIOCKILLADDR ioctl that is called when a network disconnects or an RTM_DELADDR event is received. This solution is cleaner, more robust and more flexible. The connection manager can implement SIOCKILLADDR by iterating over all connections on the deleted IP address and close all of them, but it can also close all sockets opened by a given app process (for example if the user has restricted that app from), close all of a user's TCP connections if a user has connected a secure network such as a VPN and expects all of an application's connections to be routed via the VPN, etc. Alternative schemes such as TCP keepalives in combination with "iptables -j REJECT --reject-with tcp-reset", could be used to achieve similar results, but on mobile devices TCP keepalives are very expensive, and in such a scheme detecting stuck connections has to wait for a keepalive to be sent or the application to perform a write. An explicit notification from userspace is cheaper and faster in the common case where an application is blocked on read. SOCK_DESTROY is placed behind an INET_DIAG_DESTROY configuration option, which is currently off by default. The TCP implementation of diag_destroy causes a TCP ABORT as specified by RFC 793 [1]: immediately send a RST and clear local connection state. This is what happens today if an application enables SO_LINGER with a timeout of 0 and then calls close. The first versions of the patchset did not send a RST, but that is not graceful/correct TCP behaviour. tcp_abort now does a proper RFC 793 ABORT and sends a RST to the peer. This is consistent with BSD's tcpdrop, and is more correct in general, even though in many use cases tcp_abort will only be called when sending a RST is no longer possible (e.g., the network has disconnected). The original patchset also behaved like SIOCKILADDR and closed TCP sockets with ETIMEDOUT. Tom Herbert pointed out that it would be better if applications could distinguish between a timeout and an administrative close. ECONNABORTED was chosen because it is consistent with BSD. [1] http://tools.ietf.org/html/rfc793#page-50 [2] http://svnweb.freebsd.org/base?view=revision&revision=141381 [3] https://www.freebsd.org/cgi/man.cgi?query=tcpdrop&sektion=8&manpath=FreeBSD+5.4-RELEASE [4] https://developer.apple.com/library/ios/technotes/tn2277/_index.html#//apple_ref/doc/uid/DTS40010841-CH1-SUBSECTION3 [5] http://www.spinics.net/lists/netdev/msg352775.html [6] http://www.spinics.net/lists/netdev/msg352952.html Lorenzo Colitti (4): net: diag: split inet_diag_dump_one_icsk into two net: diag: Add the ability to destroy a socket. net: diag: Support SOCK_DESTROY for inet sockets. net: diag: Support destroying TCP sockets. include/linux/inet_diag.h | 9 ++++++ include/linux/sock_diag.h | 2 ++ include/net/sock.h | 1 + include/net/tcp.h | 2 ++ include/uapi/linux/sock_diag.h | 1 + net/core/sock_diag.c | 23 +++++++++++++-- net/ipv4/Kconfig | 13 +++++++++ net/ipv4/inet_diag.c | 65 +++++++++++++++++++++++++++--------------- net/ipv4/tcp.c | 32 +++++++++++++++++++++ net/ipv4/tcp_diag.c | 19 ++++++++++++ net/ipv4/tcp_ipv4.c | 1 + net/ipv6/tcp_ipv6.c | 1 + 12 files changed, 143 insertions(+), 26 deletions(-) -- 2.6.0.rc2.230.g3dd15c0 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
