Re: Add a SOCK_DESTROY operation to close sockets from userspace

[Tom Herbert <tom@herbertland.com>]

On Tue, Nov 17, 2015 at 5:43 PM, Lorenzo Colitti <lorenzo@google.com> wrote:
> This patch series adds the ability for a privileged process to
> destroy sockets belonging to other userspace processes via the
> sock_diag interface, and implements that for TCP sockets.
>
> This functionality 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 gone. Closing the
> sockets in the protocol layer causes these calls to fail fast and
> allows applications to reconnect on another network.
>
> For many years Android kernels have done this via an out-of-tree
> SIOCKILLADDR ioctl that is called when networks disconnect, but
> this solution is cleaner, more robust and more flexible. The
> system can iterate over all connections on the deleted IP address
> and close all of them. But it can also close all sockets opened
> by a given process on a given network, for example if the user
> has restricted that process from using that network, or if a
> secure network such as a VPN is now being applied to the
> application and thus previously-established connections are
> blackholed.
>
> The patch series only implements SOCK_DESTROY for TCP sockets,
> but the mechanism can be extended to any protocol family that
> supports the sock_diag interface.
>
I assume that SIOCKILLADDR was restricted to only closing connections
related to add addresses going away, but SOCK_DESTROY seems to allow
arbitrarily killing connections without publicized cause. This
interface, even though it is for a privileged user, should be no more
powerful than it needs to be. Minimally, the application should get at
least get a clear error that the local host administratively killed
the connection, ETIMEDOUT does not provide that.

Tom

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