qemu-ga: obey LISTEN_PID when using systemd socket activation

qemu-ga's socket activation support was not obeying the LISTEN_PID
environment variable, which avoids that a process uses a socket-activation
file descriptor meant for its parent.

Mess can for example ensue if a process forks a children before consuming
the socket-activation file descriptor and therefore setting O_CLOEXEC
on it.

Luckily, qemu-nbd also got socket activation code, and its copy does
support LISTEN_PID.  Some extra fixups are needed to ensure that the
code can be used for both, but that's what this patch does.  The
main change is to replace get_listen_fds's "consume" argument with
the FIRST_SOCKET_ACTIVATION_FD macro from the qemu-nbd code.

Cc: "Richard W.M. Jones" <rjones@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/include/qemu/systemd.h b/include/qemu/systemd.h
new file mode 100644
index 0000000..e6a877e
--- /dev/null
+++ b/include/qemu/systemd.h
@@ -0,0 +1,26 @@
+/*
+ * systemd socket activation support
+ *
+ * Copyright 2017 Red Hat, Inc. and/or its affiliates
+ *
+ * Authors:
+ *  Richard W.M. Jones <rjones@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#ifndef QEMU_SYSTEMD_H
+#define QEMU_SYSTEMD_H 1
+
+#define FIRST_SOCKET_ACTIVATION_FD 3 /* defined by systemd ABI */
+
+/*
+ * Check if socket activation was requested via use of the
+ * LISTEN_FDS and LISTEN_PID environment variables.
+ *
+ * Returns 0 if no socket activation, or the number of FDs.
+ */
+unsigned int check_socket_activation(void);
+
+#endif