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>

util/systemd.c

@@ -0,0 +1,77 @@
+/*
+ * 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.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/systemd.h"
+#include "qemu/cutils.h"
+#include "qemu/error-report.h"
+
+#ifndef _WIN32
+unsigned int check_socket_activation(void)
+{
+    const char *s;
+    unsigned long pid;
+    unsigned long nr_fds;
+    unsigned int i;
+    int fd;
+    int err;
+
+    s = getenv("LISTEN_PID");
+    if (s == NULL) {
+        return 0;
+    }
+    err = qemu_strtoul(s, NULL, 10, &pid);
+    if (err) {
+        return 0;
+    }
+    if (pid != getpid()) {
+        return 0;
+    }
+
+    s = getenv("LISTEN_FDS");
+    if (s == NULL) {
+        return 0;
+    }
+    err = qemu_strtoul(s, NULL, 10, &nr_fds);
+    if (err) {
+        return 0;
+    }
+    assert(nr_fds <= UINT_MAX);
+
+    /* So these are not passed to any child processes we might start. */
+    unsetenv("LISTEN_FDS");
+    unsetenv("LISTEN_PID");
+
+    /* So the file descriptors don't leak into child processes. */
+    for (i = 0; i < nr_fds; ++i) {
+        fd = FIRST_SOCKET_ACTIVATION_FD + i;
+        if (fcntl(fd, F_SETFD, FD_CLOEXEC) == -1) {
+            /* If we cannot set FD_CLOEXEC then it probably means the file
+             * descriptor is invalid, so socket activation has gone wrong
+             * and we should exit.
+             */
+            error_report("Socket activation failed: "
+                         "invalid file descriptor fd = %d: %m",
+                         fd);
+            exit(EXIT_FAILURE);
+        }
+    }
+
+    return (unsigned int) nr_fds;
+}
+
+#else /* !_WIN32 */
+unsigned int check_socket_activation(void)
+{
+    return 0;
+}
+#endif