sockets: allow SocketAddress 'fd' to reference numeric file descriptors

The SocketAddress 'fd' kind accepts the name of a file descriptor passed
to the monitor with the 'getfd' command. This makes it impossible to use
the 'fd' kind in cases where a monitor is not available. This can apply in
handling command line argv at startup, or simply if internal code wants to
use SocketAddress and pass a numeric FD it has acquired from elsewhere.

Fortunately the 'getfd' command mandated that the FD names must not start
with a leading digit. We can thus safely extend semantics of the
SocketAddress 'fd' kind, to allow a purely numeric name to reference an
file descriptor that QEMU already has open. There will be restrictions on
when each kind can be used.

In codepaths where we are handling a monitor command (ie cur_mon != NULL),
we will only support use of named file descriptors as before. Use of FD
numbers is still not permitted for monitor commands.

In codepaths where we are not handling a monitor command (ie cur_mon ==
NULL), we will not support named file descriptors. Instead we can reference
FD numers explicitly. This allows the app spawning QEMU to intentionally
"leak" a pre-opened socket to QEMU and reference that in a SocketAddress
definition, or for code inside QEMU to pass pre-opened FDs around.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

1 files changed, 13 insertions, 3 deletions

diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
index 6dc4613855..8bd8bb64eb 100644
--- a/util/qemu-sockets.c
+++ b/util/qemu-sockets.c
@@ -1044,9 +1044,19 @@ fail:
 
 static int socket_get_fd(const char *fdstr, Error **errp)
 {
-    int fd = monitor_get_fd(cur_mon, fdstr, errp);
-    if (fd < 0) {
-        return -1;
+    int fd;
+    if (cur_mon) {
+        fd = monitor_get_fd(cur_mon, fdstr, errp);
+        if (fd < 0) {
+            return -1;
+        }
+    } else {
+        if (qemu_strtoi(fdstr, NULL, 10, &fd) < 0) {
+            error_setg_errno(errp, errno,
+                             "Unable to parse FD number %s",
+                             fdstr);
+            return -1;
+        }
     }
     if (!fd_is_socket(fd)) {
         error_setg(errp, "File descriptor '%s' is not a socket", fdstr);