migration/multifd: Ensure we're not given a socket for file migration

When doing migration using the fd: URI, QEMU will fetch the file
descriptor passed in via the monitor at
fd_start_outgoing|incoming_migration(), which means the checks at
migration_channels_and_transport_compatible() happen too soon and we
don't know at that point whether the FD refers to a plain file or a
socket.

For this reason, we've been allowing a migration channel of type
SOCKET_ADDRESS_TYPE_FD to pass the initial verifications in scenarios
where the socket migration is not supported, such as with fd + multifd.

The commit decdc76772 ("migration/multifd: Add mapped-ram support to
fd: URI") was supposed to add a second check prior to starting
migration to make sure a socket fd is not passed instead of a file fd,
but failed to do so.

Add the missing verification and update the comment explaining this
situation which is currently incorrect.

Fixes: decdc76772 ("migration/multifd: Add mapped-ram support to fd: URI")
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20240315032040.7974-2-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/fd.c

@@ -22,6 +22,7 @@
 #include "migration.h"
 #include "monitor/monitor.h"
 #include "io/channel-file.h"
+#include "io/channel-socket.h"
 #include "io/channel-util.h"
 #include "options.h"
 #include "trace.h"
@@ -95,6 +96,13 @@ void fd_start_incoming_migration(const char *fdname, Error **errp)
     }
 
     if (migrate_multifd()) {
+        if (fd_is_socket(fd)) {
+            error_setg(errp,
+                       "Multifd migration to a socket FD is not supported");
+            object_unref(ioc);
+            return;
+        }
+
         file_create_incoming_channels(ioc, errp);
     } else {
         qio_channel_set_name(ioc, "migration-fd-incoming");

migration/file.c

@@ -15,6 +15,7 @@
 #include "file.h"
 #include "migration.h"
 #include "io/channel-file.h"
+#include "io/channel-socket.h"
 #include "io/channel-util.h"
 #include "options.h"
 #include "trace.h"
@@ -58,6 +59,12 @@ bool file_send_channel_create(gpointer opaque, Error **errp)
     int fd = fd_args_get_fd();
 
     if (fd && fd != -1) {
+        if (fd_is_socket(fd)) {
+            error_setg(errp,
+                       "Multifd migration to a socket FD is not supported");
+            goto out;
+        }
+
         ioc = qio_channel_file_new_dupfd(fd, errp);
     } else {
         ioc = qio_channel_file_new_path(outgoing_args.fname, flags, 0, errp);

migration/migration.c

@@ -166,9 +166,9 @@ static bool transport_supports_seeking(MigrationAddress *addr)
     }
 
     /*
-     * At this point, the user might not yet have passed the file
+     * At this point QEMU has not yet fetched the fd passed in by the
-     * descriptor to QEMU, so we cannot know for sure whether it
+     * user, so we cannot know for sure whether it refers to a plain
-     * refers to a plain file or a socket. Let it through anyway.
+     * file or a socket. Let it through anyway and check at fd.c.
      */
     if (addr->transport == MIGRATION_ADDRESS_TYPE_SOCKET) {
         return addr->u.socket.type == SOCKET_ADDRESS_TYPE_FD;