mount: Consistently use composefs as source

The "source" field for overlayfs isn't strictly meaningful,
by default; but it's useful to identify at least the software that
created the mount.

Motivated by https://gitlab.com/redhat/centos-stream/containers/bootc/-/issues/301
where some QE testing noticed that an updated composefs using
the new mount API didn't specify the source at all, so it ends up as
`none`, but if we fall back to the legacy one it uses `overlayfs`.

Change both to be `composefs`.

Signed-off-by: Colin Walters <[email protected]>

libcomposefs/lcfs-mount.c

@@ -49,6 +49,11 @@
 #include "lcfs-utils.h"
 #include "lcfs-internal.h"
 
+// The "source" field for overlayfs isn't strictly meaningful,
+// by default; but it's useful to identify at least the software that
+// created the mount.
+#define CFS_MOUNT_SOURCE "composefs"
+
 #ifndef LOOP_CONFIGURE
 /* Snippet from util-linux/include/loopdev.h */
 /*
@@ -369,7 +374,7 @@ static errint_t lcfs_mount_ovl_legacy(struct lcfs_mount_state_s *state, char *im
 		mount_flags |= MS_SILENT;
 
 	errint_t err = 0;
-	res = mount("overlay", state->mountpoint, "overlay", mount_flags,
+	res = mount(CFS_MOUNT_SOURCE, state->mountpoint, "overlay", mount_flags,
 		    overlay_options);
 	if (res != 0) {
 		err = -errno;
@@ -404,6 +409,10 @@ static errint_t lcfs_mount_ovl(struct lcfs_mount_state_s *state, char *imagemoun
 	if (res == 0)
 		return -ENOSYS;
 
+	res = syscall_fsconfig(fd_fs, FSCONFIG_SET_STRING, "source", CFS_MOUNT_SOURCE, 0);
+	if (res < 0)
+		return -errno;
+
 	res = syscall_fsconfig(fd_fs, FSCONFIG_SET_STRING, "metacopy", "on", 0);
 	if (res < 0)
 		return -errno;