As the author said in the comment of util-linux/switch_root.c, current
implementation leaves the mount point of new root-file-system without rmdir().
As long as I experimented on a linux kernel, current process of "/" still
points old root-file-system even "/" is overmounted. So we can still access and
zap ititramfs after the directory is free from mount point.
The patch below should zap the last directory left in the initramfs.
It only swaps the timings of overmount and zapping.
diff -Naur busybox-1.31.0.org/util-linux/switch_root.c
busybox-1.31.0/util-linux/switch_root.c
--- busybox-1.31.0.org/util-linux/switch_root.c 2019-07-18 23:18:54.791346155
+0900
+++ busybox-1.31.0/util-linux/switch_root.c 2019-07-18 23:21:33.867785730
+0900
@@ -257,14 +257,14 @@
}
if (!dry_run) {
- // Zap everything out of rootdev
- delete_contents("/", rootdev);
-
// Overmount / with newdir and chroot into it
if (mount(".", "/", NULL, MS_MOVE, NULL)) {
// For example, fails when newroot is not a mountpoint
bb_perror_msg_and_die("error moving root");
}
+
+ // Zap everything out of rootdev
+ delete_contents("/", rootdev);
}
xchroot(".");
// The chdir is needed to recalculate "." and ".." links
diff -Naur busybox-1.31.0.org/util-linux/switch_root.c busybox-1.31.0/util-linux/switch_root.c
--- busybox-1.31.0.org/util-linux/switch_root.c 2019-07-18 23:18:54.791346155 +0900
+++ busybox-1.31.0/util-linux/switch_root.c 2019-07-18 23:21:33.867785730 +0900
@@ -257,14 +257,14 @@
}
if (!dry_run) {
- // Zap everything out of rootdev
- delete_contents("/", rootdev);
-
// Overmount / with newdir and chroot into it
if (mount(".", "/", NULL, MS_MOVE, NULL)) {
// For example, fails when newroot is not a mountpoint
bb_perror_msg_and_die("error moving root");
}
+
+ // Zap everything out of rootdev
+ delete_contents("/", rootdev);
}
xchroot(".");
// The chdir is needed to recalculate "." and ".." links
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