switch-root: try pivot_root() before overmounting /

We should always try to umount the old root dir if possible, instead of
overmounting it -- if that's possible.

The initial ("first") kernel rootfs can never be umounted, hence
for the usual nitrd case we never bothered using pivot_root() and
hence with fully unmounting it. However, fedup now tranisitions twice
during boot, and in that case it is highly desirable that the "second"
root dir is entirely unmounted when we switch to the "third". This patch
makes that possible.

The pivot_root() needs a directory in the "third" root dir, to move the
"second" root dir to. We use /mnt for that, under the assumption that
this directory is likely to exist, and is not itself a mount point.

1 files changed, 26 insertions, 1 deletions

diff --git a/src/core/switch-root.c b/src/core/switch-root.c
index 150332a858..ce0e41d510 100644
--- a/src/core/switch-root.c
+++ b/src/core/switch-root.c
@@ -30,6 +30,7 @@
 #include "util.h"
 #include "path-util.h"
 #include "switch-root.h"
+#include "missing.h"
 
 int switch_root(const char *new_root) {
 
@@ -44,10 +45,21 @@ int switch_root(const char *new_root) {
         struct stat new_root_stat;
         bool old_root_remove;
         const char *i;
+        _cleanup_free_ char *temporary_old_root = NULL;
 
         if (path_equal(new_root, "/"))
                 return 0;
 
+        /* When using pivot_root() we assume that /mnt exists as place
+         * we can temporarily move the old root to. As we immediately
+         * unmount it from there it doesn't matter much which
+         * directory we choose for this, but it should be more likely
+         * than not that /mnt exists and is suitable as mount point
+         * and is on the same fs as the old root dir */
+        temporary_old_root = strappend(new_root, "/mnt");
+        if (!temporary_old_root)
+                return -ENOMEM;
+
         old_root_remove = in_initrd();
 
         if (stat(new_root, &new_root_stat) < 0) {
@@ -103,7 +115,20 @@ int switch_root(const char *new_root) {
                         log_warning("Failed to open root directory: %m");
         }
 
-        if (mount(new_root, "/", NULL, MS_MOVE, NULL) < 0) {
+        /* We first try a pivot_root() so that we can umount the old
+         * root dir. In many cases (i.e. where rootfs is /), that's
+         * not possible however, and hence we simply overmount root */
+        if (pivot_root(new_root, temporary_old_root) >= 0) {
+
+                /* Immediately get rid of the old root. Since we are
+                 * running off it we need to do this lazily. */
+                if (umount2(temporary_old_root, MNT_DETACH) < 0) {
+                        r = -errno;
+                        log_error("Failed to umount old root dir %s: %m", temporary_old_root);
+                        goto fail;
+                }
+
+        } else if (mount(new_root, "/", NULL, MS_MOVE, NULL) < 0) {
                 r = -errno;
                 log_error("Failed to mount moving %s to /: %m", new_root);
                 goto fail;