From 891a4918ef75fa81e22691156c050d061bd53dd3 Mon Sep 17 00:00:00 2001

From: Lennart Poettering <lennart@poettering.net>

Date: Fri, 16 Nov 2012 18:15:30 +0100

Subject: [PATCH] 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.

---

 src/core/switch-root.c | 27 ++++++++++++++++++++++++++-

 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/src/core/switch-root.c b/src/core/switch-root.c

index 150332a..ce0e41d 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;

-- 

1.7.12.1