The current switch_root can only switch to a new root that is the root
of a mount point.
This patch adds support for "subroots", where the new root is somewhere
below a mount point. It does this by adding in a few extra steps to
chroot into the subroot after the enclosing partition has been moved
and entered.
This will be used by OLPC, who sort-of have 2 copies of Fedora stored
on a single partition under different directory trees, where the
initramfs decides which one to boot into.
---
switch_root.c | 91 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
1 files changed, 88 insertions(+), 3 deletions(-)
diff --git a/switch_root.c b/switch_root.c
index 8daacb1..8ce4aaf 100644
--- a/switch_root.c
+++ b/switch_root.c
@@ -33,11 +33,63 @@
#include <ctype.h>
#include <dirent.h>
#include <err.h>
+#include <libgen.h>
#ifndef MS_MOVE
#define MS_MOVE 8192
#endif
+/* find the enclosing mount point of a path, by examining the backing device
+ * of parent directories until we reach / or find a parent with a differing
+ * device.
+ * result must be freed.
+ */
+static char *get_parent_mount(const char *path)
+{
+ struct stat sb;
+ char dir[PATH_MAX];
+ char tmp[PATH_MAX];
+ dev_t inner_dev;
+ int r;
+
+ r = stat(path, &sb);
+ if (r != 0) {
+ warn("failed to stat %s", path);
+ return NULL;
+ }
+ inner_dev = sb.st_dev;
+
+ /* dirname has some annoying properties of modifying the input... */
+ strncpy(dir, path, PATH_MAX);
+ dir[PATH_MAX - 1] = 0; /* for safety */
+
+ while (1) {
+ char *parent;
+
+ strncpy(tmp, dir, PATH_MAX);
+ tmp[PATH_MAX - 1] = 0;
+ parent = dirname(tmp);
+
+ r = stat(parent, &sb);
+ if (r != 0) {
+ warn("failed to stat %s", parent);
+ return NULL;
+ }
+
+ /* if the parent directory's device differs then we have found a mount
+ * point */
+ if (sb.st_dev != inner_dev)
+ return strdup(dir);
+
+ strncpy(dir, parent, PATH_MAX);
+ dir[PATH_MAX - 1] = 0;
+
+ /* maybe we've reached / */
+ if (strlen(dir) == 1)
+ return strdup(dir);
+ }
+}
+
/* remove all files/directories below dirName -- don't cross mountpoints */
static int recursiveRemove(char *dirName)
{
@@ -109,7 +161,10 @@ static int switchroot(const char *newroot)
{
/* Don't try to unmount the old "/", there's no way to do it. */
const char *umounts[] = { "/dev", "/proc", "/sys", NULL };
+ char *newroot_mnt;
+ const char *chroot_path = NULL;
int i;
+ int r = -1;
for (i = 0; umounts[i] != NULL; i++) {
char newmount[PATH_MAX];
@@ -131,16 +186,46 @@ static int switchroot(const char *newroot)
recursiveRemove("/");
+ newroot_mnt = get_parent_mount(newroot);
+ if (newroot_mnt && strcmp(newroot, newroot_mnt)) {
+ /* newroot is not a mount point, so we have to MS_MOVE the parent
+ * mount point and then chroot in to the "subroot" */
+ chroot_path = newroot + strlen(newroot_mnt);
+ newroot = newroot_mnt;
+
+ if (chdir(newroot)) {
+ warn("failed to chdir to newroot mount %s", newroot);
+ goto err;
+ }
+ }
+
if (mount(newroot, "/", NULL, MS_MOVE, NULL) < 0) {
warn("failed to mount moving %s to /", newroot);
- return -1;
+ goto err;
}
if (chroot(".")) {
warn("failed to change root");
- return -1;
+ goto err;
}
- return 0;
+
+ if (chroot_path) {
+ if (chdir(chroot_path)) {
+ warn("failed to chdir to subroot %s", chroot_path);
+ goto err;
+ }
+
+ if (chroot(".")) {
+ warn("failed to change root to subroot");
+ goto err;
+ }
+ }
+
+ r = 0;
+err:
+ if (newroot_mnt)
+ free(newroot_mnt);
+ return r;
}
static void usage(FILE *output)
--
1.6.2.5
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