On Sun, Jul 12, 2009 at 12:02:43PM +0100, Daniel Drake wrote:
> 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.
Sounds crazy :-)
Applied with some minor changes. Please, review the patch below or
git://git.kernel.org/pub/scm/utils/util-linux-ng/util-linux-ng.git
Thanks.
Karel
>From a692a8745941a192528c5e2a05de97155ba586f9 Mon Sep 17 00:00:00 2001
From: Daniel Drake <dsd@xxxxxxxxxx>
Date: Tue, 14 Jul 2009 14:41:33 +0200
Subject: [PATCH] switch_root: add subroot support
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
[kzak@xxxxxxxxxx:
- port to the current u-l-ng switch_root code
- don't use static buffer for "dir" in get_parent_mount()]
CC: Peter Jones <pjones@xxxxxxxxxx>
Signed-off-by: Daniel Drake <dsd@xxxxxxxxxx>
Signed-off-by: Karel Zak <kzak@xxxxxxxxxx>
---
sys-utils/switch_root.c | 81 ++++++++++++++++++++++++++++++++++++++++++++---
1 files changed, 76 insertions(+), 5 deletions(-)
diff --git a/sys-utils/switch_root.c b/sys-utils/switch_root.c
index b947f54..b192a08 100644
--- a/sys-utils/switch_root.c
+++ b/sys-utils/switch_root.c
@@ -33,6 +33,7 @@
#include <ctype.h>
#include <dirent.h>
#include <err.h>
+#include <libgen.h>
#ifndef MS_MOVE
#define MS_MOVE 8192
@@ -108,13 +109,58 @@ done:
return rc;
}
+/* 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;
+ char tmp[PATH_MAX];
+ dev_t inner_dev;
+
+ if (stat(path, &sb) != 0) {
+ warn("failed to stat %s", path);
+ return NULL;
+ }
+
+ inner_dev = sb.st_dev;
+ dir = strdup(path);
+
+ while (dir) {
+ char *parent;
+
+ strncpy(tmp, dir, PATH_MAX);
+ tmp[PATH_MAX - 1] = '\0';
+ parent = dirname(tmp);
+
+ if (stat(parent, &sb) != 0) {
+ warn("failed to stat %s", parent);
+ return NULL;
+ }
+ if (sb.st_dev != inner_dev)
+ return dir;
+
+ strncpy(dir, parent, PATH_MAX);
+ dir[PATH_MAX - 1] = '\0';
+
+ /* maybe we've reached / */
+ if (*dir == '/' && !*(dir + 1))
+ return dir;
+ }
+ return NULL;
+}
+
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 };
int i;
- int cfd;
+ int cfd, rc = -1;
pid_t pid;
+ const char *chroot_path = NULL;
+ char *newroot_mnt;
for (i = 0; umounts[i] != NULL; i++) {
char newmount[PATH_MAX];
@@ -129,21 +175,43 @@ static int switchroot(const char *newroot)
}
}
+ 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 change directory to %s", newroot);
- return -1;
+ goto done;
}
cfd = open("/", O_RDONLY);
if (mount(newroot, "/", NULL, MS_MOVE, NULL) < 0) {
warn("failed to mount moving %s to /", newroot);
- return -1;
+ goto done;
}
+ /* move to the real root of the device */
if (chroot(".")) {
warn("failed to change root");
- return -1;
+ goto done;
+ }
+
+ /* move to the subdirectory on the root device (subroot) */
+ if (chroot_path) {
+ if (chdir(chroot_path)) {
+ warn("failed to chdir to subroot %s", chroot_path);
+ goto done;
+ }
+ if (chroot(".")) {
+ warn("failed to change root to subroot %s", chroot_path);
+ goto done;
+ }
}
if (cfd >= 0) {
@@ -155,7 +223,10 @@ static int switchroot(const char *newroot)
}
close(cfd);
}
- return 0;
+ rc = 0;
+done:
+ free(newroot_mnt);
+ return rc;
}
static void usage(FILE *output)
--
1.6.2.2
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