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@redhat.com:
    - 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@redhat.com>
Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Karel Zak <kzak@redhat.com>

diff --git a/sys-utils/switch_root.c b/sys-utils/switch_root.c
index b947f54468979c38a77ecc0aadcc457e71615cb9..b192a08e5f498c3e2077d3e7ce00bf7269397245 100644 (file)
--- 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)