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 -- To unsubscribe from this list: send the line "unsubscribe initramfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html