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pivot_root: switch to a new mechanism (v2)
This idea came from Andy Lutomirski. Instead of using a temporary directory for the pivot_root put-old, use "." both for new-root and old-root. Then fchdir into the old root temporarily in order to unmount the old-root, and finally chdir back into our '/'. Drop lxc.pivotdir from the lxc.container.conf manpage. Warn when we see a lxc.pivotdir entry (but keep it in the lxc.conf for now). Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com> Acked-by: Stéphane Graber <stgraber@ubuntu.com>
This commit is contained in:
Serge Hallyn
Stéphane Graber
parent
01db019751
commit
2d489f9e87
@ -943,20 +943,6 @@ proc proc proc nodev,noexec,nosuid 0 0
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>lxc.pivotdir</option>
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</term>
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<listitem>
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<para>
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where to pivot the original root file system under
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<option>lxc.rootfs</option>, specified relatively to
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that. The default is <filename>mnt</filename>.
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It is created if necessary, and also removed after
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unmounting everything from it during container setup.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect2>
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219
src/lxc/conf.c
219
src/lxc/conf.c
@ -1025,199 +1025,66 @@ static int setup_tty(const struct lxc_rootfs *rootfs,
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return 0;
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}
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static int setup_rootfs_pivot_root_cb(char *buffer, void *data)
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{
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struct lxc_list *mountlist, *listentry, *iterator;
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char *pivotdir, *mountpoint, *mountentry, *saveptr = NULL;
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int found;
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void **cbparm;
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mountentry = buffer;
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cbparm = (void **)data;
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mountlist = cbparm[0];
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pivotdir = cbparm[1];
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/* parse entry, first field is mountname, ignore */
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mountpoint = strtok_r(mountentry, " ", &saveptr);
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if (!mountpoint)
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return -1;
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/* second field is mountpoint */
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mountpoint = strtok_r(NULL, " ", &saveptr);
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if (!mountpoint)
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return -1;
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/* only consider mountpoints below old root fs */
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if (strncmp(mountpoint, pivotdir, strlen(pivotdir)))
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return 0;
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/* filter duplicate mountpoints */
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found = 0;
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lxc_list_for_each(iterator, mountlist) {
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if (!strcmp(iterator->elem, mountpoint)) {
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found = 1;
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break;
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}
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}
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if (found)
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return 0;
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/* add entry to list */
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listentry = malloc(sizeof(*listentry));
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if (!listentry) {
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SYSERROR("malloc for mountpoint listentry failed");
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return -1;
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}
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listentry->elem = strdup(mountpoint);
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if (!listentry->elem) {
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SYSERROR("strdup failed");
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free(listentry);
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return -1;
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}
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lxc_list_add_tail(mountlist, listentry);
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return 0;
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}
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static int umount_oldrootfs(const char *oldrootfs)
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{
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char path[MAXPATHLEN];
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void *cbparm[2];
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struct lxc_list mountlist, *iterator, *next;
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int ok, still_mounted, last_still_mounted;
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int rc;
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/* read and parse /proc/mounts in old root fs */
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lxc_list_init(&mountlist);
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/* oldrootfs is on the top tree directory now */
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rc = snprintf(path, sizeof(path), "/%s", oldrootfs);
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if (rc >= sizeof(path)) {
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ERROR("rootfs name too long");
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return -1;
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}
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cbparm[0] = &mountlist;
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cbparm[1] = strdup(path);
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if (!cbparm[1]) {
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SYSERROR("strdup failed");
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return -1;
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}
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rc = snprintf(path, sizeof(path), "%s/proc/mounts", oldrootfs);
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if (rc >= sizeof(path)) {
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ERROR("container proc/mounts name too long");
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return -1;
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}
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ok = lxc_file_for_each_line(path,
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setup_rootfs_pivot_root_cb, &cbparm);
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if (ok < 0) {
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SYSERROR("failed to read or parse mount list '%s'", path);
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return -1;
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}
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/* umount filesystems until none left or list no longer shrinks */
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still_mounted = 0;
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do {
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last_still_mounted = still_mounted;
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still_mounted = 0;
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lxc_list_for_each_safe(iterator, &mountlist, next) {
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/* umount normally */
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if (!umount(iterator->elem)) {
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DEBUG("umounted '%s'", (char *)iterator->elem);
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lxc_list_del(iterator);
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continue;
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}
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still_mounted++;
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}
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} while (still_mounted > 0 && still_mounted != last_still_mounted);
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lxc_list_for_each(iterator, &mountlist) {
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/* let's try a lazy umount */
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if (!umount2(iterator->elem, MNT_DETACH)) {
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INFO("lazy unmount of '%s'", (char *)iterator->elem);
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continue;
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}
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/* be more brutal (nfs) */
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if (!umount2(iterator->elem, MNT_FORCE)) {
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INFO("forced unmount of '%s'", (char *)iterator->elem);
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continue;
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}
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WARN("failed to unmount '%s'", (char *)iterator->elem);
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}
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return 0;
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}
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static int setup_rootfs_pivot_root(const char *rootfs, const char *pivotdir)
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{
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char path[MAXPATHLEN];
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int remove_pivotdir = 0;
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int rc;
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int oldroot = -1, newroot = -1;
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oldroot = open("/", O_DIRECTORY | O_RDONLY);
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if (oldroot < 0) {
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SYSERROR("Error opening old-/ for fchdir");
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return -1;
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}
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newroot = open(rootfs, O_DIRECTORY | O_RDONLY);
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if (newroot < 0) {
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SYSERROR("Error opening new-/ for fchdir");
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goto fail;
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}
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/* change into new root fs */
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if (chdir(rootfs)) {
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if (fchdir(newroot)) {
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SYSERROR("can't chdir to new rootfs '%s'", rootfs);
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return -1;
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goto fail;
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}
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if (!pivotdir)
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pivotdir = "lxc_putold";
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/* compute the full path to pivotdir under rootfs */
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rc = snprintf(path, sizeof(path), "%s/%s", rootfs, pivotdir);
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if (rc >= sizeof(path)) {
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ERROR("pivot dir name too long");
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return -1;
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}
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if (access(path, F_OK)) {
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if (mkdir_p(path, 0755) < 0) {
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SYSERROR("failed to create pivotdir '%s'", path);
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return -1;
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}
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remove_pivotdir = 1;
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DEBUG("created '%s' directory", path);
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}
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DEBUG("mountpoint for old rootfs is '%s'", path);
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/* pivot_root into our new root fs */
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if (pivot_root(".", path)) {
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if (pivot_root(".", ".")) {
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SYSERROR("pivot_root syscall failed");
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return -1;
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goto fail;
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}
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if (chdir("/")) {
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SYSERROR("can't chdir to / after pivot_root");
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return -1;
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/*
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* at this point the old-root is mounted on top of our new-root
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* To unmounted it we must not be chdir'd into it, so escape back
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* to old-root
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*/
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if (fchdir(oldroot) < 0) {
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SYSERROR("Error entering oldroot");
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goto fail;
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}
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if (umount2("/", MNT_DETACH) < 0) {
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SYSERROR("Error detaching old root");
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goto fail;
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}
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if (fchdir(newroot) < 0) {
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SYSERROR("Error re-entering newroot");
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goto fail;
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}
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close(oldroot);
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close(newroot);
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DEBUG("pivot_root syscall to '%s' successful", rootfs);
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/* we switch from absolute path to relative path */
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if (umount_oldrootfs(pivotdir))
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return -1;
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/* remove temporary mount point, we don't consider the removing
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* as fatal */
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if (remove_pivotdir && rmdir(pivotdir))
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WARN("can't remove mountpoint '%s': %m", pivotdir);
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return 0;
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fail:
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if (oldroot != -1)
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close(oldroot);
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if (newroot != -1)
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close(newroot);
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return -1;
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}
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/*
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@ -1662,6 +1662,7 @@ static int config_rootfs_options(const char *key, const char *value,
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static int config_pivotdir(const char *key, const char *value,
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struct lxc_conf *lxc_conf)
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{
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WARN("lxc.pivotdir is ignored. It will soon become an error.");
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return config_path_item(&lxc_conf->rootfs.pivot, value);
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}
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