% less docs/BusyBox.1
...
    pivot_root
        pivot_root NEW_ROOT PUT_OLD
        
        Move the current root file system to PUT_OLD and make
        NEW_ROOT the new root file system
...
    switch_root
        switch_root [-c /dev/console] NEW_ROOT NEW_INIT [ARGUMENTS_TO_INIT]
        
        Use from PID 1 under initramfs to free initramfs,
        chroot to NEW_ROOT, and exec NEW_INIT
        
        Options:

               -c      Redirect console to device on new root
...

% cat -n busybox-1.13.2/util-linux/pivot_root.c
...
 11  #include "libbb.h"
 12  
 13  extern int pivot_root(const char * new_root,const char * put_old);
 14  
 15  int pivot_root_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 16  int pivot_root_main(int argc, char **argv)
 17  {
 18   if (argc != 3)
 19    bb_show_usage();
 20  
 21   if (pivot_root(argv[1], argv[2]) < 0) {
 22    /* prints "pivot_root: " */
 23    bb_perror_nomsg_and_die();
 24   }
 25  
 26   return EXIT_SUCCESS;
 27  }

% find /usr/src/linux -follow  | xargs grep -i pivot_root | cat -n
  1  /usr/src/linux/arch/s390/include/asm/unistd.h:#define __NR_pivot_root         217
  2  /usr/src/linux/arch/s390/kernel/syscalls.S:SYSCALL(sys_pivot_root,sys_pivot_root,sys32_pivot_root_wrapper)
    ...
 88  /usr/src/linux/Documentation/early-userspace/README:   filesystem via linuxrc and use the pivot_root syscall.  The initrd is
 89  /usr/src/linux/Documentation/arm/IXP4xx:   a pivot_root to NFS.
 90  /usr/src/linux/usr/include/asm/unistd_64.h:#define __NR_pivot_root				155
 91  /usr/src/linux/usr/include/asm/unistd_64.h:__SYSCALL(__NR_pivot_root, sys_pivot_root)
 92  /usr/src/linux/usr/include/asm/unistd_32.h:#define __NR_pivot_root		217

% cat -n /usr/src/linux/fs/namespace.c
 ...
2157  /*
2158   * pivot_root Semantics:
2159   * Moves the root file system of the current process to the directory put_old,
2160   * makes new_root as the new root file system of the current process, and sets
2161   * root/cwd of all processes which had them on the current root to new_root.
2162   *
2163   * Restrictions:
2164   * The new_root and put_old must be directories, and  must not be on the
2165   * same file  system as the current process root. The put_old  must  be
2166   * underneath new_root,  i.e. adding a non-zero number of /.. to the string
2167   * pointed to by put_old must yield the same directory as new_root. No other
2168   * file system may be mounted on put_old. After all, new_root is a mountpoint.
2169   *
2170   * Also, the current root cannot be on the 'rootfs' (initial ramfs) filesystem.
2171   * See Documentation/filesystems/ramfs-rootfs-initramfs.txt for alternatives
2172   * in this situation.
2173   *
2174   * Notes:
2175   *  - we don't move root/cwd if they are not at the root (reason: if something
2176   *    cared enough to change them, it's probably wrong to force them elsewhere)
2177   *  - it's okay to pick a root that isn't the root of a file system, e.g.
2178   *    /nfs/my_root where /nfs is the mount point. It must be a mountpoint,
2179   *    though, so you may need to say mount --bind /nfs/my_root /nfs/my_root
2180   *    first.
2181   */
2182  SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
2183    const char __user *, put_old)
2184  {
2185   struct vfsmount *tmp;
2186   struct path new, old, parent_path, root_parent, root;
2187   int error;
2188  
2189   if (!capable(CAP_SYS_ADMIN))
2190    return -EPERM;
2191