Ubuntu Manpage: pivot_root - change the root filesystem

NAME

       pivot_root - change the root filesystem

SYNOPSIS

       int pivot_root(const char *new_root, const char *put_old);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

pivot_root() moves the root filesystem of the calling process to the directory put_old and makes new_root
the new root filesystem of the calling process.

The typical use of pivot_root() is during system startup, when the system mounts a temporary root
filesystem (e.g., an initrd), then mounts the real root filesystem, and eventually turns the latter into
the current root of all relevant processes or threads.

pivot_root() may or may not change the current root and the current working directory of any processes or
threads which use the old root directory. The caller of pivot_root() must ensure that processes with
root or current working directory at the old root operate correctly in either case. An easy way to
ensure this is to change their root and current working directory to new_root before invoking
pivot_root().

The paragraph above is intentionally vague because the implementation of pivot_root() may change in the
future. At the time of writing, pivot_root() changes root and current working directory of each process
or thread to new_root if they point to the old root directory. This is necessary in order to prevent
kernel threads from keeping the old root directory busy with their root and current working directory,
even if they never access the filesystem in any way. In the future, there may be a mechanism for kernel
threads to explicitly relinquish any access to the filesystem, such that this fairly intrusive mechanism
can be removed from pivot_root().

Note that this also applies to the calling process: pivot_root() may or may not affect its current
working directory. It is therefore recommended to call chdir("/") immediately after pivot_root().

The following restrictions apply to new_root and put_old:

- They must be directories.

- new_root and put_old must not be on the same filesystem as the current root.

- put_old must be underneath new_root, that is, adding a nonzero number of /.. to the string pointed to
by put_old must yield the same directory as new_root.

- No other filesystem may be mounted on put_old.

See also pivot_root(8) for additional usage examples.

If the current root is not a mount point (e.g., after chroot(2) or pivot_root(), see also below), not the
old root directory, but the mount point of that filesystem is mounted on put_old.

new_root does not have to be a mount point. In this case, /proc/mounts will show the mount point of the
filesystem containing new_root as root (/).

RETURN VALUE

       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS

       pivot_root() may return (in errno) any of the errors returned by stat(2).  Additionally, it may return:

       EBUSY  new_root or put_old are on the current root filesystem, or a  filesystem  is  already  mounted  on
              put_old.

       EINVAL put_old is not underneath new_root.

       ENOTDIR
              new_root or put_old is not a directory.

       EPERM  The calling process does not have the CAP_SYS_ADMIN capability.

VERSIONS

       pivot_root() was introduced in Linux 2.3.41.

CONFORMING TO

       pivot_root() is Linux-specific and hence is not portable.

NOTES

       Glibc does not provide a wrapper for this system call; call it using syscall(2).

BUGS

       pivot_root()  should  not have to change root and current working directory of all other processes in the
       system.

       Some of the more obscure uses of pivot_root() may quickly lead to insanity.

COLOPHON

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       information   about   reporting   bugs,   and   the  latest  version  of  this  page,  can  be  found  at
       http://www.kernel.org/doc/man-pages/.