[MAN] pivot_root

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PIVOT_ROOT

Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
Index Return to Main Contents

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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.  

SEE ALSO

chdir(2), chroot(2), stat(2), initrd(4), pivot_root(8), switch_root(8)  

COLOPHON

This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

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Index

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

VERSIONS

CONFORMING TO

NOTES

BUGS

SEE ALSO

COLOPHON

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PIVOT_ROOT

Section: System Administration (8)
Updated: August 2011
Index Return to Main Contents

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NAME

pivot_root - change the root filesystem  

SYNOPSIS

pivot_root new_root put_old  

DESCRIPTION

pivot_root moves the root file system of the current process to the directory put_old and makes new_root the new root file system. Since pivot_root(8) simply calls pivot_root(2), we refer to the man page of the latter for further details.

Note that, depending on the implementation of pivot_root, root and cwd of the caller may or may not change. The following is a sequence for invoking pivot_root that works in either case, assuming that pivot_root and chroot are in the current PATH:

cd new_root
pivot_root . put_old
exec chroot . command

Note that chroot must be available under the old root and under the new root, because pivot_root may or may not have implicitly changed the root directory of the shell.

Note that exec chroot changes the running executable, which is necessary if the old root directory should be unmounted afterwards. Also note that standard input, output, and error may still point to a device on the old root file system, keeping it busy. They can easily be changed when invoking chroot (see below; note the absence of leading slashes to make it work whether pivot_root has changed the shell's root or not).  

OPTIONS

-V, --version

Display version information and exit.

-h, --help

Display help text and exit.

 

EXAMPLES

Change the root file system to /dev/hda1 from an interactive shell:

mount /dev/hda1 /new-root
cd /new-root
pivot_root . old-root
exec chroot . sh <dev/console >dev/console 2>&1
umount /old-root

Mount the new root file system over NFS from 10.0.0.1:/my_root and run init:

ifconfig lo 127.0.0.1 up   # for portmap
# configure Ethernet or such
portmap   # for lockd (implicitly started by mount)
mount -o ro 10.0.0.1:/my_root /mnt
killall portmap   # portmap keeps old root busy
cd /mnt
pivot_root . old_root
exec chroot . sh -c 'umount /old_root; exec /sbin/init' \
  <dev/console >dev/console 2>&1

 

SEE ALSO

chroot(1), pivot_root(2), mount(8), switch_root(8), umount(8)  

AVAILABILITY

The pivot_root command is part of the util-linux package and is available from https://www.kernel.org/pub/linux/utils/util-linux/.

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Index

NAME

SYNOPSIS

DESCRIPTION

OPTIONS

EXAMPLES

SEE ALSO

AVAILABILITY

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This document was created by man2html, using the manual pages.
Time: 04:45:58 GMT, September 16, 2022