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pivot_root() - Unix, Linux System Call


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NAME

pivot_root - change the root file system

SYNOPSIS

int pivot_root(const char *new_root, const char *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 of the current process.

The typical use of pivot_root() is during system startup, when the system mounts a temporary root file system (e.g. an initrd), then mounts the real root file system, 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 (cwd) of any processes or threads which use the old root directory. The caller of pivot_root() must ensure that processes with root or cwd at the old root operate correctly in either case. An easy way to ensure this is to change their root and cwd 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 cwd 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 cwd, even if they never access the file system in any way. In the future, there may be a mechanism for kernel threads to explicitly relinquish any access to the file system, such that this fairly intrusive mechanism can be removed from pivot_root().

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

The following restrictions apply to new_root and put_old:

TagDescription
- They must be directories.
- new_root and put_old must not be on the same file system as the current root.
- put_old must be underneath new_root, i.e. adding a non-zero number of /.. to the string pointed to by put_old must yield the same directory as new_root.
- No other file system 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 file system 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 file system 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:
TagDescription
EBUSY new_root or put_old are on the current root file system, or a file system 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 current 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.

BUGS

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

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

NOTES

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

SEE ALSO



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