pivot_root - change the root file system
#include <linux/unistd.h>
_syscall2(int,pivot_root,const char *,new_root,const char *,put_old)
int pivot_root(const char *new_root, const char *put_old);
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:
- 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 (/).
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
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 file system, or a
file system is already mounted on put_old.
EINVAL put_old is not underneath new_root.
ENOTDIR [Toc] [Back]
new_root or put_old is not a directory.
EPERM The current process does not have the administrator capability.
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.
pivot_root is Linux-specific and hence is not portable.
pivot_root was introduced in Linux 2.3.41.
chdir(2), chroot(2), initrd(4), pivot_root(8), stat(2)
Linux 2000-02-23 PIVOT_ROOT(2)
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