This patchset adds a user namespace, which allows a process to
unshare it's user_struct table,  allowing for separate accounting
per user namespace.  It appends a user namespace to vfsmounts and
fown_structs, so that uid1==uid2 checks can be extended to be
false if uid1 and uid2 are in different namespaces.

A vfsmount generally cannot be accessed by another user namespace
than that in which it was mounted.  A vfsmount can be mounted
"shared-ns", in which case it can be accessed by any user namespace.
This is needed at least to bootstrap a container so it can get far
enough to create it's own private file system tree, and can be
used in conjunction with read-only bind mounts to provide shared
/usr trees, for instance.  However, for more useful, more fine-grained
sharing accross user namespaces, it has been suggested that a new
filesystem specifying global userid's be used.

Patches are as follows:

	1. make exit_task_namespaces extern to prevent future
	compile failure.
	2. add userns framework.
	3. add userns pointer to vfsmount
	4. hook permission to check current against vfsmount userns
	5. prepare for copy_mnt and copy_tree to return -EPERM
	6. implement shared mounts which can cross user namespaces
	7. implement user ns checks for file sigio
	8. implement user namespace unshare

#include <unistd.h>
#include <linux/unistd.h>
#include <stdio.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <errno.h>
#include <signal.h>

#ifndef MNT_DETACH
#define MNT_DETACH 2
#endif

#ifndef MS_SHARE_NS
#define MS_SHARE_NS 1<<22
#endif

#ifndef CLONE_NEWUSER
#define CLONE_NEWUSER 0x10000000
#endif

#define PIPEREAD 0
#define PIPEWRITE 1

static inline _syscall2(int, clone, int, flags, int, foo)
int to1[2], from1[1], to2[2], from2[2];

void do_test1(void)
{
	int ret;
	int fd;
	char buf[100];

	rmdir("/mnt1");
	ret = mkdir("/mnt1", 0666);
	if (ret == -1) {
		perror("mkdir mnt1");
		_exit(1);
	}
	ret = mount("/", "/mnt1", "none", MS_BIND, "");
	if (ret == -1) {
		perror("mount mnt1");
		_exit(1);
	}
	fd = open("/mnt1/testme", O_RDWR|O_CREAT);
	if (fd == -1) {
		printf("thread 1: unable to write /testme: ERROR\n");
	} else {
		write(fd, "a", 1);
		printf("thread 1: able to write /testme: GOOD\n");
		close(fd);
	}
	write(from1[PIPEWRITE], "mount", 6);

	read(to1[PIPEREAD], buf, 2);
	umount("/mnt1");
	printf("thread 1: exiting.\n");
	write(from1[PIPEWRITE], "done", 5);
	rmdir("/mnt1");
}

void do_test2(void)
{
	int ret;
	char buf[100];
	int fd;

	rmdir("/mnt2");
	ret = mkdir("/mnt2", 0666);
	if (ret == -1) {
		perror("mkdir mnt2");
		_exit(1);
	}
	ret = read(to2[PIPEREAD], buf, 10);
	fd = open("/testme", O_RDWR|O_CREAT);
	if (fd == -1) {
		printf("thread 2: unable to write /testme: ERROR\n");
	} else {
		write(fd, "b", 1);
		printf("thread 2: able to write /testme: GOOD\n");
		close(fd);
	}

	fd = open("/mnt1/testme", O_RDWR|O_CREAT);
	if (fd == -1) {
		printf("thread 2: unable to open /mnt1/testme: GOOD\n");
	} else {
		write(fd, "c", 1);
		printf("thread 2: able to open /mnt1/testme: ERROR\n");
		close(fd);
	}

	/* now try remounting /mnt1 to /mnt2 */
	ret = mount("/mnt1", "/mnt2", "none", MS_BIND, "");
	if (ret == -1) {
		printf("thread2: unable to remount /mnt1 to /mnt2: GOOD\n");
	} else {
		perror("thread2: able to remount /mnt1 to /mnt2: BAD\n");
		fd = open("/mnt2/testme", O_RDWR|O_CREAT);
		if (fd == -1) {
			printf("thread 2: unable to open /mnt2/testme: GOOD\n");
		} else {
			write(fd, "d", 1);
			printf("thread 2: able to open /mnt2/testme: ERROR\n");
			close(fd);
		}
		umount ("/mnt2");
	}

	write(from2[PIPEWRITE], "done", 5);
	printf("thread 2: exiting\n");
	rmdir("/mnt2");
}

int main(int argc, char *argv[])
{
	int childpid1, childpid2;
	int ret;
	char buf[100];

	ret = mount("/", "/mnt", "none", MS_SHARE_NS | MS_BIND, "");
	if (ret == -1) {
		perror("failed to mount /mnt shared_ns.\n");
		_exit(1);
	}

	chdir("/mnt");
	chroot("/mnt");

	ret = pipe(to1);
	if (ret == -1) {
		perror("failed to create child pipe 1\n");
		_exit(1);
	}
	ret = pipe(to2);
	if (ret == -1) {
		perror("failed to create child pipe 2\n");
		_exit(1);
	}
	ret = pipe(from1);
	if (ret == -1) {
		perror("failed to create child pipe 1\n");
		_exit(1);
	}
	ret = pipe(from2);
	if (ret == -1) {
		perror("failed to create child pipe 2\n");
		_exit(1);
	}

	childpid1 = clone(CLONE_NEWUSER | SIGCHLD, 0);
	if (childpid1 != 0) {
		do_test1();
		_exit(0);
	}
	childpid2 = clone(CLONE_NEWUSER | SIGCHLD, 0);
	if (childpid2 != 0) {
		do_test2();
		_exit(0);
	}

	read(from1[PIPEREAD], buf, 5);
	write(to2[PIPEWRITE], "go", 3);
	read(from2[PIPEREAD], buf, 4);
	write(to1[PIPEWRITE], "go", 3);
	read(from1[PIPEREAD], buf, 4);
	return 0;
}
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