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Subject[PATCH 0/8] user namespace: Introduction
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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