UTS Namespace 详解

struct uts_namespace {
	struct kref kref;
	struct new_utsname name;                // UTS 主要存储的信息
	struct user_namespace *user_ns;
	struct ucounts *ucounts;
	struct ns_common ns;
} __randomize_layout;

↓

struct new_utsname {
	char sysname[__NEW_UTS_LEN + 1];
	char nodename[__NEW_UTS_LEN + 1];       // host name
	char release[__NEW_UTS_LEN + 1];
	char version[__NEW_UTS_LEN + 1];
	char machine[__NEW_UTS_LEN + 1];
	char domainname[__NEW_UTS_LEN + 1];     // domain name
};

pwl@ubuntu:~$ hostname
ubuntu                                      // 当前 hostname 为 ubuntu

pwl@ubuntu:~$ sudo unshare --uts /bin/bash
[sudo] password for pwl:
root@ubuntu:~# hostname
ubuntu
root@ubuntu:~# hostname test
root@ubuntu:~# hostname
test                                        // 在新的 UTS namespace 中更改 hostname 为 test
pwl@ubuntu:~$ hostname
ubuntu                                      // 在旧的 UTS namespace 中的 hostname 仍然为 ubuntu

pwl@ubuntu:~$ domainname
(none)                                      // 当前 hostname 为空

pwl@ubuntu:~$ domainname
(none)
pwl@ubuntu:~$ sudo unshare --uts /bin/bash
root@ubuntu:~# domainname
(none)
root@ubuntu:~# domainname test
root@ubuntu:~# domainname
test                                        // 在新的 UTS namespace 中更改 hostname 为 test
root@ubuntu:~#
pwl@ubuntu:~$ domainname
(none)                                      // 在旧的 UTS namespace 中的 hostname 仍然为空

pwl@ubuntu:~$ uname -a
Linux ubuntu 4.15.0-123-generic #126-Ubuntu SMP Wed Oct 21 09:40:11 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
pwl@ubuntu:~$ uname --help
Usage: uname [OPTION]...
Print certain system information.  With no OPTION, same as -s.

  -a, --all                print all information, in the following order,
                             except omit -p and -i if unknown:
  -s, --kernel-name        print the kernel name
  -n, --nodename           print the network node hostname
  -r, --kernel-release     print the kernel release
  -v, --kernel-version     print the kernel version
  -m, --machine            print the machine hardware name
  -p, --processor          print the processor type (non-portable)
  -i, --hardware-platform  print the hardware platform (non-portable)
  -o, --operating-system   print the operating system
      --help     display this help and exit
      --version  output version information and exit

struct uts_namespace *copy_utsname(unsigned long flags,
	struct user_namespace *user_ns, struct uts_namespace *old_ns)
{
	struct uts_namespace *new_ns;

	BUG_ON(!old_ns);
	get_uts_ns(old_ns);

	if (!(flags & CLONE_NEWUTS))
		return old_ns;

	new_ns = clone_uts_ns(user_ns, old_ns);

	put_uts_ns(old_ns);
	return new_ns;
}

↓

static struct uts_namespace *clone_uts_ns(struct user_namespace *user_ns,
					  struct uts_namespace *old_ns)
{
	struct uts_namespace *ns;
	struct ucounts *ucounts;
	int err;

	err = -ENOSPC;
	ucounts = inc_uts_namespaces(user_ns);
	if (!ucounts)
		goto fail;

	err = -ENOMEM;
    /* (1) 创建一个新的 uts namespace 结构 */
	ns = create_uts_ns();
	if (!ns)
		goto fail_dec;

    /* (2) 分配一个新的 namespace 编号 */
	err = ns_alloc_inum(&ns->ns);
	if (err)
		goto fail_free;

	ns->ucounts = ucounts;
	ns->ns.ops = &utsns_operations;

	down_read(&uts_sem);
    /* (3) 拷贝旧的 uts namespace 的内容 */
	memcpy(&ns->name, &old_ns->name, sizeof(ns->name));
	ns->user_ns = get_user_ns(user_ns);
	up_read(&uts_sem);
	return ns;

fail_free:
	kfree(ns);
fail_dec:
	dec_uts_namespaces(ucounts);
fail:
	return ERR_PTR(err);
}

SYSCALL_DEFINE2(sethostname, char __user *, name, int, len)
{
	int errno;
	char tmp[__NEW_UTS_LEN];

	if (!ns_capable(current->nsproxy->uts_ns->user_ns, CAP_SYS_ADMIN))
		return -EPERM;

	if (len < 0 || len > __NEW_UTS_LEN)
		return -EINVAL;
	errno = -EFAULT;
	if (!copy_from_user(tmp, name, len)) {
		struct new_utsname *u;

		down_write(&uts_sem);
		u = utsname();
        /* (1) 设置当前 uts namespace 中的 uts_ns->name->nodename */
		memcpy(u->nodename, tmp, len);
		memset(u->nodename + len, 0, sizeof(u->nodename) - len);
		errno = 0;
		uts_proc_notify(UTS_PROC_HOSTNAME);
		up_write(&uts_sem);
	}
	return errno;
}

↓

static inline struct new_utsname *utsname(void)
{
	return &current->nsproxy->uts_ns->name;
}

SYSCALL_DEFINE2(gethostname, char __user *, name, int, len)
{
	int i;
	struct new_utsname *u;
	char tmp[__NEW_UTS_LEN + 1];

	if (len < 0)
		return -EINVAL;
	down_read(&uts_sem);
	u = utsname();
	i = 1 + strlen(u->nodename);
	if (i > len)
		i = len;
    /* (1) 获取当前 uts namespace 中的 uts_ns->name->nodename */
	memcpy(tmp, u->nodename, i);
	up_read(&uts_sem);
	if (copy_to_user(name, tmp, i))
		return -EFAULT;
	return 0;
}

SYSCALL_DEFINE2(setdomainname, char __user *, name, int, len)
{
	int errno;
	char tmp[__NEW_UTS_LEN];

	if (!ns_capable(current->nsproxy->uts_ns->user_ns, CAP_SYS_ADMIN))
		return -EPERM;
	if (len < 0 || len > __NEW_UTS_LEN)
		return -EINVAL;

	errno = -EFAULT;
	if (!copy_from_user(tmp, name, len)) {
		struct new_utsname *u;

		down_write(&uts_sem);
		u = utsname();
        /* (1) 设置当前 uts namespace 中的 uts_ns->name->domainname */
		memcpy(u->domainname, tmp, len);
		memset(u->domainname + len, 0, sizeof(u->domainname) - len);
		errno = 0;
		uts_proc_notify(UTS_PROC_DOMAINNAME);
		up_write(&uts_sem);
	}
	return errno;
}

SYSCALL_DEFINE1(newuname, struct new_utsname __user *, name)
{
	struct new_utsname tmp;

	down_read(&uts_sem);
    /* (1) 获取当前 uts namespace 中的所有信息 */
	memcpy(&tmp, utsname(), sizeof(tmp));
	up_read(&uts_sem);
	if (copy_to_user(name, &tmp, sizeof(tmp)))
		return -EFAULT;

	if (override_release(name->release, sizeof(name->release)))
		return -EFAULT;
	if (override_architecture(name))
		return -EFAULT;
	return 0;
}