1.Linux内核namespace机制
Linux Namespaces机制提供一种资源隔离方案。PID,IPC,Network等系统资源不再是全局性的，而是属于某个特定的Namespace。每个namespace下的资源对于其他namespace下的资源都是透明，不可见的。因此在操作系统层面上看，就会出现多个相同pid的进程。系统中可以同时存在两个进程号为0,1,2的进程，由于属于不同的namespace，所以它们之间并不冲突。而在用户层面上只能看到属于用户自己namespace下的资源，例如使用ps命令只能列出自己namespace下的进程。这样每个namespace看上去就像一个单独的Linux系统。
2 .Linux内核中namespace结构体
在Linux内核中提供了多个namespace，其中包括fs (mount), uts, network, sysvipc, 等。一个进程可以属于多个namesapce,既然namespace和进程相关，那么在task_struct结构体中就会包含和namespace相关联的变量。在task_struct 结构中有一个指向namespace结构体的指针nsproxy。
struct task_struct {
……..
/* namespaces */
struct nsproxy *nsproxy;
…….
}
再看一下nsproxy是如何定义的，在include/linux/nsproxy.h文件中，这里一共定义了5个各自的命名空间结构体，在该结构体中定义了5个指向各个类型namespace的指针，由于多个进程可以使用同一个namespace，所以nsproxy可以共享使用，count字段是该结构的引用计数。
/* 'count' is the number of tasks holding a reference.
* The count for each namespace, then, will be the number
* of nsproxies pointing to it, not the number of tasks.
* The nsproxy is shared by tasks which share all namespaces.
* As soon as a single namespace is cloned or unshared, the
* nsproxy is copied
*/
struct nsproxy {
atomic_t count;
struct uts_namespace *uts_ns;
struct ipc_namespace *ipc_ns;
struct mnt_namespace *mnt_ns;
struct pid_namespace *pid_ns_for_children;
struct net*net_ns;
};
(1)UTS命名空间包含了运行内核的名称、版本、底层体系结构类型等信息。UTS是UNIX Timesharing System的简称。
(2)保存在struct ipc_namespace中的所有与进程间通信（IPC）有关的信息。
(3)已经装载的文件系统的视图，在struct mnt_namespace中给出。
(4)有关进程ID的信息，由struct pid_namespace提供。
(5)struct net_ns包含所有网络相关的命名空间参数。
系统中有一个默认的nsproxy，init_nsproxy，该结构在task初始化是也会被初始化。#define INIT_TASK(tsk)\
{
.nsproxy= &init_nsproxy,
}
其中init_nsproxy的定义为：
static struct kmem_cache *nsproxy_cachep;
struct nsproxy init_nsproxy = {
.count= ATOMIC_INIT(1),
.uts_ns= &init_uts_ns,
#if defined(CONFIG_POSIX_MQUEUE) || defined(CONFIG_SYSVIPC)
.ipc_ns= &init_ipc_ns,
#endif
.mnt_ns= NULL,
.pid_ns_for_children= &init_pid_ns,
#ifdef CONFIG_NET
.net_ns= &init_net,
#endif
};
对于.mnt_ns没有进行初始化，其余的namespace都进行了系统默认初始。
3. 使用clone创建自己的Namespace
如果要创建自己的命名空间，可以使用系统调用clone(),它在用户空间的原型为
int clone(int (*fn)(void *), void *child_stack, int flags, void *arg)
这里fn是函数指针，这个就是指向函数的指针，, child_stack是为子进程分配系统堆栈空间,flags就是标志用来描述你需要从父进程继承那些资源， arg就是传给子进程的参数也就是fn指向的函数参数。下面是flags可以取的值。这里只关心和namespace相关的参数。
CLONE_FS子进程与父进程共享相同的文件系统，包括root、当前目录、umask
CLONE_NEWNS当clone需要自己的命名空间时设置这个标志，不能同时设置CLONE_NEWS和CLONE_FS。
Clone()函数是在libc库中定义的一个封装函数，它负责建立新轻量级进程的堆栈并且调用对编程者隐藏了clone系统条用。实现clone()系统调用的sys_clone()服务例程并没有fn和arg参数。封装函数把fn指针存放在子进程堆栈的每个位置处，该位置就是该封装函数本身返回地址存放的位置。Arg指针正好存放在子进程堆栈中的fn的下面。当封装函数结束时，CPU从堆栈中取出返回地址，然后执行fn(arg)函数。
/* Prototype for the glibc wrapper function */
#include <sched.h>
int clone(int (*fn)(void *), void *child_stack,
int flags, void *arg, ...
/* pid_t *ptid, struct user_desc *tls, pid_t *ctid */ );
/* Prototype for the raw system call */
long clone(unsigned long flags, void *child_stack,
void *ptid, void *ctid,
struct pt_regs *regs);
我们在Linux内核中看到的实现函数，是经过libc库进行封装过的，在Linux内核中的fork.c文件中，有下面的定义，最终调用的都是do_fork()函数。
#ifdef __ARCH_WANT_SYS_CLONE
#ifdef CONFIG_CLONE_BACKWARDS
SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
int __user *, parent_tidptr,
int, tls_val,
int __user *, child_tidptr)
#elif defined(CONFIG_CLONE_BACKWARDS2)
SYSCALL_DEFINE5(clone, unsigned long, newsp, unsigned long, clone_flags,
int __user *, parent_tidptr,
int __user *, child_tidptr,
int, tls_val)
#elif defined(CONFIG_CLONE_BACKWARDS3)
SYSCALL_DEFINE6(clone, unsigned long, clone_flags, unsigned long, newsp,
int, stack_size,
int __user *, parent_tidptr,
int __user *, child_tidptr,
int, tls_val)
#else
SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
int __user *, parent_tidptr,
int __user *, child_tidptr,
int, tls_val)
#endif
{
return do_fork(clone_flags, newsp, 0, parent_tidptr, child_tidptr);
}
#endif
3.1do_fork函数
在clone()函数中调用do_fork函数进行真正的处理，在do_fork函数中调用copy_process进程处理。
long do_fork(unsigned long clone_flags,
unsigned long stack_start,
unsigned long stack_size,
int __user *parent_tidptr,
int __user *child_tidptr)
{
struct task_struct *p;
int trace = 0;
long nr;
/*
* Determine whether and which event to report to ptracer.When
* called from kernel_thread or CLONE_UNTRACED is explicitly
* requested, no event is reported; otherwise, report if the event
* for the type of forking is enabled.
*/
if (!(clone_flags & CLONE_UNTRACED)) {
if (clone_flags & CLONE_VFORK)
trace = PTRACE_EVENT_VFORK;
else if ((clone_flags & CSIGNAL) != SIGCHLD)
trace = PTRACE_EVENT_CLONE;
else
trace = PTRACE_EVENT_FORK;
if (likely(!ptrace_event_enabled(current, trace)))
trace = 0;
}
p = copy_process(clone_flags, stack_start, stack_size,
child_tidptr, NULL, trace);
/*
* Do this prior waking up the new thread - the thread pointer
* might get invalid after that point, if the thread exits quickly.
*/
if (!IS_ERR(p)) {
struct completion vfork;
struct pid *pid;
trace_sched_process_fork(current, p);
pid = get_task_pid(p, PIDTYPE_PID);
nr = pid_vnr(pid);
if (clone_flags & CLONE_PARENT_SETTID)
put_user(nr, parent_tidptr);
if (clone_flags & CLONE_VFORK) {
p->vfork_done = &vfork;
init_completion(&vfork);
get_task_struct(p);
}
wake_up_new_task(p);
/* forking complete and child started to run, tell ptracer */
if (unlikely(trace))
ptrace_event_pid(trace, pid);
if (clone_flags & CLONE_VFORK) {
if (!wait_for_vfork_done(p, &vfork))
ptrace_event_pid(PTRACE_EVENT_VFORK_DONE, pid);
}
put_pid(pid);
} else {
nr = PTR_ERR(p);
}
return nr;
}
3.2copy_process函数
在copy_process函数中调用copy_namespaces函数。
static struct task_struct *copy_process(unsigned long clone_flags,
unsigned long stack_start,
unsigned long stack_size,
int __user *child_tidptr,
struct pid *pid,
int trace)
{
int retval;
struct task_struct *p;
/*下面的代码是对clone_flag标志进行检查，有部分表示是互斥的，例如CLONE_NEWNS和CLONENEW_FS*/
if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
return ERR_PTR(-EINVAL);
if ((clone_flags & (CLONE_NEWUSER|CLONE_FS)) == (CLONE_NEWUSER|CLONE_FS))
return ERR_PTR(-EINVAL);
if ((clone_flags & CLONE_THREAD) && !(clone_flags & CLONE_SIGHAND))
return ERR_PTR(-EINVAL);
if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))
return ERR_PTR(-EINVAL);
if ((clone_flags & CLONE_PARENT) &&
current->signal->flags & SIGNAL_UNKILLABLE)
return ERR_PTR(-EINVAL);
……
retval = copy_namespaces(clone_flags, p);
if (retval)
goto bad_fork_cleanup_mm;
retval = copy_io(clone_flags, p);
if (retval)
goto bad_fork_cleanup_namespaces;
retval = copy_thread(clone_flags, stack_start, stack_size, p);
if (retval)
goto bad_fork_cleanup_io;
/*do_fork中调用copy_process函数，该函数中pid参数为NULL，所以这里的if判断是成立的。为进程所在的namespace分配pid，在3.0的内核之前还有一个关键函数，就是namespace创建后和cgroup的关系，
if (current->nsproxy != p->nsproxy) {
retval = ns_cgroup_clone(p, pid);
if (retval)
goto bad_fork_free_pid;
但在3.0内核以后给删掉了，具体请参考remove the ns_cgroup*/
if (pid != &init_struct_pid) {
retval = -ENOMEM;
pid = alloc_pid(p->nsproxy->pid_ns_for_children);
if (!pid)
goto bad_fork_cleanup_io;
}…..
}
3.3copy_namespaces 函数
在kernel/nsproxy.c文件中定义了copy_namespaces函数。
int copy_namespaces(unsigned long flags, struct task_struct *tsk)
{
struct nsproxy *old_ns = tsk->nsproxy;
struct user_namespace *user_ns = task_cred_xxx(tsk, user_ns);
struct nsproxy *new_ns;
/*首先检查flag，如果flag标志不是下面的五种之一，就会调用get_nsproxy对old_ns递减引用计数，然后直接返回0*/
if (likely(!(flags & (CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC |
CLONE_NEWPID | CLONE_NEWNET)))) {
get_nsproxy(old_ns);
return 0;
}
/*当前进程是否有超级用户的权限*/
if (!ns_capable(user_ns, CAP_SYS_ADMIN))
return -EPERM;
/*
* CLONE_NEWIPC must detach from the undolist: after switching
* to a new ipc namespace, the semaphore arrays from the old
* namespace are unreachable.In clone parlance, CLONE_SYSVSEM
* means share undolist with parent, so we must forbid using
* it along with CLONE_NEWIPC.
对CLONE_NEWIPC进行特殊的判断，*/
if ((flags & (CLONE_NEWIPC | CLONE_SYSVSEM)) ==
(CLONE_NEWIPC | CLONE_SYSVSEM))
return -EINVAL;
/*为进程创建新的namespace*/
new_ns = create_new_namespaces(flags, tsk, user_ns, tsk->fs);
if (IS_ERR(new_ns))
returnPTR_ERR(new_ns);
tsk->nsproxy = new_ns;
return 0;
}
3.4create_new_namespaces函数
create_new_namespaces创建新的namespace
static struct nsproxy *create_new_namespaces(unsigned long flags,
struct task_struct *tsk, struct user_namespace *user_ns,
struct fs_struct *new_fs)
{
struct nsproxy *new_nsp;
int err;
/*为新的nsproxy分配内存空间，并对其引用计数设置为初始1*/
new_nsp = create_nsproxy();
if (!new_nsp)
return ERR_PTR(-ENOMEM);
/*如果Namespace中的各个标志位进行了设置，则会调用相应的namespace进行创建*/
new_nsp->mnt_ns = copy_mnt_ns(flags, tsk->nsproxy->mnt_ns, user_ns, new_fs);
if (IS_ERR(new_nsp->mnt_ns)) {
err = PTR_ERR(new_nsp->mnt_ns);
goto out_ns;
}
new_nsp->uts_ns = copy_utsname(flags, user_ns, tsk->nsproxy->uts_ns);
if (IS_ERR(new_nsp->uts_ns)) {
err = PTR_ERR(new_nsp->uts_ns);
goto out_uts;
}
new_nsp->ipc_ns = copy_ipcs(flags, user_ns, tsk->nsproxy->ipc_ns);
if (IS_ERR(new_nsp->ipc_ns)) {
err = PTR_ERR(new_nsp->ipc_ns);
goto out_ipc;
}
new_nsp->pid_ns_for_children =
copy_pid_ns(flags, user_ns, tsk->nsproxy->pid_ns_for_children);
if (IS_ERR(new_nsp->pid_ns_for_children)) {
err = PTR_ERR(new_nsp->pid_ns_for_children);
goto out_pid;
}
new_nsp->net_ns = copy_net_ns(flags, user_ns, tsk->nsproxy->net_ns);
if (IS_ERR(new_nsp->net_ns)) {
err = PTR_ERR(new_nsp->net_ns);
goto out_net;
}
return new_nsp;
out_net:
if (new_nsp->pid_ns_for_children)
put_pid_ns(new_nsp->pid_ns_for_children);
out_pid:
if (new_nsp->ipc_ns)
put_ipc_ns(new_nsp->ipc_ns);
out_ipc:
if (new_nsp->uts_ns)
put_uts_ns(new_nsp->uts_ns);
out_uts:
if (new_nsp->mnt_ns)
put_mnt_ns(new_nsp->mnt_ns);
out_ns:
kmem_cache_free(nsproxy_cachep, new_nsp);
return ERR_PTR(err);
}
3.4.1 create_nsproxy函数
static inline struct nsproxy *create_nsproxy(void)
{
struct nsproxy *nsproxy;
nsproxy = kmem_cache_alloc(nsproxy_cachep, GFP_KERNEL);
if (nsproxy)
atomic_set(&nsproxy->count, 1);
return nsproxy;
}
例子1：namespace pid的例子
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sched.h>
#include <string.h>
static int fork_child(void *arg)
{
int a = (int)arg;
int i;
pid_t pid;
char *cmd= "ps -el;
printf("In the container, my pid is: %d\n", getpid());
/*ps命令是解析procfs的内容得到结果的，而procfs根目录的进程pid目录是基于mount当时的pid namespace的，这个在procfs的get_sb回调中体现的。因此只需要重新mount一下proc, mount -t proc proc /proc*/
mount("proc", "/proc", "proc", 0, "");
for (i = 0; i <a; i++) {
pid = fork();
if (pid <0)
return pid;
else if (pid)
printf("pid of my child is %d\n", pid);
else if (pid == 0) {
sleep(30);
exit(0);
}
}
execl("/bin/bash", "/bin/bash","-c",cmd, NULL);
return 0;
}
int main(int argc, char *argv[])
{
int cpid;
void *childstack, *stack;
int flags;
int ret = 0;
int stacksize = getpagesize() * 4;
if (argc != 2) {
fprintf(stderr, "Wrong usage.\n");
return -1;
}
stack = malloc(stacksize);
if(stack == NULL)
{
return -1;
}
printf("Out of the container, my pid is: %d\n", getpid());
childstack = stack + stacksize;
flags = CLONE_NEWPID | CLONE_NEWNS;
cpid = clone(fork_child, childstack, flags, (void *)atoi(argv[1]));
printf("cpid: %d\n", cpid);
if (cpid <0) {
perror("clone");
ret = -1;
goto out;
}
fprintf(stderr, "Parent sleeping 20 seconds\n");
sleep(20);
ret = 0;
out:
free(stack);
return ret;
}
}
运行结果：
root@Ubuntu:~/c_program# ./namespace 7
Out of the container, my pid is: 8684
cpid: 8685
Parent sleeping 20 seconds
In the container, my pid is: 1
pid of my child is 2
pid of my child is 3
pid of my child is 4
pid of my child is 5
pid of my child is 6
pid of my child is 7
pid of my child is 8
F SUIDPIDPPIDC PRINI ADDR SZ WCHANTTYTIME CMD
4 R0100800 -1085 -pts/000:00:00 ps
1 S0210800 -458 hrtime pts/000:00:00 namespace
1 S0310800 -458 hrtime pts/000:00:00 namespace
1 S0410800 -458 hrtime pts/000:00:00 namespace
1 S0510800 -458 hrtime pts/000:00:00 namespace
1 S0610800 -458 hrtime pts/000:00:00 namespace
1 S0710800 -458 hrtime pts/000:00:00 namespace
1 S0810800 -458 hrtime pts/000:00:00 namespace
例子2：UTS的例子
#define _GNU_SOURCE
#include <sys/wait.h>
#include <sys/utsname.h>
#include <sched.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define errExit(msg)do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
static int/* Start function for cloned child */
childFunc(void *arg)
{
struct utsname uts;
/* Change hostname in UTS namespace of child */
if (sethostname(arg, strlen(arg)) == -1)
errExit("sethostname");
/* Retrieve and display hostname */
if (uname(&uts) == -1)
errExit("uname");
printf("uts.nodename in child:%s\n", uts.nodename);
/* Keep the namespace open for a while, by sleeping.
*This allows some experimentation--for example, another
*process might join the namespace. */
sleep(200);
return 0;/* Child terminates now */
}
#define STACK_SIZE (1024 * 1024)/* Stack size for cloned child */
int
main(int argc, char *argv[])
{
char *stack;/* Start of stack buffer */
char *stackTop;/* End of stack buffer */
pid_t pid;
struct utsname uts;
if (argc < 2) {
fprintf(stderr, "Usage: %s <child-hostname>\n", argv[0]);
exit(EXIT_SUCCESS);
}
/* Allocate stack for child */
stack = malloc(STACK_SIZE);
if (stack == NULL)
errExit("malloc");
stackTop = stack + STACK_SIZE;/* Assume stack grows downward */
/* Create child that has its own UTS namespace;
*child commences execution in childFunc() */
pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]);
if (pid == -1)
errExit("clone");
printf("clone() returned %ld\n", (long) pid);
/* Parent falls through to here */
sleep(1);/* Give child time to change its hostname */
/* Display hostname in parent's UTS namespace. This will be
*different from hostname in child's UTS namespace. */
if (uname(&uts) == -1)
errExit("uname");
printf("uts.nodename in parent: %s\n", uts.nodename);
if (waitpid(pid, NULL, 0) == -1)/* Wait for child */
errExit("waitpid");
printf("child has terminated\n");
exit(EXIT_SUCCESS);
}
root@ubuntu:~/c_program# ./namespace_1 test
clone() returned 4101
uts.nodename in child:test
uts.nodename in parent: ubuntu