Docker 基础技术之 Linux namespace 源码分析

内容预览:
  • int copy_namespaces(unsigned long flags, struct task_struct *tsk);v...~
  • 创建新的 namespace 初始化完之后,下面看看如何创建一个新的 namespace...~
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上篇我们从进程 clone 的角度,结合代码简单分析了 Linux 提供的 6 种 namespace,本篇从源码上进一步分析 Linux namespace,让你对 Docker namespace 的隔离机制有更深的认识。我用的是 Linux-4.1.19 的版本,由于 namespace 模块更新都比较少,所以,只要 3.0 以上的版本都是差不多的。

从内核进程描述符 task_struct 开始切入

由于 Linux namespace 是用来做进程资源隔离的,所以在进程描述符中,一定有 namespace 所对应的信息,我们可以从这里开始切入代码。

首先找到描述进程信息 task_struct,找到指向 namespace 的结构 struct *nsproxy(sched.h):

struct task_struct {
......
/* namespaces */
struct nsproxy *nsproxy;
......
}

其中 nsproxy 结构体定义在 nsproxy.h 中:

/*
* A structure to contain pointers to all per-process
* namespaces - fs (mount), uts, network, sysvipc, etc.
*
* '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;
struct net *net_ns;
};
extern struct nsproxy init_nsproxy;

这个结构是被所有 namespace 所共享的,只要一个 namespace 被 clone 了,nsproxy 也会被 clone。注意到,由于 user namespace 是和其他 namespace 耦合在一起的,所以没出现在上述结构中。

同时,nsproxy.h 中还定义了一些对 namespace 的操作,包括 copy_namespaces 等。

int copy_namespaces(unsigned long flags, struct task_struct *tsk);
void exit_task_namespaces(struct task_struct *tsk);
void switch_task_namespaces(struct task_struct *tsk, struct nsproxy *new);
void free_nsproxy(struct nsproxy *ns);
int unshare_nsproxy_namespaces(unsigned long, struct nsproxy **,
struct fs_struct *);

task_struct,nsproxy,几种 namespace 之间的关系如下所示:

各个 namespace 的初始化

在各个 namespace 结构定义下都有个 init 函数,nsproxy 也有个 init_nsproxy 函数,init_nsproxy 在 task 初始化的时候会被初始化,附带的,init_nsproxy 中定义了各个 namespace 的 init 函数,如下:

在 init_task 函数中(init_task.h):

/*
* INIT_TASK is used to set up the first task table, touch at
* your own risk!. Base=0, limit=0x1fffff (=2MB)
*/
#define INIT_TASK(tsk)
{
......
.nsproxy = &init_nsproxy,
......
}

继续跟进 init_nsproxy,在 nsproxy.c 中:

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
};

可见,init_nsproxy 中,对 uts, ipc, pid, net 都进行了初始化,但 mount 却没有。

创建新的 namespace

初始化完之后,下面看看如何创建一个新的 namespace,通过前面的文章,我们知道是通过 clone 函数来完成的,在 Linux kernel 中,fork/vfork() 对 clone 进行了封装。如下:

#ifdef __ARCH_WANT_SYS_FORK
SYSCALL_DEFINE0(fork)
{
#ifdef CONFIG_MMU
return do_fork(SIGCHLD, 0, 0, NULL, NULL);
#else
/* can not support in nommu mode */
return -EINVAL;
#endif
}
#endif

#ifdef __ARCH_WANT_SYS_VFORK
SYSCALL_DEFINE0(vfork)
{
return do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, 0,
0, NULL, NULL);
}
#endif

#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

可以看到,无论是 fork() 还是 vfork(),最终都会调用到 do_fork() 函数:

/*
* Ok, this is the main fork-routine.
*
* It copies the process, and if successful kick-starts
* it and waits for it to finish using the VM if required.
*/
long do_fork(unsigned long clone_flags,
unsigned long stack_start,
unsigned long stack_size,
int __user *parent_tidptr,
int __user *child_tidptr)
{
// 创建进程描述符指针
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;
}

// 复制进程描述符,返回值是 task_struct
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);

// 得到新进程描述符的 pid
pid = get_task_pid(p, PIDTYPE_PID);
nr = pid_vnr(pid);

if (clone_flags & CLONE_PARENT_SETTID)
put_user(nr, parent_tidptr);

// 调用 vfork() 方法,完成相关的初始化工作
if (clone_flags & CLONE_VFORK) {
p->vfork_done = &vfork;
init_completion(&vfork);
get_task_struct(p);
}

// 将新进程加入到调度器中,为其分配 CPU,准备执行
wake_up_new_task(p);

// fork() 完成,子进程开始运行,并让 ptrace 跟踪
/* forking complete and child started to run, tell ptracer */
if (unlikely(trace))
ptrace_event_pid(trace, pid);

// 如果是 vfork(),将父进程加入等待队列,等待子进程完成
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;
}

do_fork() 首先调用 copy_process 将父进程信息复制给子进程,然后调用 vfork() 完成相关的初始化工作,接着调用 wake_up_new_task() 将进程加入调度器中,为之分配 CPU。最后,等待子进程退出。

copy_process():

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;
// 创建进程描述符指针
struct task_struct *p;

// 检查 clone flags 的合法性,比如 CLONE_NEWNS 与 CLONE_FS 是互斥的
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);

/*
* Thread groups must share signals as well, and detached threads
* can only be started up within the thread group.
*/
if ((clone_flags & CLONE_THREAD) && !(clone_flags & CLONE_SIGHAND))
return ERR_PTR(-EINVAL);

/*
* Shared signal handlers imply shared VM. By way of the above,
* thread groups also imply shared VM. Blocking this case allows
* for various simplifications in other code.
*/
if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))
return ERR_PTR(-EINVAL);

/*
* Siblings of global init remain as zombies on exit since they are
* not reaped by their parent (swapper). To solve this and to avoid
* multi-rooted process trees, prevent global and container-inits
* from creating siblings.
*/
// 比如CLONE_PARENT时得检查当前signal flags是否为SIGNAL_UNKILLABLE,防止kill init进程。
if ((clone_flags & CLONE_PARENT) &&
current->signal->flags & SIGNAL_UNKILLABLE)
return ERR_PTR(-EINVAL);

/*
* If the new process will be in a different pid or user namespace
* do not allow it to share a thread group or signal handlers or
* parent with the forking task.
*/
if (clone_flags & CLONE_SIGHAND) {
if ((clone_flags & (CLONE_NEWUSER | CLONE_NEWPID)) ||
(task_active_pid_ns(current) !=
current->nsproxy->pid_ns_for_children))
return ERR_PTR(-EINVAL);
}

retval = security_task_create(clone_flags);
if (retval)
goto fork_out;

retval = -ENOMEM;
// 复制当前的 task_struct
p = dup_task_struct(current);
if (!p)
goto fork_out;

ftrace_graph_init_task(p);

rt_mutex_init_task(p);

#ifdef CONFIG_PROVE_LOCKING
DEBUG_LOCKS_WARN_ON(!p->hardirqs_enabled);
DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);
#endif
retval = -EAGAIN;

// 检查进程是否超过限制,由 OS 定义
if (atomic_read(&p->real_cred->user->processes) >=
task_rlimit(p, RLIMIT_NPROC)) {
if (p->real_cred->user != INIT_USER &&
!capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
goto bad_fork_free;
}
current->flags &= ~PF_NPROC_EXCEEDED;

retval = copy_creds(p, clone_flags);
if (retval < 0)
goto bad_fork_free;

/*
* If multiple threads are within copy_process(), then this check
* triggers too late. This doesn't hurt, the check is only there
* to stop root fork bombs.
*/
retval = -EAGAIN;
// 检查进程数是否超过 max_threads,由内存大小定义
if (nr_threads >= max_threads)
goto bad_fork_cleanup_count;

// ......

// 初始化 io 计数器
task_io_accounting_init(&p->ioac);
acct_clear_integrals(p);

// 初始化 CPU 定时器
posix_cpu_timers_init(p);

// ......

// 初始化进程数据结构,并为进程分配 CPU,进程状态设置为 TASK_RUNNING
/* Perform scheduler related setup. Assign this task to a CPU. */
retval = sched_fork(clone_flags, p);

if (retval)
goto bad_fork_cleanup_policy;

retval = perf_event_init_task(p);
if (retval)
goto bad_fork_cleanup_policy;
retval = audit_alloc(p);
if (retval)
goto bad_fork_cleanup_perf;
/* copy all the process information */
// 复制所有进程信息,包括文件系统,信号处理函数、信号、内存管理等
shm_init_task(p);
retval = copy_semundo(clone_flags, p);
if (retval)
goto bad_fork_cleanup_audit;
retval = copy_files(clone_flags, p);
if (retval)
goto bad_fork_cleanup_semundo;
retval = copy_fs(clone_flags, p);
if (retval)
goto bad_fork_cleanup_files;
retval = copy_sighand(clone_flags, p);
if (retval)
goto bad_fork_cleanup_fs;
retval = copy_signal(clone_flags, p);
if (retval)
goto bad_fork_cleanup_sighand;
retval = copy_mm(clone_flags, p);
if (retval)
goto bad_fork_cleanup_signal;
// !!! 复制 namespace
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;
// 初始化子进程内核栈
retval = copy_thread(clone_flags, stack_start, stack_size, p);
if (retval)
goto bad_fork_cleanup_io;
// 为新进程分配新的 pid
if (pid != &init_struct_pid) {
pid = alloc_pid(p->nsproxy->pid_ns_for_children);
if (IS_ERR(pid)) {
retval = PTR_ERR(pid);
goto bad_fork_cleanup_io;
}
}

// ......

// 返回新进程 p
return p;
}

copy_process 主要分为三步:首先调用 dup_task_struct() 复制当前的进程描述符信息 task_struct,为新进程分配新的堆栈,第二步调用 sched_fork() 初始化进程数据结构,为其分配 CPU,把进程状态设置为 TASK_RUNNING,最后一步就是调用 copy_namespaces() 复制 namesapces。我们重点关注最后一步 copy_namespaces():

/*
* called from clone. This now handles copy for nsproxy and all
* namespaces therein.
*/
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;

if (likely(!(flags & (CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC |
CLONE_NEWPID | CLONE_NEWNET)))) {
get_nsproxy(old_ns);
return 0;
}

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.
*/
if ((flags & (CLONE_NEWIPC | CLONE_SYSVSEM)) ==
(CLONE_NEWIPC | CLONE_SYSVSEM))
return -EINVAL;

new_ns = create_new_namespaces(flags, tsk, user_ns, tsk->fs);
if (IS_ERR(new_ns))
return PTR_ERR(new_ns);

tsk->nsproxy = new_ns;
return 0;
}

可见,copy_namespace() 主要基于“旧的” namespace 创建“新的” namespace,核心函数在于 create_new_namespaces:

/*
* Create new nsproxy and all of its the associated namespaces.
* Return the newly created nsproxy. Do not attach this to the task,
* leave it to the caller to do proper locking and attach it to task.
*/
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;

// 创建新的 nsproxy
new_nsp = create_nsproxy();
if (!new_nsp)
return ERR_PTR(-ENOMEM);

//创建 mnt namespace
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;
}
//创建 uts namespace
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;
}
//创建 ipc namespace
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;
}
//创建 pid namespace
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;
}
//创建 network namespace
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;
// 出错处理
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);
}

在create_new_namespaces()中,分别调用 create_nsproxy(), create_utsname(), create_ipcs(), create_pid_ns(), create_net_ns(), create_mnt_ns() 来创建 nsproxy 结构,uts,ipcs,pid,mnt,net。

具体的函数我们就不再分析,基本到此为止,我们从子进程创建,到子进程相关的信息的初始化,包括文件系统,CPU,内存管理等,再到各个 namespace 的创建,都走了一遍,下面附上 namespace 创建的代码流程图。

具体流程图和更多的细节(包括各个 namespace 的创建过程)大家可以关注我的公众号阅读,那里的阅读体验会更好一些。

PS:对云计算感兴趣的小伙伴可以关注我的微信公众号:aCloudDeveloper,专注云计算领域,坚持分享干货。

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