Linux内核分析：实验六

安常青   原创作品转载请注明出处    《Linux内核分析》MOOC课程http://mooc.study.163.com/course/USTC-1000029000 

1.什么是进程

所谓进程，就是一个程序的一个运行的动态实体，每个进程都有自己的进程控制块，里面记录了进程的各种信息，并通过pid唯一的标识。操作系统就是通过获取和改变进程控制块里面的成员变量，从而控制进程的运行。

2.进程的创建

进程可以通过多个系统调用进行创建，fork，vfork，clone都可以创建一个进程，但它们本质上都调用do_fork来实现的。而do_fork主要调用了copy_process来创建一个新的进程。

copy_process主要完成了以下几部分：

1.调用dup_task_struct复制当前的task_struct

2.调用sched_fork初始化数据结构，并把进程状态设置为TASK_RUNNING

3.复制父进程的所有信息

4.调用copy_thread初始化子进程的内核栈

5.为新的进程分配并设置行的pid

下面是copy_process的代码：

[cpp]  view plain  copy

 print ?

 /*
  * This creates a new process as a copy of the old one,
  * but does not actually start it yet.
  *
  * It copies the registers, and all the appropriate
  * parts of the process environment (as per the clone
  * flags). The actual kick-off is left to the caller.
  */
 static struct task_struct *copy_process(unsigned long clone_flags,
                     unsigned long stack_start,
                     struct pt_regs *regs,
                     unsigned long stack_size,
                     int __user *child_tidptr,
                     struct pid *pid,
                     int trace)
 {
     int retval;
     struct task_struct *p;
     int cgroup_callbacks_done = 0;
     /*下面为参数有效性检查*/
     if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|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.
      */
     if ((clone_flags & CLONE_PARENT) &&
                 current->signal->flags & SIGNAL_UNKILLABLE)
         return ERR_PTR(-EINVAL);
     /*Check permission before creating a child process.  See the clone(2)
  *  manual page for definitions of the @clone_flags.
  *  @clone_flags contains the flags indicating what should be shared.
  *  Return 0 if permission is granted.*/
     retval = security_task_create(clone_flags);
     if (retval)
         goto fork_out;

     retval = -ENOMEM;
     /*为新进程创建一个内核栈、thread_iofo和task_struct,
     这里完全copy父进程的内容，所以到目前为止，
     父进程和子进程是没有任何区别的。,从这里也
     可以看出，这个版本的内核在创建时需要为自己
     分配内核栈*/
     p = dup_task_struct(current);
     if (!p)
         goto fork_out;
     /*task结构中ftrace_ret_stack结构变量的初始化，即函数
     返回用的栈*/
     ftrace_graph_init_task(p);
     /*task中互斥变量的初始化*/
     rt_mutex_init_task(p);

 #ifdef CONFIG_PROVE_LOCKING/*not set*/
     DEBUG_LOCKS_WARN_ON(!p->hardirqs_enabled);
     DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);
 #endif
     retval = -EAGAIN;
     /*首先看前面的两个if，第一个if里面的rlim数组包含在task_sturct数组中。
     对进程占用的资源数做出限制，rlim[RLIMIT_NPROC]限制了改进程用户
     可以拥有的总进程数量，如果当前用户所拥有的进程数量超过了
     规定的最大拥有进程数量，在内核中就直接goto bad_fork_free了。
     第2个if使用了capable（）函数来对权限做出检查，检查是否有权
     对指定的资源进行操作，该函数返回0则代表无权操作。*/
     if (atomic_read(&p->real_cred->user->processes) >=
             p->signal->rlim[RLIMIT_NPROC].rlim_cur) {
         if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_RESOURCE) &&
             p->real_cred->user != INIT_USER)
             goto bad_fork_free;
     }
     /*Copy credentials for the new process*/
     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;
     /*检查创建的进程是否超过了系统进程总量*/
     if (nr_threads >= max_threads)
         goto bad_fork_cleanup_count;

     if (!try_module_get(task_thread_info(p)->exec_domain->module))
         goto bad_fork_cleanup_count;

     p->did_exec = 0;
     delayacct_tsk_init(p);  /* Must remain after dup_task_struct() */
     /*更新task_struct结构中flags成员*/
     copy_flags(clone_flags, p);
     INIT_LIST_HEAD(&p->children);
     INIT_LIST_HEAD(&p->sibling);
     rcu_copy_process(p);/*rcu相关变量的初始化*/
     p->vfork_done = NULL;
     spin_lock_init(&p->alloc_lock);

     init_sigpending(&p->pending);

     p->utime = cputime_zero;
     p->stime = cputime_zero;
     p->gtime = cputime_zero;
     p->utimescaled = cputime_zero;
     p->stimescaled = cputime_zero;
     p->prev_utime = cputime_zero;
     p->prev_stime = cputime_zero;

     p->default_timer_slack_ns = current->timer_slack_ns;

     task_io_accounting_init(&p->ioac);/*task中io数据记录的初始化*/
     acct_clear_integrals(p);/*clear the mm integral fields in task_struct*/

     posix_cpu_timers_init(p);/*timer初始化*/

     p->lock_depth = -1;      /* -1 = no lock */
     do_posix_clock_monotonic_gettime(&p->start_time);
     p->real_start_time = p->start_time;
     monotonic_to_bootbased(&p->real_start_time);
     p->io_context = NULL;
     p->audit_context = NULL;
     cgroup_fork(p);/*attach newly forked task to its parents cgroup.*/
 #ifdef CONFIG_NUMA/*not set*/
     p->mempolicy = mpol_dup(p->mempolicy);
     if (IS_ERR(p->mempolicy)) {
         retval = PTR_ERR(p->mempolicy);
         p->mempolicy = NULL;
         goto bad_fork_cleanup_cgroup;
     }
     mpol_fix_fork_child_flag(p);
 #endif
 #ifdef CONFIG_TRACE_IRQFLAGS/*yes*/
     p->irq_events = 0;
 #ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW/*not found*/
     p->hardirqs_enabled = 1;
 #else
     p->hardirqs_enabled = 0;
 #endif
     p->hardirq_enable_ip = 0;
     p->hardirq_enable_event = 0;
     p->hardirq_disable_ip = _THIS_IP_;
     p->hardirq_disable_event = 0;
     p->softirqs_enabled = 1;
     p->softirq_enable_ip = _THIS_IP_;
     p->softirq_enable_event = 0;
     p->softirq_disable_ip = 0;
     p->softirq_disable_event = 0;
     p->hardirq_context = 0;
     p->softirq_context = 0;
 #endif
 #ifdef CONFIG_LOCKDEP/*yes*/
     p->lockdep_depth = 0; /* no locks held yet */
     p->curr_chain_key = 0;
     p->lockdep_recursion = 0;
 #endif

 #ifdef CONFIG_DEBUG_MUTEXES/*not set */
     p->blocked_on = NULL; /* not blocked yet */
 #endif

     p->bts = NULL;

     p->stack_start = stack_start;

     /* Perform scheduler related setup. Assign this task to a CPU. */
     sched_fork(p, clone_flags);/*调度相关初始化*/
     /*
     * Initialize the perf_event context in task_struct
     */
     retval = perf_event_init_task(p);
     if (retval)
         goto bad_fork_cleanup_policy;

     if ((retval = audit_alloc(p)))
         goto bad_fork_cleanup_policy;

     /*下面的操作中，根据flag中是否设置了相关标志
     进行重新分配或者共享父进程的内容*/
     /* copy all the process information */
     if ((retval = copy_semundo(clone_flags, p)))
         goto bad_fork_cleanup_audit;
     if ((retval = copy_files(clone_flags, p)))
         goto bad_fork_cleanup_semundo;
     if ((retval = copy_fs(clone_flags, p)))
         goto bad_fork_cleanup_files;
     if ((retval = copy_sighand(clone_flags, p)))
         goto bad_fork_cleanup_fs;
     if ((retval = copy_signal(clone_flags, p)))
         goto bad_fork_cleanup_sighand;
     if ((retval = copy_mm(clone_flags, p)))
         goto bad_fork_cleanup_signal;
     if ((retval = copy_namespaces(clone_flags, p)))
         goto bad_fork_cleanup_mm;
     if ((retval = copy_io(clone_flags, p)))
         goto bad_fork_cleanup_namespaces;
     retval = copy_thread(clone_flags, stack_start, stack_size, p, regs);
     if (retval)
         goto bad_fork_cleanup_io;

     if (pid != &init_struct_pid) {
         retval = -ENOMEM;
         pid = alloc_pid(p->nsproxy->pid_ns);
         if (!pid)
             goto bad_fork_cleanup_io;

         if (clone_flags & CLONE_NEWPID) {
             retval = pid_ns_prepare_proc(p->nsproxy->pid_ns);
             if (retval < 0)
                 goto bad_fork_free_pid;
         }
     }

     p->pid = pid_nr(pid);
     p->tgid = p->pid;
     /*如果设置了同在一个线程组则继承TGID。
     对于普通进程来说TGID和PID相等，
     对于线程来说，同一线程组内的所有线程的TGID都相等，
     这使得这些多线程可以通过调用getpid()获得相同的PID。*/
     if (clone_flags & CLONE_THREAD)
         p->tgid = current->tgid;
     /*如果命名空间不相等，clone the cgroup the given subsystem is attached to*/
     if (current->nsproxy != p->nsproxy) {
         retval = ns_cgroup_clone(p, pid);
         if (retval)
             goto bad_fork_free_pid;
     }
     /*如果设置了CLONE_CHILD_SETTID,将变量设置为参数*/
     p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
     /*
      * Clear TID on mm_release()?
      */
     p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr: NULL;
 #ifdef CONFIG_FUTEX/*yes*/
     p->robust_list = NULL;
 #ifdef CONFIG_COMPAT/*not found*/
     p->compat_robust_list = NULL;
 #endif
     INIT_LIST_HEAD(&p->pi_state_list);
     p->pi_state_cache = NULL;
 #endif
     /*
      * sigaltstack should be cleared when sharing the same VM
      */
     if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM)
         p->sas_ss_sp = p->sas_ss_size = 0;

     /*
      * Syscall tracing should be turned off in the child regardless
      * of CLONE_PTRACE.
      */
     clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE);
 #ifdef TIF_SYSCALL_EMU/*not found*/
     clear_tsk_thread_flag(p, TIF_SYSCALL_EMU);
 #endif
     clear_all_latency_tracing(p);

     /* ok, now we should be set up.. */
     p->exit_signal = (clone_flags & CLONE_THREAD) ? -1 : (clone_flags & CSIGNAL);
     p->pdeath_signal = 0;
     p->exit_state = 0;

     /*
      * Ok, make it visible to the rest of the system.
      * We dont wake it up yet.
      */
     p->group_leader = p;
     INIT_LIST_HEAD(&p->thread_group);

     /* Now that the task is set up, run cgroup callbacks if
      * necessary. We need to run them before the task is visible
      * on the tasklist. */
     cgroup_fork_callbacks(p);
     cgroup_callbacks_done = 1;

     /* Need tasklist lock for parent etc handling! */
     write_lock_irq(&tasklist_lock);

     /*
      * The task hasn't been attached yet, so its cpus_allowed mask will
      * not be changed, nor will its assigned CPU.
      *
      * The cpus_allowed mask of the parent may have changed after it was
      * copied first time - so re-copy it here, then check the child's CPU
      * to ensure it is on a valid CPU (and if not, just force it back to
      * parent's CPU). This avoids alot of nasty races.
      */
     p->cpus_allowed = current->cpus_allowed;
     p->rt.nr_cpus_allowed = current->rt.nr_cpus_allowed;
     if (unlikely(!cpu_isset(task_cpu(p), p->cpus_allowed) ||
             !cpu_online(task_cpu(p))))
         set_task_cpu(p, smp_processor_id());

     /* CLONE_PARENT re-uses the old parent */
     /*如果这两个标志设定了，那么和父进程
     有相同的父进程*/
     if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
         p->real_parent = current->real_parent;
         p->parent_exec_id = current->parent_exec_id;
     } else {/*父进程为实际父进程*/
         p->real_parent = current;
         p->parent_exec_id = current->self_exec_id;
     }

     spin_lock(¤t->sighand->siglock);

     /*
      * Process group and session signals need to be delivered to just the
      * parent before the fork or both the parent and the child after the
      * fork. Restart if a signal comes in before we add the new process to
      * it's process group.
      * A fatal signal pending means that current will exit, so the new
      * thread can't slip out of an OOM kill (or normal SIGKILL).
      */
     recalc_sigpending();
     if (signal_pending(current)) {
         spin_unlock(¤t->sighand->siglock);
         write_unlock_irq(&tasklist_lock);
         retval = -ERESTARTNOINTR;
         goto bad_fork_free_pid;
     }
     /*如果和父进程有相同的线程组*/
     if (clone_flags & CLONE_THREAD) {
         atomic_inc(¤t->signal->count);
         atomic_inc(¤t->signal->live);
         p->group_leader = current->group_leader;/*和父进程相同的组领导*/
         /*将进程加入组leader的链表尾部*/
         list_add_tail_rcu(&p->thread_group, &p->group_leader->thread_group);
     }

     if (likely(p->pid)) {
         /*保持进程应有的关系*/
         list_add_tail(&p->sibling, &p->real_parent->children);
         /*ptrace的相关初始化*/
         tracehook_finish_clone(p, clone_flags, trace);

         if (thread_group_leader(p)) {/*如果进程p时线程组leader*/
             if (clone_flags & CLONE_NEWPID)
                 /*如果是新创建的pid命名空间，那么将
                 命名空间的child_reaper变量置为p,从这里看出
                 这个变量设置为创建者进程控制块*/
                 p->nsproxy->pid_ns->child_reaper = p;

             p->signal->leader_pid = pid;
             tty_kref_put(p->signal->tty);/*释放tty引用*/
             p->signal->tty = tty_kref_get(current->signal->tty);/*得到tty引用*/
             /*下面两行为加入对应的pid哈希表，关于pid
             的组织参考前一篇文章*/
             attach_pid(p, PIDTYPE_PGID, task_pgrp(current));
             attach_pid(p, PIDTYPE_SID, task_session(current));
             list_add_tail_rcu(&p->tasks, &init_task.tasks);/*加入队列*/
             __get_cpu_var(process_counts)++;/*将per cpu变量加一*/
         }
         attach_pid(p, PIDTYPE_PID, pid);/*维护pid变量*/
         nr_threads++;/*线程数加一*/
     }

     total_forks++;
     spin_unlock(¤t->sighand->siglock);
     write_unlock_irq(&tasklist_lock);
     proc_fork_connector(p);
     /*Adds the task to the list running through its css_set if necessary.*/
     cgroup_post_fork(p);
     perf_event_fork(p);
     return p;

 bad_fork_free_pid:
     if (pid != &init_struct_pid)
         free_pid(pid);
 bad_fork_cleanup_io:
     put_io_context(p->io_context);
 bad_fork_cleanup_namespaces:
     exit_task_namespaces(p);
 bad_fork_cleanup_mm:
     if (p->mm)
         mmput(p->mm);
 bad_fork_cleanup_signal:
     if (!(clone_flags & CLONE_THREAD))
         __cleanup_signal(p->signal);
 bad_fork_cleanup_sighand:
     __cleanup_sighand(p->sighand);
 bad_fork_cleanup_fs:
     exit_fs(p); /* blocking */
 bad_fork_cleanup_files:
     exit_files(p); /* blocking */
 bad_fork_cleanup_semundo:
     exit_sem(p);
 bad_fork_cleanup_audit:
     audit_free(p);
 bad_fork_cleanup_policy:
     perf_event_free_task(p);
 #ifdef CONFIG_NUMA
     mpol_put(p->mempolicy);
 bad_fork_cleanup_cgroup:
 #endif
     cgroup_exit(p, cgroup_callbacks_done);
     delayacct_tsk_free(p);
     module_put(task_thread_info(p)->exec_domain->module);
 bad_fork_cleanup_count:
     atomic_dec(&p->cred->user->processes);
     exit_creds(p);
 bad_fork_free:
     free_task(p);
 fork_out:
     return ERR_PTR(retval);
 }

子进程创建后，我们就有两个问题，子进程的返回值为什么是0？子进程从哪里开始执行？

其实，这两个问题在copy_thread中解决了。

在copy_thread中，childregs->ax=0，把子进程的eax赋值为0，这样，子进程的返回值就是0.

在copy_thread中，p->thread.ip=(unsigned long)ret_from_fork，将子进程的ip设为ret_from_fork的首地址，因此子进程是从ret_form_fork执行的。

代码如下：

int copy_thread(int nr, unsigned long clone_flags, unsigned long esp,
 unsigned long unused,
 struct task_struct * p, struct pt_regs * regs)
{
 struct pt_regs * childregs;
 childregs = ((struct pt_regs *) (THREAD_SIZE + (unsigned long) p)) - 1;
 struct_cpy(childregs, regs);
 childregs->eax = 0;
 childregs->esp = esp;
 p->thread.esp = (unsigned long) childregs;
 p->thread.esp0 = (unsigned long) (childregs+1);
 p->thread.eip = (unsigned long) ret_from_fork;
 savesegment(fs,p->thread.fs);
 savesegment(gs,p->thread.gs);
 unlazy_fpu(current);
 struct_cpy(&p->thread.i387, ¤t->thread.i387);
 return 0;
}

3.总结

通过这次实验，对进程和进程的创建有了更深的理解。