do_fork-->copy_process（二）

不褪色 · 发表于 2024-1-5 19:57:11

1 /*
2 * This creates a new process as a copy of the old one,
3 * but does not actually start it yet.
4 *
5 * It copies the registers, and all the appropriate
6 * parts of the process environment (as per the clone
7 * flags). The actual kick-off is left to the caller.
8 */
9 struct task_struct *copy_process(unsigned long clone_flags,
10 unsigned long stack_start,
11 struct pt_regs *regs,
12 unsigned long stack_size,
13 int __user *parent_tidptr,
14 int __user *child_tidptr)
15 {
16 int retval;
17 struct task_struct *p = NULL;
18
19 if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
20 return ERR_PTR(-EINVAL);
21
22 /*
23 * Thread groups must share signals as well, and detached threads
24 * can only be started up within the thread group.
25 */
26 if ((clone_flags & CLONE_THREAD) && !(clone_flags & CLONE_SIGHAND))
27 return ERR_PTR(-EINVAL);
28
29 /*
30 * Shared signal handlers imply shared VM. By way of the above,
31 * thread groups also imply shared VM. Blocking this case allows
32 * for various simplifications in other code.
33 */
34 if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))
35 return ERR_PTR(-EINVAL);
36
37 /*
38 * CLONE_DETACHED must match CLONE_THREAD: it's a historical
39 * thing.
40 */
41 if (!(clone_flags & CLONE_DETACHED) != !(clone_flags & CLONE_THREAD)) {
42 /* Warn about the old no longer supported case so that we see it */
43 if (clone_flags & CLONE_THREAD) {
44 static int count;
45 if (count < 5) {
46 count++;
47 printk(KERN_WARNING "%s trying to use CLONE_THREAD without CLONE_DETACH\n", current->comm);
48 }
49 }
50 return ERR_PTR(-EINVAL);
51 }
52
53 retval = security_task_create(clone_flags);
54 if (retval)
55 goto fork_out;
56
57 retval = -ENOMEM;
58 p = dup_task_struct(current);
59 if (!p)
60 goto fork_out;
61
62 retval = -EAGAIN;
63 if (atomic_read(&p->user->processes) >=
64 p->rlim[RLIMIT_NPROC].rlim_cur) {
65 if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_RESOURCE) &&
66 p->user != &root_user)
67 goto bad_fork_free;
68 }
69
70 atomic_inc(&p->user->__count);
71 atomic_inc(&p->user->processes);
72
73 /*
74 * If multiple threads are within copy_process(), then this check
75 * triggers too late. This doesn't hurt, the check is only there
76 * to stop root fork bombs.
77 */
78 if (nr_threads >= max_threads)
79 goto bad_fork_cleanup_count;
80
81 if (!try_module_get(p->thread_info->exec_domain->module))
82 goto bad_fork_cleanup_count;
83
84 if (p->binfmt && !try_module_get(p->binfmt->module))
85 goto bad_fork_cleanup_put_domain;
86
87 #ifdef CONFIG_PREEMPT
88 /*
89 * schedule_tail drops this_rq()->lock so we compensate with a count
90 * of 1. Also, we want to start with kernel preemption disabled.
91 */
92 p->thread_info->preempt_count = 1;
93 #endif
94 p->did_exec = 0;
95 p->state = TASK_UNINTERRUPTIBLE;
96
97 copy_flags(clone_flags, p);
98 if (clone_flags & CLONE_IDLETASK)
99 p->pid = 0;
100 else {
101 p->pid = alloc_pidmap();
102 if (p->pid == -1)
103 goto bad_fork_cleanup;
104 }
105 retval = -EFAULT;
106 if (clone_flags & CLONE_PARENT_SETTID)
107 if (put_user(p->pid, parent_tidptr))
108 goto bad_fork_cleanup;
109
110 p->proc_dentry = NULL;
111
112 INIT_LIST_HEAD(&p->run_list);
113
114 INIT_LIST_HEAD(&p->children);
115 INIT_LIST_HEAD(&p->sibling);
116 INIT_LIST_HEAD(&p->posix_timers);
117 init_waitqueue_head(&p->wait_chldexit);
118 p->vfork_done = NULL;
119 spin_lock_init(&p->alloc_lock);
120 spin_lock_init(&p->switch_lock);
121 spin_lock_init(&p->proc_lock);
122
123 clear_tsk_thread_flag(p, TIF_SIGPENDING);
124 init_sigpending(&p->pending);
125
126 p->it_real_value = p->it_virt_value = p->it_prof_value = 0;
127 p->it_real_incr = p->it_virt_incr = p->it_prof_incr = 0;
128 init_timer(&p->real_timer);
129 p->real_timer.data = (unsigned long) p;
130
131 p->leader = 0; /* session leadership doesn't inherit */
132 p->tty_old_pgrp = 0;
133 p->utime = p->stime = 0;
134 p->cutime = p->cstime = 0;
135 p->array = NULL;
136 p->lock_depth = -1; /* -1 = no lock */
137 p->start_time = get_jiffies_64();
138 p->security = NULL;
139 p->io_context = NULL;
140
141 retval = -ENOMEM;
142 if ((retval = security_task_alloc(p)))
143 goto bad_fork_cleanup;
144 /* copy all the process information */
145 if ((retval = copy_semundo(clone_flags, p)))
146 goto bad_fork_cleanup_security;
147 if ((retval = copy_files(clone_flags, p)))
148 goto bad_fork_cleanup_semundo;
149 if ((retval = copy_fs(clone_flags, p)))
150 goto bad_fork_cleanup_files;
151 if ((retval = copy_sighand(clone_flags, p)))
152 goto bad_fork_cleanup_fs;
153 if ((retval = copy_signal(clone_flags, p)))
154 goto bad_fork_cleanup_sighand;
155 if ((retval = copy_mm(clone_flags, p)))
156 goto bad_fork_cleanup_signal;
157 if ((retval = copy_namespace(clone_flags, p)))
158 goto bad_fork_cleanup_mm;
159 retval = copy_thread(0, clone_flags, stack_start, stack_size, p, regs);
160 if (retval)
161 goto bad_fork_cleanup_namespace;
162
163 p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL;
164 /*
165 * Clear TID on mm_release()?
166 */
167 p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr: NULL;
168
169 /*
170 * Syscall tracing should be turned off in the child regardless
171 * of CLONE_PTRACE.
172 */
173 clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE);
174
175 /* Our parent execution domain becomes current domain
176 These must match for thread signalling to apply */
177
178 p->parent_exec_id = p->self_exec_id;
179
180 /* ok, now we should be set up.. */
181 p->exit_signal = (clone_flags & CLONE_THREAD) ? -1 : (clone_flags & CSIGNAL);
182 p->pdeath_signal = 0;
183
184 /*
185 * Share the timeslice between parent and child, thus the
186 * total amount of pending timeslices in the system doesn't change,
187 * resulting in more scheduling fairness.
188 */
189 local_irq_disable();
190 p->time_slice = (current->time_slice + 1) >> 1;
191 /*
192 * The remainder of the first timeslice might be recovered by
193 * the parent if the child exits early enough.
194 */
195 p->first_time_slice = 1;
196 current->time_slice >>= 1;
197 p->timestamp = sched_clock();
198 if (!current->time_slice) {
199 /*
200 * This case is rare, it happens when the parent has only
201 * a single jiffy left from its timeslice. Taking the
202 * runqueue lock is not a problem.
203 */
204 current->time_slice = 1;
205 preempt_disable();
206 scheduler_tick(0, 0);
207 local_irq_enable();
208 preempt_enable();
209 } else
210 local_irq_enable();
211 /*
212 * Ok, add it to the run-queues and make it
213 * visible to the rest of the system.
214 *
215 * Let it rip!
216 */
217 p->tgid = p->pid;
218 p->group_leader = p;
219 INIT_LIST_HEAD(&p->ptrace_children);
220 INIT_LIST_HEAD(&p->ptrace_list);
221
222 /* Need tasklist lock for parent etc handling! */
223 write_lock_irq(&tasklist_lock);
224 /*
225 * Check for pending SIGKILL! The new thread should not be allowed
226 * to slip out of an OOM kill. (or normal SIGKILL.)
227 */
228 if (sigismember(&current->pending.signal, SIGKILL)) {
229 write_unlock_irq(&tasklist_lock);
230 retval = -EINTR;
231 goto bad_fork_cleanup_namespace;
232 }
233
234 /* CLONE_PARENT re-uses the old parent */
235 if (clone_flags & CLONE_PARENT)
236 p->real_parent = current->real_parent;
237 else
238 p->real_parent = current;
239 p->parent = p->real_parent;
240
241 if (clone_flags & CLONE_THREAD) {
242 spin_lock(&current->sighand->siglock);
243 /*
244 * Important: if an exit-all has been started then
245 * do not create this new thread - the whole thread
246 * group is supposed to exit anyway.
247 */
248 if (current->signal->group_exit) {
249 spin_unlock(&current->sighand->siglock);
250 write_unlock_irq(&tasklist_lock);
251 retval = -EAGAIN;
252 goto bad_fork_cleanup_namespace;
253 }
254 p->tgid = current->tgid;
255 p->group_leader = current->group_leader;
256
257 if (current->signal->group_stop_count > 0) {
258 /*
259 * There is an all-stop in progress for the group.
260 * We ourselves will stop as soon as we check signals.
261 * Make the new thread part of that group stop too.
262 */
263 current->signal->group_stop_count++;
264 set_tsk_thread_flag(p, TIF_SIGPENDING);
265 }
266
267 spin_unlock(&current->sighand->siglock);
268 }
269
270 SET_LINKS(p);
271 if (p->ptrace & PT_PTRACED)
272 __ptrace_link(p, current->parent);
273
274 attach_pid(p, PIDTYPE_PID, p->pid);
275 if (thread_group_leader(p)) {
276 attach_pid(p, PIDTYPE_TGID, p->tgid);
277 attach_pid(p, PIDTYPE_PGID, process_group(p));
278 attach_pid(p, PIDTYPE_SID, p->session);
279 if (p->pid)
280 __get_cpu_var(process_counts)++;
281 } else
282 link_pid(p, p->pids + PIDTYPE_TGID, &p->group_leader->pids[PIDTYPE_TGID].pid);
283
284 nr_threads++;
285 write_unlock_irq(&tasklist_lock);
286 retval = 0;
287
288 fork_out:
289 if (retval)
290 return ERR_PTR(retval);
291 return p;
292
293 bad_fork_cleanup_namespace:
294 exit_namespace(p);
295 bad_fork_cleanup_mm:
296 exit_mm(p);
297 bad_fork_cleanup_signal:
298 exit_signal(p);
299 bad_fork_cleanup_sighand:
300 exit_sighand(p);
301 bad_fork_cleanup_fs:
302 exit_fs(p); /* blocking */
303 bad_fork_cleanup_files:
304 exit_files(p); /* blocking */
305 bad_fork_cleanup_semundo:
306 exit_sem(p);
307 bad_fork_cleanup_security:
308 security_task_free(p);
309 bad_fork_cleanup:
310 if (p->pid > 0)
311 free_pidmap(p->pid);
312 if (p->binfmt)
313 module_put(p->binfmt->module);
314 bad_fork_cleanup_put_domain:
315 module_put(p->thread_info->exec_domain->module);
316 bad_fork_cleanup_count:
317 atomic_dec(&p->user->processes);
318 free_uid(p->user);
319 bad_fork_free:
320 free_task(p);
321 goto fork_out;
322 }

复制代码

17行：struct task_struct 结构体包含了进程相关的所有属性和信息（也叫进程控制块， Process Control Block， PCB）。包含：进程属性相关信息，进程间关系，进程调度信息，内存管理信息，文件管理信息，信号处理相关信息，资源限制相关信息。
19,26,34,41行：检查flags标记位, clone_flags 是在调用do_fork时的入参，不同的函数调用，参数不同。（通常对应的是不同的系统调用，fork，vfork）

53行：安全性检查，询问Linux Security Moudule(LSM)看当前任务是否可以创建一个新任务。
58行：为进程分配物理页面。其中调用（alloc_task_struct，alloc_thread_info再调用__get_free_pages申请物理页面）
63,64行：检查进程资源限制。user指针指向user_struct 结构体，一共用户通常有多个进程，共享一个结构体。rlim指向资源限制结构体。

97行：复制flags, CLONE_IDLETASK代表0号进程。如果不是0号进程，申请pid。
101行：pid循环使用，使用pid位图来管理。默认pid最大值是32767，在64位系统中，用户可以通过写/proc/sys/kernel/pid_max文件，扩展到4194303。
142行：LSM Linux安全模块（后续学习）
145行：复制IPC信息。通过get_undo_list申请IPC结构体内存空间（是一个链表，并将链表放入undo_list中，将支持ipc的进程链接到一起。不支持ipc则设置为NULL.）

147行：复制已打开文件的控制结构，只有在CLONE_FILES标记位为0时才进行，否则共享父进程的结构。共享和复制的区别在于，如果是共享，子进程对文件操作会影响到父进程（比如lseek()）。
149行：复制进程目录，权限等信息。（与copy_files() 类似）
151,153行：复制信号相关的数据。
155行：复制内存相关的数据。（内存相关的比较复杂，后续需要深究。）
157行：复制命名空间。（参考：https://cloud.tencent.com/developer/article/2129136）
159行：拷贝进程堆栈。
163,167行：set_child_tid 指向子进程的pid.当新进程执行时，将该进程pid。

178行：parent_exec_id 是父进程的执行域， self_exec_id 是本进程的执行域。
181,182行：exit_signal 是当前进程退出时向父进程发出的信号，pdeath_signal是父进程退出时，向子进程发出的信号。
190,196行：time_slice 是时间片。将当前进程的时间片分成两份，一份给当前进程，一份给子进程。
197行：获取进程时间戳。
217-220行：将进程链接到一起，加入到进程队列中。
235-239行：设置父进程，考虑到被调试的情况，需要parent 和 real_parent。
270行：将子进程的task_struct 链入到内核的进程队列中。
274-282行：处理进程关系（还没搞清楚）

来源:https://www.cnblogs.com/atest/p/17941382
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do_fork-->copy_process（二）

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