一.进程控制块PCB-stack_struct
进程在操作系统中都有一个结构，用于表示这个进程。这就是进程控制块(PCB)，在Linux中具体实现是task_struct数据结构，它主要记录了以下信息:
- 状态信息，例如可执行状态、就绪状态、阻塞状态等。
- 性质，由于unix有很多变种，进行有自己独特的性质。
- 资源，资源的链接比如内存，还有资源的限制和权限等。
- 组织，例如按照家族关系建立起来的树(父进程、子进程等)。
task_struct结构体内容非常庞大,暂时没有去分析源代码，以后有时间再去研究。
二.Linux fork执行的过程
在menu中添加一个fork的系统调用，然后用gdb开始调试.执行以下命令
qemu -kernel linux-3.18.6/arch/x86/boot/bzImage -initrd rootfs.img -s -s gdb file linux-3.18.6/vmlinux target remote:1234
然后在sys_fork、sys_clone处设置断点，再逐步调试，观察fork系统调用的执行过程。
具体分析fork系统调用执行过程.
1.fork、vfork和clone三个系统调用都可以创建一个新进程，而且它们都是通过调用do_fork来实现进程的创建，do_fork通过传递不同的clone_flags来实现fork、clone、vfork。
long do_fork(unsigned long clone_flags,
1624 unsigned long stack_start,
1625 unsigned long stack_size,
1626 int __user *parent_tidptr,
1627 int __user *child_tidptr)
1628{
1629 struct task_struct *p;
1630 int trace = 0;
1631 long nr;
1632
1633 /*
1634 * Determine whether and which event to report to ptracer. When
1635 * called from kernel_thread or CLONE_UNTRACED is explicitly
1636 * requested, no event is reported; otherwise, report if the event
1637 * for the type of forking is enabled.
1638 */
1639 if (!(clone_flags & CLONE_UNTRACED)) {
1640 if (clone_flags & CLONE_VFORK)
1641 trace = PTRACE_EVENT_VFORK;
1642 else if ((clone_flags & CSIGNAL) != SIGCHLD)
1643 trace = PTRACE_EVENT_CLONE;
1644 else
1645 trace = PTRACE_EVENT_FORK;
1646
1647 if (likely(!ptrace_event_enabled(current, trace)))
1648 trace = 0;
1649 }
1650
1651 p = copy_process(clone_flags, stack_start, stack_size,
1652 child_tidptr, NULL, trace); #进程复制，核心函数
1653 /*
1654 * Do this prior waking up the new thread - the thread pointer
1655 * might get invalid after that point, if the thread exits quickly.
1656 */
1657 if (!IS_ERR(p)) {
1658 struct completion vfork;
1659 struct pid *pid;
1660
1661 trace_sched_process_fork(current, p);
1662
1663 pid = get_task_pid(p, PIDTYPE_PID);
1664 nr = pid_vnr(pid);
1665
1666 if (clone_flags & CLONE_PARENT_SETTID)
1667 put_user(nr, parent_tidptr);
1668
1669 if (clone_flags & CLONE_VFORK) {
1670 p->vfork_done = &vfork;
1671 init_completion(&vfork);
1672 get_task_struct(p);
1673 }
1674
1675 wake_up_new_task(p);
1676
1677 /* forking complete and child started to run, tell ptracer */
1678 if (unlikely(trace))
1679 ptrace_event_pid(trace, pid);
1680
1681 if (clone_flags & CLONE_VFORK) {
1682 if (!wait_for_vfork_done(p, &vfork))
1683 ptrace_event_pid(PTRACE_EVENT_VFORK_DONE, pid);
1684 }
1685
1686 put_pid(pid);
1687 } else {
1688 nr = PTR_ERR(p);
1689 }
1690 return nr;
1691}
1692
do_fork()函数的核心是copy_process()，该函数完成了进程创建的绝大部分。
/*
1175 * This creates a new process as a copy of the old one,
1176 * but does not actually start it yet.
1177 *
1178 * It copies the registers, and all the appropriate
1179 * parts of the process environment (as per the clone
1180 * flags). The actual kick-off is left to the caller.
1181 */
1182static struct task_struct *copy_process(unsigned long clone_flags,
1183 unsigned long stack_start,
1184 unsigned long stack_size,
1185 int __user *child_tidptr,
1186 struct pid *pid,
1187 int trace)
1188{
1189 int retval;
1190 struct task_struct *p;
1191
1192 if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
1193 return ERR_PTR(-EINVAL);
1194
1195 if ((clone_flags & (CLONE_NEWUSER|CLONE_FS)) == (CLONE_NEWUSER|CLONE_FS))
1196 return ERR_PTR(-EINVAL);
1197
1198 /*
1199 * Thread groups must share signals as well, and detached threads
1200 * can only be started up within the thread group.
1201 */
1202 if ((clone_flags & CLONE_THREAD) && !(clone_flags & CLONE_SIGHAND))
1203 return ERR_PTR(-EINVAL);
1204
1205 /*
1206 * Shared signal handlers imply shared VM. By way of the above,
1207 * thread groups also imply shared VM. Blocking this case allows
1208 * for various simplifications in other code.
1209 */
1210 if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))
1211 return ERR_PTR(-EINVAL);
1212
1213 /*
1214 * Siblings of global init remain as zombies on exit since they are
1215 * not reaped by their parent (swapper). To solve this and to avoid
1216 * multi-rooted process trees, prevent global and container-inits
1217 * from creating siblings.
1218 */
1219 if ((clone_flags & CLONE_PARENT) &&
1220 current->signal->flags & SIGNAL_UNKILLABLE)
1221 return ERR_PTR(-EINVAL);
1222
1223 /*
1224 * If the new process will be in a different pid or user namespace
1225 * do not allow it to share a thread group or signal handlers or
1226 * parent with the forking task.
1227 */
1228 if (clone_flags & CLONE_SIGHAND) {
1229 if ((clone_flags & (CLONE_NEWUSER | CLONE_NEWPID)) ||
1230 (task_active_pid_ns(current) !=
1231 current->nsproxy->pid_ns_for_children))
1232 return ERR_PTR(-EINVAL);
1233 }
1234
1235 retval = security_task_create(clone_flags);
1236 if (retval)
1237 goto fork_out;
1238
1239 retval = -ENOMEM;
1240 p = dup_task_struct(current); #为子进程创建一个新的内核栈,复制task_struct和thread_info结构，此时子进程的进程控制块和父进程完全一致。
1241 if (!p)
1242 goto fork_out;
1243
1244 ftrace_graph_init_task(p);
1245
1246 rt_mutex_init_task(p);
1247
1248#ifdef CONFIG_PROVE_LOCKING
1249 DEBUG_LOCKS_WARN_ON(!p->hardirqs_enabled);
1250 DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);
1251#endif
1252 retval = -EAGAIN;
1253 if (atomic_read(&p->real_cred->user->processes) >=
1254 task_rlimit(p, RLIMIT_NPROC)) {
1255 if (p->real_cred->user != INIT_USER &&
1256 !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
1257 goto bad_fork_free;
1258 }
1259 current->flags &= ~PF_NPROC_EXCEEDED;
1260
1261 retval = copy_creds(p, clone_flags);
1262 if (retval < 0)
1263 goto bad_fork_free;
1264
1265 /*
1266 * If multiple threads are within copy_process(), then this check
1267 * triggers too late. This doesn‘t hurt, the check is only there
1268 * to stop root fork bombs.
1269 */
1270 retval = -EAGAIN;
1271 if (nr_threads >= max_threads)
1272 goto bad_fork_cleanup_count;
1273
1274 if (!try_module_get(task_thread_info(p)->exec_domain->module))
1275 goto bad_fork_cleanup_count;
1276
1277 delayacct_tsk_init(p); /* Must remain after dup_task_struct() */
1278 p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER);
1279 p->flags |= PF_FORKNOEXEC;
1280 INIT_LIST_HEAD(&p->children);
1281 INIT_LIST_HEAD(&p->sibling);
1282 rcu_copy_process(p);
1283 p->vfork_done = NULL;
1284 spin_lock_init(&p->alloc_lock);
1285
1286 init_sigpending(&p->pending);
1287
1288 p->utime = p->stime = p->gtime = 0;
....
通过dup_task_struct()函数，为子进程创建一个新的内核栈,复制task_struct和thread_info结构。
ti=alloc_thread_info_node(task,node); tsk->stack=ti; setup_thread_stack(tsk,orig); //这里只是复制了thread_info
重点关注下,fork()创建子进程后，父进程从系统调用中返回，而子进程从哪开始返回.
这主要是在copy_process()中copy_thread()代码.
int copy_thread(unsigned long clone_flags, unsigned long sp,
133 unsigned long arg, struct task_struct *p)
134{
135 struct pt_regs *childregs = task_pt_regs(p);
136 struct task_struct *tsk;
137 int err;
138
139 p->thread.sp = (unsigned long) childregs; #记录进程切换时的堆栈指针
140 p->thread.sp0 = (unsigned long) (childregs+1);
141 memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));
142
143 if (unlikely(p->flags & PF_KTHREAD)) {
144 /* kernel thread */
145 memset(childregs, 0, sizeof(struct pt_regs));
146 p->thread.ip = (unsigned long) ret_from_kernel_thread;
147 task_user_gs(p) = __KERNEL_STACK_CANARY;
148 childregs->ds = __USER_DS;
149 childregs->es = __USER_DS;
150 childregs->fs = __KERNEL_PERCPU;
151 childregs->bx = sp; /* function */
152 childregs->bp = arg;
153 childregs->orig_ax = -1;
154 childregs->cs = __KERNEL_CS | get_kernel_rpl();
155 childregs->flags = X86_EFLAGS_IF | X86_EFLAGS_FIXED;
156 p->thread.io_bitmap_ptr = NULL;
157 return 0;
158 }
159 *childregs = *current_pt_regs();#复制内核堆栈
160 childregs->ax = 0; #这也是为什么子进程的fork返回0
161 if (sp)
162 childregs->sp = sp;
163
164 p->thread.ip = (unsigned long) ret_from_fork; #子进程开始执行处
165 task_user_gs(p) = get_user_gs(current_pt_regs());
166
167 p->thread.io_bitmap_ptr = NULL;
168 tsk = current;
169 err = -ENOMEM;
170
171 if (unlikely(test_tsk_thread_flag(tsk, TIF_IO_BITMAP))) {
172 p->thread.io_bitmap_ptr = kmemdup(tsk->thread.io_bitmap_ptr,
173 IO_BITMAP_BYTES, GFP_KERNEL);
174 if (!p->thread.io_bitmap_ptr) {
175 p->thread.io_bitmap_max = 0;
176 return -ENOMEM;
177 }
178 set_tsk_thread_flag(p, TIF_IO_BITMAP);
179 }
180
181 err = 0;
182
183 /*
184 * Set a new TLS for the child thread?
185 */
186 if (clone_flags & CLONE_SETTLS)
187 err = do_set_thread_area(p, -1,
188 (struct user_desc __user *)childregs->si, 0);
189
190 if (err && p->thread.io_bitmap_ptr) {
191 kfree(p->thread.io_bitmap_ptr);
192 p->thread.io_bitmap_max = 0;
193 }
194 return err;
}
然后回到do_fork()函数中,唤醒子进程并开始运行。至此，一个进程创建就完成了。
三.实验总结
中间虽然的很多细节还不是很清楚，但是对linux 创建子进程的大体流程有了一个宏观的认识，更加深刻地理解了底层Linux 内核进程运行的机制。
时间： 2024-10-10 10:55:52