fork 系统调用的执行过程与调试
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casualet + 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 ” 前言: 在linux中，我们可以通过fork系统调用来处理进程创建的任务。对于进程的创建， 可以sys_clone, sys_vfork，以及sys_fork. 这些系统调用的内部都使用了do_fork.函数。对于do_fork函数， 会copy tast_struct， 设置内核堆栈， 并且对一些特定的数据结构进行修改。其中里面还有copy_thread 函数， 会设置这个进程的cs和ip。这个是在进程的thread_info中保持的。这里的ip设置成了ret_from_fork函数（在ret_from_frok里面有一个jmp system_exit). 后面，fork系统调用本身可以进入到之前系统调用的部分讲的system_exit部分。 这样fork 系统调用在这里就会有一个进程调度的时机。schedule 对比自己写的多道时间片轮转的问题，进程调度大致的流程是，找stask_struct链表， 找里面可以用的进程，找到以后， 找里面保持的ip， 这里就是刚才设置的ret_from_fork, 从这里开始， jmp 到system_exit， 就可以ret restore_all， 恢复到父进程那个位置的代码，开始执行。 |
下面展示调试的基本：
首先，进入menuos， 然后开始设置一下断点:
然后，我们运行fork命令，发现fork命令停止到sys_clone断点：
这个过程是中断处理过程， 我们调用的fork系统函数， 通过int 0x80 陷入内核。 在sys_call_table中， 通过eax传递参数，获得中断向量表中的对应的中断处理函数，并且开始执行。 执行到sys_clone函数。 我们可以
看到，sys_clone代码在fork.c中。
1724SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
1725 int __user *, parent_tidptr,
1726 int, tls_val,
1727 int __user *, child_tidptr)
1728#elif defined(CONFIG_CLONE_BACKWARDS2)
1729SYSCALL_DEFINE5(clone, unsigned long, newsp, unsigned long, clone_flags,
1730 int __user *, parent_tidptr,
1731 int __user *, child_tidptr,
1732 int, tls_val)
1733#elif defined(CONFIG_CLONE_BACKWARDS3)
1734SYSCALL_DEFINE6(clone, unsigned long, clone_flags, unsigned long, newsp,
1735 int, stack_size,
1736 int __user *, parent_tidptr,
1737 int __user *, child_tidptr,
1738 int, tls_val)
1739#else
1740SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
1741 int __user *, parent_tidptr,
1742 int __user *, child_tidptr,
1743 int, tls_val)
1744#endif
1745{
1746 return do_fork(clone_flags, newsp, 0, parent_tidptr, child_tidptr);
1747}
1748#endif
我们输入next，就开始进入do_fork断点，然后我们输入step进入do_fork函数。 对于创建进程的几个函数， 其最后的实际工作都是通过do_fork函数来完成的，所以我们现在重点关注一下这个函数。
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); 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}
这个函数会进行一些pcb的拷贝工作。 我们主要关注里面的copy_process函数:这个函数会进行一些实际的内容拷贝工作。
static 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); 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; 1289 p->utimescaled = p->stimescaled = 0; 1290#ifndef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE 1291 p->prev_cputime.utime = p->prev_cputime.stime = 0; 1292#endif 1293#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN 1294 seqlock_init(&p->vtime_seqlock); 1295 p->vtime_snap = 0; 1296 p->vtime_snap_whence = VTIME_SLEEPING; 1297#endif 1298 1299#if defined(SPLIT_RSS_COUNTING) 1300 memset(&p->rss_stat, 0, sizeof(p->rss_stat)); 1301#endif 1302 1303 p->default_timer_slack_ns = current->timer_slack_ns; 1304 1305 task_io_accounting_init(&p->ioac); 1306 acct_clear_integrals(p); 1307 1308 posix_cpu_timers_init(p); 1309 1310 p->start_time = ktime_get_ns(); //设置了一些和时间相关的数量 1311 p->real_start_time = ktime_get_boot_ns(); 1312 p->io_context = NULL; 1313 p->audit_context = NULL; 1314 if (clone_flags & CLONE_THREAD) 1315 threadgroup_change_begin(current); 1316 cgroup_fork(p); 1317#ifdef CONFIG_NUMA 1318 p->mempolicy = mpol_dup(p->mempolicy); 1319 if (IS_ERR(p->mempolicy)) { 1320 retval = PTR_ERR(p->mempolicy); 1321 p->mempolicy = NULL; 1322 goto bad_fork_cleanup_threadgroup_lock; 1323 } 1324#endif 1325#ifdef CONFIG_CPUSETS 1326 p->cpuset_mem_spread_rotor = NUMA_NO_NODE; 1327 p->cpuset_slab_spread_rotor = NUMA_NO_NODE; 1328 seqcount_init(&p->mems_allowed_seq); 1329#endif 1330#ifdef CONFIG_TRACE_IRQFLAGS 1331 p->irq_events = 0; 1332 p->hardirqs_enabled = 0; 1333 p->hardirq_enable_ip = 0; 1334 p->hardirq_enable_event = 0; 1335 p->hardirq_disable_ip = _THIS_IP_; 1336 p->hardirq_disable_event = 0; 1337 p->softirqs_enabled = 1; 1338 p->softirq_enable_ip = _THIS_IP_; 1339 p->softirq_enable_event = 0; 1340 p->softirq_disable_ip = 0; 1341 p->softirq_disable_event = 0; 1342 p->hardirq_context = 0; 1343 p->softirq_context = 0; 1344#endif 1345#ifdef CONFIG_LOCKDEP 1346 p->lockdep_depth = 0; /* no locks held yet */ 1347 p->curr_chain_key = 0; 1348 p->lockdep_recursion = 0; 1349#endif 1350 1351#ifdef CONFIG_DEBUG_MUTEXES 1352 p->blocked_on = NULL; /* not blocked yet */ 1353#endif 1354#ifdef CONFIG_BCACHE 1355 p->sequential_io = 0; 1356 p->sequential_io_avg = 0; 1357#endif 1358 1359 /* Perform scheduler related setup. Assign this task to a CPU. */ 1360 retval = sched_fork(clone_flags, p); 1361 if (retval) 1362 goto bad_fork_cleanup_policy; 1363 1364 retval = perf_event_init_task(p); 1365 if (retval) 1366 goto bad_fork_cleanup_policy; 1367 retval = audit_alloc(p); 1368 if (retval) 1369 goto bad_fork_cleanup_perf; 1370 /* copy all the process information */ 1371 shm_init_task(p); 1372 retval = copy_semundo(clone_flags, p); 1373 if (retval) 1374 goto bad_fork_cleanup_audit; 1375 retval = copy_files(clone_flags, p); 1376 if (retval) 1377 goto bad_fork_cleanup_semundo; 1378 retval = copy_fs(clone_flags, p); 1379 if (retval) 1380 goto bad_fork_cleanup_files; 1381 retval = copy_sighand(clone_flags, p); 1382 if (retval) 1383 goto bad_fork_cleanup_fs; 1384 retval = copy_signal(clone_flags, p); 1385 if (retval) 1386 goto bad_fork_cleanup_sighand; 1387 retval = copy_mm(clone_flags, p); 1388 if (retval) 1389 goto bad_fork_cleanup_signal; 1390 retval = copy_namespaces(clone_flags, p); 1391 if (retval) 1392 goto bad_fork_cleanup_mm; 1393 retval = copy_io(clone_flags, p); 1394 if (retval) 1395 goto bad_fork_cleanup_namespaces; 1396 retval = copy_thread(clone_flags, stack_start, stack_size, p); 1397 if (retval) 1398 goto bad_fork_cleanup_io; 1399 1400 if (pid != &init_struct_pid) { 1401 retval = -ENOMEM; 1402 pid = alloc_pid(p->nsproxy->pid_ns_for_children); 1403 if (!pid) 1404 goto bad_fork_cleanup_io; 1405 } 1406 1407 p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? child_tidptr : NULL; 1408 /* 1409 * Clear TID on mm_release()? 1410 */ 1411 p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? child_tidptr : NULL; 1412#ifdef CONFIG_BLOCK 1413 p->plug = NULL; 1414#endif 1415#ifdef CONFIG_FUTEX 1416 p->robust_list = NULL; 1417#ifdef CONFIG_COMPAT 1418 p->compat_robust_list = NULL; 1419#endif 1420 INIT_LIST_HEAD(&p->pi_state_list); 1421 p->pi_state_cache = NULL; 1422#endif 1423 /* 1424 * sigaltstack should be cleared when sharing the same VM 1425 */ 1426 if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM) 1427 p->sas_ss_sp = p->sas_ss_size = 0; 1428 1429 /* 1430 * Syscall tracing and stepping should be turned off in the 1431 * child regardless of CLONE_PTRACE. 1432 */ 1433 user_disable_single_step(p); 1434 clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE); 1435#ifdef TIF_SYSCALL_EMU 1436 clear_tsk_thread_flag(p, TIF_SYSCALL_EMU); 1437#endif 1438 clear_all_latency_tracing(p); 1439 1440 /* ok, now we should be set up.. */ 1441 p->pid = pid_nr(pid); 1442 if (clone_flags & CLONE_THREAD) { 1443 p->exit_signal = -1; 1444 p->group_leader = current->group_leader; 1445 p->tgid = current->tgid; 1446 } else { 1447 if (clone_flags & CLONE_PARENT) 1448 p->exit_signal = current->group_leader->exit_signal; 1449 else 1450 p->exit_signal = (clone_flags & CSIGNAL); 1451 p->group_leader = p; 1452 p->tgid = p->pid; 1453 } 1454 1455 p->nr_dirtied = 0; 1456 p->nr_dirtied_pause = 128 >> (PAGE_SHIFT - 10); 1457 p->dirty_paused_when = 0; 1458 1459 p->pdeath_signal = 0; 1460 INIT_LIST_HEAD(&p->thread_group); 1461 p->task_works = NULL; 1462 1463 /* 1464 * Make it visible to the rest of the system, but dont wake it up yet. 1465 * Need tasklist lock for parent etc handling! 1466 */ 1467 write_lock_irq(&tasklist_lock); 1468 1469 /* CLONE_PARENT re-uses the old parent */ 1470 if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) { 1471 p->real_parent = current->real_parent; 1472 p->parent_exec_id = current->parent_exec_id; 1473 } else { 1474 p->real_parent = current; 1475 p->parent_exec_id = current->self_exec_id; 1476 } 1477 1478 spin_lock(¤t->sighand->siglock); 1479 1480 /* 1481 * Copy seccomp details explicitly here, in case they were changed 1482 * before holding sighand lock. 1483 */ 1484 copy_seccomp(p); 1485 1486 /* 1487 * Process group and session signals need to be delivered to just the 1488 * parent before the fork or both the parent and the child after the 1489 * fork. Restart if a signal comes in before we add the new process to 1490 * it's process group. 1491 * A fatal signal pending means that current will exit, so the new 1492 * thread can't slip out of an OOM kill (or normal SIGKILL). 1493 */ 1494 recalc_sigpending(); 1495 if (signal_pending(current)) { 1496 spin_unlock(¤t->sighand->siglock); 1497 write_unlock_irq(&tasklist_lock); 1498 retval = -ERESTARTNOINTR; 1499 goto bad_fork_free_pid; 1500 } 1501 1502 if (likely(p->pid)) { 1503 ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace); 1504 1505 init_task_pid(p, PIDTYPE_PID, pid); 1506 if (thread_group_leader(p)) { 1507 init_task_pid(p, PIDTYPE_PGID, task_pgrp(current)); 1508 init_task_pid(p, PIDTYPE_SID, task_session(current)); 1509 1510 if (is_child_reaper(pid)) { 1511 ns_of_pid(pid)->child_reaper = p; 1512 p->signal->flags |= SIGNAL_UNKILLABLE; 1513 } 1514 1515 p->signal->leader_pid = pid; 1516 p->signal->tty = tty_kref_get(current->signal->tty); 1517 list_add_tail(&p->sibling, &p->real_parent->children); 1518 list_add_tail_rcu(&p->tasks, &init_task.tasks); 1519 attach_pid(p, PIDTYPE_PGID); 1520 attach_pid(p, PIDTYPE_SID); 1521 __this_cpu_inc(process_counts); 1522 } else { 1523 current->signal->nr_threads++; 1524 atomic_inc(¤t->signal->live); 1525 atomic_inc(¤t->signal->sigcnt); 1526 list_add_tail_rcu(&p->thread_group, 1527 &p->group_leader->thread_group); 1528 list_add_tail_rcu(&p->thread_node, 1529 &p->signal->thread_head); 1530 } 1531 attach_pid(p, PIDTYPE_PID); 1532 nr_threads++; 1533 } 1534 1535 total_forks++; 1536 spin_unlock(¤t->sighand->siglock); 1537 syscall_tracepoint_update(p); 1538 write_unlock_irq(&tasklist_lock); 1539 1540 proc_fork_connector(p); 1541 cgroup_post_fork(p); 1542 if (clone_flags & CLONE_THREAD) 1543 threadgroup_change_end(current); 1544 perf_event_fork(p); 1545 1546 trace_task_newtask(p, clone_flags); 1547 uprobe_copy_process(p, clone_flags); 1548 1549 return p; 1550 1551bad_fork_free_pid: 1552 if (pid != &init_struct_pid) 1553 free_pid(pid); 1554bad_fork_cleanup_io: 1555 if (p->io_context) 1556 exit_io_context(p); 1557bad_fork_cleanup_namespaces: 1558 exit_task_namespaces(p); 1559bad_fork_cleanup_mm: 1560 if (p->mm) 1561 mmput(p->mm); 1562bad_fork_cleanup_signal: 1563 if (!(clone_flags & CLONE_THREAD)) 1564 free_signal_struct(p->signal); 1565bad_fork_cleanup_sighand: 1566 __cleanup_sighand(p->sighand); 1567bad_fork_cleanup_fs: 1568 exit_fs(p); /* blocking */ 1569bad_fork_cleanup_files: 1570 exit_files(p); /* blocking */ 1571bad_fork_cleanup_semundo: 1572 exit_sem(p); 1573bad_fork_cleanup_audit: 1574 audit_free(p); 1575bad_fork_cleanup_perf: 1576 perf_event_free_task(p); 1577bad_fork_cleanup_policy: 1578#ifdef CONFIG_NUMA 1579 mpol_put(p->mempolicy); 1580bad_fork_cleanup_threadgroup_lock: 1581#endif 1582 if (clone_flags & CLONE_THREAD) 1583 threadgroup_change_end(current); 1584 delayacct_tsk_free(p); 1585 module_put(task_thread_info(p)->exec_domain->module); 1586bad_fork_cleanup_count: 1587 atomic_dec(&p->cred->user->processes); 1588 exit_creds(p); 1589bad_fork_free: 1590 free_task(p); 1591fork_out: 1592 return ERR_PTR(retval); 1593}
我们可以看到1240行的函数，dup_task_struct. 这个函数利用了当前进程的current指针，进行了pcb的拷贝工作。除此之外， 后面还会进行一系列的copy操作， 包括mm等部分。 最后，这部分的函数执行结束，返回到do_fork函数继续执行。在上面的1396
行有一个copy_thread函数，这个函数对于不同的体系结构， 有不同的实现，在这里我们选择32位的x86架构，我们来查看一下它的代码:
132int 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; 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(); 160 childregs->ax = 0; 161 if (sp) 162 childregs->sp = sp; 163 164 p->thread.ip = (unsigned long) ret_from_fork; 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; 195}
我们可以看到， 在copy_process中有一句p->thread.ip=(unsigned long)ret_from_work; 这个设置了进程的ip。
在do_fork执行完成以后(return nr)，会返回sys_clone的宏定义部分， 然后进入core.c中的schedule函数， 进行进程调度。 调度的时候， 会选中刚才创建的子进程的PCB。这样可以由刚才设置的ip进入ret_from_fork函数进行执行。
ENTRY(ret_from_fork) 291 CFI_STARTPROC 292 pushl_cfi %eax 293 call schedule_tail 294 GET_THREAD_INFO(%ebp) 295 popl_cfi %eax 296 pushl_cfi $0x0202 # Reset kernel eflags 297 popfl_cfi 298 jmp syscall_exit 299 CFI_ENDPROC 300 END(ret_from_fork)
在这里使用call schedule_tail， 又会调用schedule函数。进行一些相关的处理。至此，我们完成了fork函数的基本执行。
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