fork 在 Linux 内核里面的实现
太闲了看了一下 fork 在内核里面的工作
内核版本：Linux kernel 5.6.14
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L2522
SYSCALL_DEFINE0(fork)
{
#ifdef CONFIG_MMU
struct kernel_clone_args args = {
.exit_signal = SIGCHLD,
};
return _do_fork(&args);
#else
/* can not support in nommu mode */
return -EINVAL;
#endif
}
其实在内核里面 fork vfork clone 最终都是会调用 _do_fork
直接跟进去
_do_frok
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L2403
long _do_fork(struct kernel_clone_args *args)
{
// 取出 flag 参数
u64 clone_flags = args->flags;
struct completion vfork;
struct pid *pid;
// 创建一个 进程描述符
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.
*/
// 对 flag 标志进行检查，详细说明参见下面的 标志含义
if (!(clone_flags & CLONE_UNTRACED)) {
if (clone_flags & CLONE_VFORK)
trace = PTRACE_EVENT_VFORK;
else if (args->exit_signal != SIGCHLD)
trace = PTRACE_EVENT_CLONE;
else
trace = PTRACE_EVENT_FORK;
// 程序是否支持调试 trace
/*
这是 ptrace_event_enabled 的源码
* ptrace_event_enabled - test whether a ptrace event is enabled
* @task: ptracee of interest
* @event: %PTRACE_EVENT_* to test
*
* Test whether @event is enabled for ptracee @task.
*
* Returns %true if @event is enabled, %false otherwise.
static inline bool ptrace_event_enabled(struct task_struct *task, int event)
{
return task->ptrace & PT_EVENT_FLAG(event);
}
*/
if (likely(!ptrace_event_enabled(current, trace)))
trace = 0;
}
// fork 的主要工作 copy_process 生成新的进程 p
p = copy_process(NULL, trace, NUMA_NO_NODE, args);
add_latent_entropy();
if (IS_ERR(p))
return PTR_ERR(p);
/*
* Do this prior waking up the new thread - the thread pointer
* might get invalid after that point, if the thread exits quickly.
*/
trace_sched_process_fork(current, p);
pid = get_task_pid(p, PIDTYPE_PID);
nr = pid_vnr(pid);
if (clone_flags & CLONE_PARENT_SETTID)
put_user(nr, args->parent_tid);
if (clone_flags & CLONE_VFORK) {
p->vfork_done = &vfork;
init_completion(&vfork);
get_task_struct(p);
}
wake_up_new_task(p);
/* forking complete and child started to run, tell ptracer */
if (unlikely(trace))
ptrace_event_pid(trace, pid);
if (clone_flags & CLONE_VFORK) {
if (!wait_for_vfork_done(p, &vfork))
ptrace_event_pid(PTRACE_EVENT_VFORK_DONE, pid);
}
put_pid(pid);
return nr;
}
标志含义
CLONE_PARENT 创建的子进程的父进程是调用者的父进程，新进程与创建它的进程成了“兄弟”而不是“父子”
CLONE_FS 子进程与父进程共享相同的文件系统，包括 root 、当前目录、 umask
CLONE_FILES 子进程与父进程共享相同的文件描述符（file descriptor）表
CLONE_NEWNS 在新的 namespace 启动子进程， namespace 描述了进程的文件 hierarchy
CLONE_SIGHAND 子进程与父进程共享相同的信号处理（signal handler）表
CLONE_PTRACE 若父进程被 trace ，子进程也被 trace
CLONE_VFORK 父进程被挂起，直至子进程释放虚拟内存资源
CLONE_VM 子进程与父进程运行于相同的内存空间
CLONE_PID 子进程在创建时 PID 与父进程一致
CLONE_THREAD Linux 2.4 中增加以支持 POSIX 线程标准，子进程与父进程共享相同的线程群
copy_process
生成新的进程
p = copy_process(NULL, trace, NUMA_NO_NODE, args);
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L1824
这个函数有点长得可怕 在 Linux 5.6.14 下面一共 538 行，不能每一行都分析了，选重点吧
/*
* 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 __latent_entropy struct task_struct *copy_process(
struct pid *pid,
int trace,
int node,
struct kernel_clone_args *args)
{
int pidfd = -1, retval;
struct task_struct *p;
struct multiprocess_signals delayed;
struct file *pidfile = NULL;
u64 clone_flags = args->flags;
struct nsproxy *nsp = current->nsproxy;
/*
* Don't allow sharing the root directory with processes in a different
* namespace
*/
// 不同的命名空间（namespace）不允许共享 根目录（/） 其实共享根目录了那还玩啥，全透明了都
// CLONE_NEWNS 在新的 namespace 启动子进程， namespace 描述了进程的文件 hierarchy
// CLONE_FS 子进程与父进程共享相同的文件系统，包括 root 、当前目录、 umask
if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
return ERR_PTR(-EINVAL);
// namespace 用户隔离，这个东西曾经出了一个提权漏洞
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.
*/
//CLONE_THREAD 子进程与父进程共享相同的线程群
//共享相同的线程组不共享 signals 冲突
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.
*/
//CLONE_SIGHAND 子进程与父进程共享相同的信号处理（signal handler）表
//CLONE_VM 子进程与父进程运行于相同的内存空间
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);
/*
* If the new process will be in a different pid or user namespace
* do not allow it to share a thread group with the forking task.
*/
//新进程有独立的 pid
//运行于另一个命名空间（namespace）的进程
//都不允许共享线程组
if (clone_flags & CLONE_THREAD) {
if ((clone_flags & (CLONE_NEWUSER | CLONE_NEWPID)) ||
(task_active_pid_ns(current) != nsp->pid_ns_for_children))
return ERR_PTR(-EINVAL);
}
/*
* If the new process will be in a different time namespace
* do not allow it to share VM or a thread group with the forking task.
*/
if (clone_flags & (CLONE_THREAD | CLONE_VM)) {
if (nsp->time_ns != nsp->time_ns_for_children)
return ERR_PTR(-EINVAL);
}
if (clone_flags & CLONE_PIDFD) {
/*
* - CLONE_DETACHED is blocked so that we can potentially
* reuse it later for CLONE_PIDFD.
* - CLONE_THREAD is blocked until someone really needs it.
*/
if (clone_flags & (CLONE_DETACHED | CLONE_THREAD))
return ERR_PTR(-EINVAL);
}
/*
* Force any signals received before this point to be delivered
* before the fork happens. Collect up signals sent to multiple
* processes that happen during the fork and delay them so that
* they appear to happen after the fork.
*/
sigemptyset(&delayed.signal);
INIT_HLIST_NODE(&delayed.node);
spin_lock_irq(¤t->sighand->siglock);
if (!(clone_flags & CLONE_THREAD))
hlist_add_head(&delayed.node, ¤t->signal->multiprocess);
recalc_sigpending();
spin_unlock_irq(¤t->sighand->siglock);
retval = -ERESTARTNOINTR;
if (signal_pending(current))
goto fork_out;
retval = -ENOMEM;
// 重点操作，就是在里面复制 父进程 的进程描述符，详细分析见下面
p = dup_task_struct(current, node);
if (!p)
goto fork_out;
/*
* This _must_ happen before we call free_task(), i.e. before we jump
* to any of the bad_fork_* labels. This is to avoid freeing
* p->set_child_tid which is (ab)used as a kthread's data pointer for
* kernel threads (PF_KTHREAD).
*/
p->set_child_tid = (clone_flags & CLONE_CHILD_SETTID) ? args->child_tid : NULL;
/*
* Clear TID on mm_release()?
*/
p->clear_child_tid = (clone_flags & CLONE_CHILD_CLEARTID) ? args->child_tid : NULL;
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;
// RLIMIT_NPROC 每个 real id（ruid） 可拥有的最大子进程数
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;
// 顾名思义，复制 cred，详细源码下面分析
// 官方描述：Copy credentials for the new process created by fork()
// cred 结构体其实就是将原先 task_struct 中的一些涉及安全和信任的字段包装成了一个结构体
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;
if (nr_threads >= max_threads)
goto bad_fork_cleanup_count;
delayacct_tsk_init(p); /* Must remain after dup_task_struct() */
// task_struct 的 flag 字段
// via：https://elixir.bootlin.com/linux/v5.6.14/source/include/linux/sched.h#L1461
// PF_SUPERPRIV 使用超级用户权限
// PF_IDLE 标志进程空闲
// PF_WQ_WORKER 标志是工作者线程
// PF_FORKNOEXEC fork 但是不执行
p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER | PF_IDLE);
p->flags |= PF_FORKNOEXEC;
// 初始化进程亲属关系链表
INIT_LIST_HEAD(&p->children);
INIT_LIST_HEAD(&p->sibling);
rcu_copy_process(p);
// vfork 会用到的字段
p->vfork_done = NULL;
spin_lock_init(&p->alloc_lock);
/*
static inline void sigemptyset(sigset_t *set)
{
switch (_NSIG_WORDS) {
default:
memset(set, 0, sizeof(sigset_t));
break;
case 2: set->sig[1] = 0;
// fall through
case 1: set->sig[0] = 0;
break;
}
}
分支说明：
x86：case 1
x64：case 2
其他：default
pending 字段：进程上还需要处理的信号
*/
init_sigpending(&p->pending);
// 初始化 时间数据成员
p->utime = p->stime = p->gtime = 0;
#ifdef CONFIG_ARCH_HAS_SCALED_CPUTIME
p->utimescaled = p->stimescaled = 0;
#endif
prev_cputime_init(&p->prev_cputime);
#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
seqcount_init(&p->vtime.seqcount);
//starttime 进程的开始执行时间
p->vtime.starttime = 0;
p->vtime.state = VTIME_INACTIVE;
#endif
#if defined(SPLIT_RSS_COUNTING)
memset(&p->rss_stat, 0, sizeof(p->rss_stat));
#endif
// 复制父进程的 时间延迟值
p->default_timer_slack_ns = current->timer_slack_ns;
#ifdef CONFIG_PSI
p->psi_flags = 0;
#endif
// 等价 memset(ioac, 0, sizeof(p->ioac));
task_io_accounting_init(&p->ioac);
acct_clear_integrals(p);
posix_cputimers_init(&p->posix_cputimers);
p->io_context = NULL;
audit_set_context(p, NULL);
// cgroup 和 init_css_set
// via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/cgroup/cgroup.c#L5870
cgroup_fork(p);
#ifdef CONFIG_NUMA
p->mempolicy = mpol_dup(p->mempolicy);
if (IS_ERR(p->mempolicy)) {
retval = PTR_ERR(p->mempolicy);
p->mempolicy = NULL;
goto bad_fork_cleanup_threadgroup_lock;
}
#endif
#ifdef CONFIG_CPUSETS
p->cpuset_mem_spread_rotor = NUMA_NO_NODE;
p->cpuset_slab_spread_rotor = NUMA_NO_NODE;
seqcount_init(&p->mems_allowed_seq);
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
// 初始化中断请求
p->irq_events = 0;
p->hardirqs_enabled = 0;
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
p->pagefault_disabled = 0;
#ifdef CONFIG_LOCKDEP
lockdep_init_task(p);
#endif
#ifdef CONFIG_DEBUG_MUTEXES
p->blocked_on = NULL; /* not blocked yet */
#endif
#ifdef CONFIG_BCACHE
p->sequential_io = 0;
p->sequential_io_avg = 0;
#endif
/* Perform scheduler related setup. Assign this task to a CPU. */
// 把进程加入调度队列
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
shm_init_task(p); // #define shm_init_task(task) INIT_LIST_HEAD(&(task)->sysvshm.shm_clist)
// 用 kzalloc 给 security 分配内存
retval = security_task_alloc(p, clone_flags);
if (retval)
goto bad_fork_cleanup_audit;
// 这个函数直接返回 0
// via：https://elixir.bootlin.com/linux/v5.6.14/source/include/linux/sem.h#L25
retval = copy_semundo(clone_flags, p);
if (retval)
goto bad_fork_cleanup_security;
// 复制父进程打开的文件信息，详细见下面分析
retval = copy_files(clone_flags, p);
if (retval)
goto bad_fork_cleanup_semundo;
// 复制父进程 fs_struct 信息，详细见下面分析
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;
// 复制父进程所接收的信号
retval = copy_signal(clone_flags, p);
if (retval)
goto bad_fork_cleanup_sighand;
// 复制父进程的内存管理相关信息，详细见下面分析
retval = copy_mm(clone_flags, p);
if (retval)
goto bad_fork_cleanup_signal;
// 复制父进程的 namespaces
retval = copy_namespaces(clone_flags, p);
if (retval)
goto bad_fork_cleanup_mm;
// 复制父进程的 io_context 上下文信息，详细见下面分析
retval = copy_io(clone_flags, p);
if (retval)
goto bad_fork_cleanup_namespaces;
// 复制线程的
retval = copy_thread_tls(clone_flags, args->stack, args->stack_size, p,
args->tls);
if (retval)
goto bad_fork_cleanup_io;
stackleak_task_init(p);
if (pid != &init_struct_pid) {
pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid,
args->set_tid_size);
if (IS_ERR(pid)) {
retval = PTR_ERR(pid);
goto bad_fork_cleanup_thread;
}
}
/*
* This has to happen after we've potentially unshared the file
* descriptor table (so that the pidfd doesn't leak into the child
* if the fd table isn't shared).
*/
if (clone_flags & CLONE_PIDFD) {
retval = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
if (retval < 0)
goto bad_fork_free_pid;
pidfd = retval;
pidfile = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
O_RDWR | O_CLOEXEC);
if (IS_ERR(pidfile)) {
put_unused_fd(pidfd);
retval = PTR_ERR(pidfile);
goto bad_fork_free_pid;
}
get_pid(pid); /* held by pidfile now */
retval = put_user(pidfd, args->pidfd);
if (retval)
goto bad_fork_put_pidfd;
}
#ifdef CONFIG_BLOCK
p->plug = NULL;
#endif
futex_init_task(p);
/*
* sigaltstack should be cleared when sharing the same VM
*/
if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM)
sas_ss_reset(p);
/*
* Syscall tracing and stepping should be turned off in the
* child regardless of CLONE_PTRACE.
*/
user_disable_single_step(p);
clear_tsk_thread_flag(p, TIF_SYSCALL_TRACE);
#ifdef TIF_SYSCALL_EMU
clear_tsk_thread_flag(p, TIF_SYSCALL_EMU);
#endif
clear_tsk_latency_tracing(p);
/* ok, now we should be set up.. */
p->pid = pid_nr(pid);
if (clone_flags & CLONE_THREAD) {
p->exit_signal = -1;
p->group_leader = current->group_leader;
p->tgid = current->tgid;
} else {
if (clone_flags & CLONE_PARENT)
p->exit_signal = current->group_leader->exit_signal;
else
p->exit_signal = args->exit_signal;
p->group_leader = p;
p->tgid = p->pid;
}
p->nr_dirtied = 0;
p->nr_dirtied_pause = 128 >> (PAGE_SHIFT - 10);
p->dirty_paused_when = 0;
p->pdeath_signal = 0;
INIT_LIST_HEAD(&p->thread_group);
p->task_works = NULL;
cgroup_threadgroup_change_begin(current);
/*
* Ensure that the cgroup subsystem policies allow the new process to be
* forked. It should be noted the the new process's css_set can be changed
* between here and cgroup_post_fork() if an organisation operation is in
* progress.
*/
retval = cgroup_can_fork(p);
if (retval)
goto bad_fork_cgroup_threadgroup_change_end;
/*
* From this point on we must avoid any synchronous user-space
* communication until we take the tasklist-lock. In particular, we do
* not want user-space to be able to predict the process start-time by
* stalling fork(2) after we recorded the start_time but before it is
* visible to the system.
*/
p->start_time = ktime_get_ns();
p->start_boottime = ktime_get_boottime_ns();
/*
* Make it visible to the rest of the system, but dont wake it up yet.
* Need tasklist lock for parent etc handling!
*/
write_lock_irq(&tasklist_lock);
/* CLONE_PARENT re-uses the old parent */
if (clone_flags & (CLONE_PARENT|CLONE_THREAD)) {
p->real_parent = current->real_parent;
p->parent_exec_id = current->parent_exec_id;
} else {
p->real_parent = current;
p->parent_exec_id = current->self_exec_id;
}
klp_copy_process(p);
spin_lock(¤t->sighand->siglock);
/*
* Copy seccomp details explicitly here, in case they were changed
* before holding sighand lock.
*/
copy_seccomp(p);
rseq_fork(p, clone_flags);
/* Don't start children in a dying pid namespace */
if (unlikely(!(ns_of_pid(pid)->pid_allocated & PIDNS_ADDING))) {
retval = -ENOMEM;
goto bad_fork_cancel_cgroup;
}
/* Let kill terminate clone/fork in the middle */
if (fatal_signal_pending(current)) {
retval = -EINTR;
goto bad_fork_cancel_cgroup;
}
/* past the last point of failure */
if (pidfile)
fd_install(pidfd, pidfile);
init_task_pid_links(p);
if (likely(p->pid)) {
ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace);
init_task_pid(p, PIDTYPE_PID, pid);
if (thread_group_leader(p)) {
init_task_pid(p, PIDTYPE_TGID, pid);
init_task_pid(p, PIDTYPE_PGID, task_pgrp(current));
init_task_pid(p, PIDTYPE_SID, task_session(current));
if (is_child_reaper(pid)) {
ns_of_pid(pid)->child_reaper = p;
p->signal->flags |= SIGNAL_UNKILLABLE;
}
p->signal->shared_pending.signal = delayed.signal;
p->signal->tty = tty_kref_get(current->signal->tty);
/*
* Inherit has_child_subreaper flag under the same
* tasklist_lock with adding child to the process tree
* for propagate_has_child_subreaper optimization.
*/
p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
p->real_parent->signal->is_child_subreaper;
list_add_tail(&p->sibling, &p->real_parent->children);
list_add_tail_rcu(&p->tasks, &init_task.tasks);
attach_pid(p, PIDTYPE_TGID);
attach_pid(p, PIDTYPE_PGID);
attach_pid(p, PIDTYPE_SID);
__this_cpu_inc(process_counts);
} else {
current->signal->nr_threads++;
atomic_inc(¤t->signal->live);
refcount_inc(¤t->signal->sigcnt);
task_join_group_stop(p);
list_add_tail_rcu(&p->thread_group,
&p->group_leader->thread_group);
list_add_tail_rcu(&p->thread_node,
&p->signal->thread_head);
}
attach_pid(p, PIDTYPE_PID);
nr_threads++;
}
total_forks++;
hlist_del_init(&delayed.node);
spin_unlock(¤t->sighand->siglock);
syscall_tracepoint_update(p);
write_unlock_irq(&tasklist_lock);
proc_fork_connector(p);
cgroup_post_fork(p);
cgroup_threadgroup_change_end(current);
perf_event_fork(p);
trace_task_newtask(p, clone_flags);
uprobe_copy_process(p, clone_flags);
return p;
bad_fork_cancel_cgroup:
spin_unlock(¤t->sighand->siglock);
write_unlock_irq(&tasklist_lock);
cgroup_cancel_fork(p);
bad_fork_cgroup_threadgroup_change_end:
cgroup_threadgroup_change_end(current);
bad_fork_put_pidfd:
if (clone_flags & CLONE_PIDFD) {
fput(pidfile);
put_unused_fd(pidfd);
}
bad_fork_free_pid:
if (pid != &init_struct_pid)
free_pid(pid);
bad_fork_cleanup_thread:
exit_thread(p);
bad_fork_cleanup_io:
if (p->io_context)
exit_io_context(p);
bad_fork_cleanup_namespaces:
exit_task_namespaces(p);
bad_fork_cleanup_mm:
if (p->mm) {
mm_clear_owner(p->mm, p);
mmput(p->mm);
}
bad_fork_cleanup_signal:
if (!(clone_flags & CLONE_THREAD))
free_signal_struct(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_security:
security_task_free(p);
bad_fork_cleanup_audit:
audit_free(p);
bad_fork_cleanup_perf:
perf_event_free_task(p);
bad_fork_cleanup_policy:
lockdep_free_task(p);
#ifdef CONFIG_NUMA
mpol_put(p->mempolicy);
bad_fork_cleanup_threadgroup_lock:
#endif
delayacct_tsk_free(p);
bad_fork_cleanup_count:
atomic_dec(&p->cred->user->processes);
exit_creds(p);
bad_fork_free:
p->state = TASK_DEAD;
put_task_stack(p);
delayed_free_task(p);
fork_out:
spin_lock_irq(¤t->sighand->siglock);
hlist_del_init(&delayed.node);
spin_unlock_irq(¤t->sighand->siglock);
return ERR_PTR(retval);
}
dup_task_struct
p = dup_task_struct(current, node);
node == NUMA_NO_NODE
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L859
static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
{
struct task_struct *tsk;
unsigned long *stack;
struct vm_struct *stack_vm_area __maybe_unused;
int err;
if (node == NUMA_NO_NODE)
node = tsk_fork_get_node(orig);
/*
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L167
static struct kmem_cache *task_struct_cachep;
static inline struct task_struct *alloc_task_struct_node(int node)
{
return kmem_cache_alloc_node(task_struct_cachep, GFP_KERNEL, node);
}
就是分配内存
kmem_cache_alloc_node 如果指定的 NUMA 节点与本处理器所在节点不一致，则先从指定节点上获取 slab，替换处理器活动 slab，然后分配对象。
*/
tsk = alloc_task_struct_node(node);
if (!tsk)
return NULL;
/*
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L305
static unsigned long *alloc_thread_stack_node(struct task_struct *tsk,
int node)
{
unsigned long *stack;
stack = kmem_cache_alloc_node(thread_stack_cache, THREADINFO_GFP, node);
tsk->stack = stack;
return stack;
}
给新进程分配栈内存
*/
stack = alloc_thread_stack_node(tsk, node);
if (!stack)
goto free_tsk;
// 源码有点长，放下面了
if (memcg_charge_kernel_stack(tsk))
goto free_stack;
// return t->stack_vm_area; 获取 tsk 的 stack 的 vm_area
stack_vm_area = task_stack_vm_area(tsk);
// 重点，这里就是真正的复制父进程的 task_struct，其实源码很简单
/*
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L844
int __weak arch_dup_task_struct(struct task_struct *dst,
struct task_struct *src)
{
*dst = *src;
return 0;
}
orig 就是指向父进程的 task_struct 指针
tsk 就是指向子进程的 task_struct 指针
解引用，复制值
*/
err = arch_dup_task_struct(tsk, orig);
/*
* arch_dup_task_struct() clobbers the stack-related fields. Make
* sure they're properly initialized before using any stack-related
* functions again.
*/
// 子进程肯定用的是自己的栈，没有这个一句子进程就会父进程共用一个栈
// 这个就是为什么上面要分配内存，并取出这个两个字段
tsk->stack = stack;
#ifdef CONFIG_VMAP_STACK
tsk->stack_vm_area = stack_vm_area;
#endif
#ifdef CONFIG_THREAD_INFO_IN_TASK
refcount_set(&tsk->stack_refcount, 1);
#endif
if (err)
goto free_stack;
#ifdef CONFIG_SECCOMP
/*
* We must handle setting up seccomp filters once we're under
* the sighand lock in case orig has changed between now and
* then. Until then, filter must be NULL to avoid messing up
* the usage counts on the error path calling free_task.
*/
tsk->seccomp.filter = NULL;
#endif
/* 设置线程栈
via：https://elixir.bootlin.com/linux/v5.6.14/source/include/linux/sched/task_stack.h#L24
展开的源码是
static inline void setup_thread_stack(struct task_struct *p, struct task_struct *org)
{
*((struct thread_info *)(p)->stack) = *(((struct thread_info *)(org)->stack));
(&p->thread_info)->task = p;
}
其实就是复制了父进程的 栈 ，然后设置子进程的 thread_info 的 task 字段
这里补充一点：一个进程的 task_strcut 的 thread_info 字段存的是进程的 thread_info，然后 thread_info 里的 task 字段存的是进程的 task_struct
*/
setup_thread_stack(tsk, orig);
// 把进程加入调度队列（就是把 thread_info 的 标志位（flag） 设置成 TIF_NEED_RESCHED 1 （rescheduling necessary））
clear_user_return_notifier(tsk);
clear_tsk_need_resched(tsk);
/*
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L851
#define STACK_END_MAGIC 0x57AC6E9D
void set_task_stack_end_magic(struct task_struct *tsk)
{
unsigned long *stackend;
stackend = end_of_stack(tsk);
*stackend = STACK_END_MAGIC; // for overflow detection
}
在栈底设置一个 魔数 以检测是否发生栈溢出（有点像 canary ，但是这个是固定的数，只是用来检测意外溢出，不是用来防止 overflow exploit）
*/
set_task_stack_end_magic(tsk);
#ifdef CONFIG_STACKPROTECTOR
// 顾名思义，canary，随机数，详细算法
// via：https://elixir.bootlin.com/linux/v5.6.14/source/drivers/char/random.c#L2162
tsk->stack_canary = get_random_canary();
#endif
if (orig->cpus_ptr == &orig->cpus_mask)
tsk->cpus_ptr = &tsk->cpus_mask;
/*
* One for the user space visible state that goes away when reaped.
* One for the scheduler.
*/
refcount_set(&tsk->rcu_users, 2);
/* One for the rcu users */
refcount_set(&tsk->usage, 1);
#ifdef CONFIG_BLK_DEV_IO_TRACE
tsk->btrace_seq = 0;
#endif
tsk->splice_pipe = NULL;
tsk->task_frag.page = NULL;
tsk->wake_q.next = NULL;
account_kernel_stack(tsk, 1);
kcov_task_init(tsk);
#ifdef CONFIG_FAULT_INJECTION
tsk->fail_nth = 0;
#endif
#ifdef CONFIG_BLK_CGROUP
tsk->throttle_queue = NULL;
tsk->use_memdelay = 0;
#endif
#ifdef CONFIG_MEMCG
tsk->active_memcg = NULL;
#endif
return tsk;
free_stack:
free_thread_stack(tsk);
free_tsk:
free_task_struct(tsk);
return NULL;
}
memcg_charge_kernel_stack
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L405
static int memcg_charge_kernel_stack(struct task_struct *tsk)
{
#ifdef CONFIG_VMAP_STACK
struct vm_struct *vm = task_stack_vm_area(tsk);
int ret;
if (vm) {
int i;
for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++) {
/*
* If memcg_kmem_charge() fails, page->mem_cgroup
* pointer is NULL, and both memcg_kmem_uncharge()
* and mod_memcg_page_state() in free_thread_stack()
* will ignore this page. So it's safe.
*/
ret = memcg_kmem_charge(vm->pages[i], GFP_KERNEL, 0);
if (ret)
return ret;
mod_memcg_page_state(vm->pages[i],
MEMCG_KERNEL_STACK_KB,
PAGE_SIZE / 1024);
}
}
#endif
return 0;
}
copy_creds
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/cred.c#L330
/*
* Copy credentials for the new process created by fork()
*
* We share if we can, but under some circumstances we have to generate a new
* set.
*
* The new process gets the current process's subjective credentials as its
* objective and subjective credentials
*/
int copy_creds(struct task_struct *p, unsigned long clone_flags)
{
struct cred *new;
int ret;
#ifdef CONFIG_KEYS_REQUEST_CACHE
p->cached_requested_key = NULL;
#endif
if (
#ifdef CONFIG_KEYS
!p->cred->thread_keyring &&
#endif
clone_flags & CLONE_THREAD
) {
//设置 p 的 real cred 为 cred
p->real_cred = get_cred(p->cred);
get_cred(p->cred);
alter_cred_subscribers(p->cred, 2);
kdebug("share_creds(%p{%d,%d})",
p->cred, atomic_read(&p->cred->usage),
read_cred_subscribers(p->cred));
// 原子性增加 p进程对应的 用户（其实是 ruid） 的进程数
atomic_inc(&p->cred->user->processes);
return 0;
}
// 官方注释：prepare_creds - Prepare a new set of credentials for modification
// 详细分析见下面
new = prepare_creds();
if (!new)
return -ENOMEM;
// namespace 相关。要是指定用新用户的身份去启动子进程，就要修改 cred
if (clone_flags & CLONE_NEWUSER) {
//
ret = create_user_ns(new);
if (ret < 0)
goto error_put;
}
#ifdef CONFIG_KEYS
/* new threads get their own thread keyrings if their parent already
* had one */
if (new->thread_keyring) {
key_put(new->thread_keyring);
new->thread_keyring = NULL;
if (clone_flags & CLONE_THREAD)
install_thread_keyring_to_cred(new);
}
/* The process keyring is only shared between the threads in a process;
* anything outside of those threads doesn't inherit.
*/
if (!(clone_flags & CLONE_THREAD)) {
key_put(new->process_keyring);
new->process_keyring = NULL;
}
#endif
atomic_inc(&new->user->processes);
p->cred = p->real_cred = get_cred(new);
alter_cred_subscribers(new, 2);
validate_creds(new);
return 0;
error_put:
put_cred(new);
return ret;
}
prepare_creds
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/cred.c#L250
/**
* prepare_creds - Prepare a new set of credentials for modification
*
* Prepare a new set of task credentials for modification. A task's creds
* shouldn't generally be modified directly, therefore this function is used to
* prepare a new copy, which the caller then modifies and then commits by
* calling commit_creds().
*
* Preparation involves making a copy of the objective creds for modification.
*
* Returns a pointer to the new creds-to-be if successful, NULL otherwise.
*
* Call commit_creds() or abort_creds() to clean up.
*/
struct cred *prepare_creds(void)
{
//获取父进程的进程描述符指针（task_struct）
struct task_struct *task = current;
const struct cred *old;
struct cred *new;
validate_process_creds();
// 分配内存
new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
if (!new)
return NULL;
kdebug("prepare_creds() alloc %p", new);
// 保存父进程的 cred
old = task->cred;
// 直接把父进程的 cred 拷贝给子进程
memcpy(new, old, sizeof(struct cred));
new->non_rcu = 0;
atomic_set(&new->usage, 1);
set_cred_subscribers(new, 0);
// get_group_info(new->group_info); 相当于 atomic_inc(&gi->usage);
get_group_info(new->group_info);
// refcount_inc(&u->__count);
get_uid(new->user);
/*
static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
{
if (ns)
atomic_inc(&ns->count);
return ns;
}
namespace 相关
*/
get_user_ns(new->user_ns);
#ifdef CONFIG_KEYS
key_get(new->session_keyring);
key_get(new->process_keyring);
key_get(new->thread_keyring);
key_get(new->request_key_auth);
#endif
#ifdef CONFIG_SECURITY
new->security = NULL;
#endif
if (security_prepare_creds(new, old, GFP_KERNEL_ACCOUNT) < 0)
goto error;
validate_creds(new);
return new;
error:
abort_creds(new);
return NULL;
}
EXPORT_SYMBOL(prepare_creds);
create_user_ns
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/user_namespace.c#L69
int create_user_ns(struct cred *new)
{
// 取出父进程的 user_namespace
struct user_namespace *ns, *parent_ns = new->user_ns;
// euid 用于系统决定用户对系统资源的权限。也就是说当用户做任何一个操作时，最终看它有没有权限
kuid_t owner = new->euid;
kgid_t group = new->egid;
struct ucounts *ucounts;
int ret, i;
ret = -ENOSPC;
// user ns 是可以层级关系的，但是最高不允许超过 32 层
if (parent_ns->level > 32)
goto fail;
ucounts = inc_user_namespaces(parent_ns, owner);
if (!ucounts)
goto fail;
/*
* Verify that we can not violate the policy of which files
* may be accessed that is specified by the root directory,
* by verifing that the root directory is at the root of the
* mount namespace which allows all files to be accessed.
*/
ret = -EPERM;
// 判断是不是 chroot 环境
if (current_chrooted())
goto fail_dec;
/* The creator needs a mapping in the parent user namespace
* or else we won't be able to reasonably tell userspace who
* created a user_namespace.
*/
ret = -EPERM;
// 检查映射
/*
via：https://elixir.bootlin.com/linux/v5.6.14/source/include/linux/uidgid.h#L179
via：https://elixir.bootlin.com/linux/v5.6.14/source/include/linux/uidgid.h#L111
return __kuid_val(uid) != (uid_t) -1;
这里的 -1 就是表示不映射
*/
if (!kuid_has_mapping(parent_ns, owner) ||
!kgid_has_mapping(parent_ns, group))
goto fail_dec;
ret = -ENOMEM;
ns = kmem_cache_zalloc(user_ns_cachep, GFP_KERNEL);
if (!ns)
goto fail_dec;
ret = ns_alloc_inum(&ns->ns);
if (ret)
goto fail_free;
ns->ns.ops = &userns_operations;
atomic_set(&ns->count, 1);
/* Leave the new->user_ns reference with the new user namespace. */
// 把子进程的 namespce 的 parent 字段设置为父进程的 namespace
ns->parent = parent_ns;
ns->level = parent_ns->level + 1;
ns->owner = owner;
ns->group = group;
INIT_WORK(&ns->work, free_user_ns);
for (i = 0; i < UCOUNT_COUNTS; i++) {
ns->ucount_max[i] = INT_MAX;
}
ns->ucounts = ucounts;
/* Inherit USERNS_SETGROUPS_ALLOWED from our parent */
mutex_lock(&userns_state_mutex);
ns->flags = parent_ns->flags;
mutex_unlock(&userns_state_mutex);
#ifdef CONFIG_KEYS
INIT_LIST_HEAD(&ns->keyring_name_list);
init_rwsem(&ns->keyring_sem);
#endif
ret = -ENOMEM;
if (!setup_userns_sysctls(ns))
goto fail_keyring;
// 设置 crediential ，就是所谓的 cap
/*
static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
{
// Start with the same capabilities as init but useless for doing
// anything as the capabilities are bound to the new user namespace.
// 设置和 init 一样的权限，但是由于这些功能已绑定到新的用户 namespace ，因此这些权限只在用户命名空间有效。
cred->securebits = SECUREBITS_DEFAULT;
cred->cap_inheritable = CAP_EMPTY_SET;
cred->cap_permitted = CAP_FULL_SET;
cred->cap_effective = CAP_FULL_SET;
cred->cap_ambient = CAP_EMPTY_SET;
cred->cap_bset = CAP_FULL_SET;
#ifdef CONFIG_KEYS
key_put(cred->request_key_auth);
cred->request_key_auth = NULL;
#endif
// tgcred will be cleared in our caller bc CLONE_THREAD won't be set
cred->user_ns = user_ns;
}
*/
set_cred_user_ns(new, ns);
return 0;
fail_keyring:
#ifdef CONFIG_PERSISTENT_KEYRINGS
key_put(ns->persistent_keyring_register);
#endif
ns_free_inum(&ns->ns);
fail_free:
kmem_cache_free(user_ns_cachep, ns);
fail_dec:
dec_user_namespaces(ucounts);
fail:
return ret;
}
copy_files
复制父进程的 文件描述符
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L1449
static int copy_files(unsigned long clone_flags, struct task_struct *tsk)
{
struct files_struct *oldf, *newf;
int error = 0;
/*
* A background process may not have any files ...
*/
// 获取父进程的 files 结构体指针
oldf = current->files;
if (!oldf)
goto out;
if (clone_flags & CLONE_FILES) {
atomic_inc(&oldf->count);
goto out;
}
// 直接用 dup_fd 复制
// via：https://elixir.bootlin.com/linux/latest/source/fs/file.c#L272
newf = dup_fd(oldf, &error);
if (!newf)
goto out;
// 更新子进程的 files 为父进程的 files_struct 的副本
tsk->files = newf;
error = 0;
out:
return error;
}
copy_fs
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L1429
static int copy_fs(unsigned long clone_flags, struct task_struct *tsk)
{
struct fs_struct *fs = current->fs;
if (clone_flags & CLONE_FS) {
/* tsk->fs is already what we want */
spin_lock(&fs->lock);
if (fs->in_exec) {
spin_unlock(&fs->lock);
return -EAGAIN;
}
fs->users++;
spin_unlock(&fs->lock);
return 0;
}
tsk->fs = copy_fs_struct(fs);
if (!tsk->fs)
return -ENOMEM;
return 0;
}
copy_fs_struct
copy_fs 的实际操作
struct fs_struct *copy_fs_struct(struct fs_struct *old)
{
// 分配内存
struct fs_struct *fs = kmem_cache_alloc(fs_cachep, GFP_KERNEL);
/* We don't need to lock fs - think why ;-) */
if (fs) {
fs->users = 1;
fs->in_exec = 0;
spin_lock_init(&fs->lock);
seqcount_init(&fs->seq);
// 复制父进程的 umask
fs->umask = old->umask;
spin_lock(&old->lock);
// 根目录
fs->root = old->root;
path_get(&fs->root);
// 当前目录
fs->pwd = old->pwd;
path_get(&fs->pwd);
spin_unlock(&old->lock);
}
return fs;
}
copy_mm
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L1382
static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
{
struct mm_struct *mm, *oldmm;
int retval;
tsk->min_flt = tsk->maj_flt = 0;
tsk->nvcsw = tsk->nivcsw = 0;
#ifdef CONFIG_DETECT_HUNG_TASK
tsk->last_switch_count = tsk->nvcsw + tsk->nivcsw;
tsk->last_switch_time = 0;
#endif
tsk->mm = NULL;
tsk->active_mm = NULL;
/*
* Are we cloning a kernel thread?
*
* We need to steal a active VM for that..
*/
// 获取父进程的 mm_struct（mm_struct 是用来描述进程的内存空间的）
oldmm = current->mm;
if (!oldmm)
return 0;
/* initialize the new vmacache entries */
vmacache_flush(tsk);
// 如果子进程与父进程运行于相同的内存空间
if (clone_flags & CLONE_VM) {
mmget(oldmm); // &oldmm->mm_users 增加 1
mm = oldmm; // 直接让子进程的 mm_struct 指向父进程的 mm_struct
goto good_mm;
}
retval = -ENOMEM;
// 复制父进程 mm_struct 的内容
mm = dup_mm(tsk, current->mm);
if (!mm)
goto fail_nomem;
good_mm:
tsk->mm = mm;
tsk->active_mm = mm;
return 0;
fail_nomem:
return retval;
}
dup _mm
via：https://elixir.bootlin.com/linux/latest/source/kernel/fork.c#L1345
static struct mm_struct *dup_mm(struct task_struct *tsk,
struct mm_struct *oldmm)
{
struct mm_struct *mm;
int err;
mm = allocate_mm();
if (!mm)
goto fail_nomem;
// 拷贝父进程的 mm 的内容到 子进程（不同于 CLONE_VM）
memcpy(mm, oldmm, sizeof(*mm));
// 初始化 mm 的其他字段（我有点累暂时不看）
if (!mm_init(mm, tsk, mm->user_ns))
goto fail_nomem;
// 拷贝父进程地址空间
err = dup_mmap(mm, oldmm);
if (err)
goto free_pt;
mm->hiwater_rss = get_mm_rss(mm);
mm->hiwater_vm = mm->total_vm;
if (mm->binfmt && !try_module_get(mm->binfmt->module))
goto free_pt;
return mm;
free_pt:
/* don't put binfmt in mmput, we haven't got module yet */
mm->binfmt = NULL;
mm_init_owner(mm, NULL);
mmput(mm);
fail_nomem:
return NULL;
}
dup_mmap
拷贝父进程地址空间
via：https://elixir.bootlin.com/linux/latest/source/kernel/fork.c#L481
static __latent_entropy int dup_mmap(struct mm_struct *mm,
struct mm_struct *oldmm)
{
struct vm_area_struct *mpnt, *tmp, *prev, **pprev;
struct rb_node **rb_link, *rb_parent;
int retval;
unsigned long charge;
LIST_HEAD(uf);
uprobe_start_dup_mmap();
// 获取 线性区 的信号量
if (down_write_killable(&oldmm->mmap_sem)) {
retval = -EINTR;
goto fail_uprobe_end;
}
flush_cache_dup_mm(oldmm);
uprobe_dup_mmap(oldmm, mm);
/*
* Not linked in yet - no deadlock potential:
*/
down_write_nested(&mm->mmap_sem, SINGLE_DEPTH_NESTING);
/* No ordering required: file already has been exposed. */
RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm));
mm->total_vm = oldmm->total_vm; // 复制父进程的进程地址空间的的页数
mm->data_vm = oldmm->data_vm;
mm->exec_vm = oldmm->exec_vm; // 复制父进程的可执行内存映射中的页数
mm->stack_vm = oldmm->stack_vm;// 复制父进程的用户态栈堆中的页数
// 红黑树。。。。
// rblink 存的是 VMA 的根节点
rb_link = &mm->mm_rb.rb_node;
rb_parent = NULL;
pprev = &mm->mmap;
retval = ksm_fork(mm, oldmm);
if (retval)
goto out;
retval = khugepaged_fork(mm, oldmm);
if (retval)
goto out;
prev = NULL;
// 遍历父进程的 VMA
for (mpnt = oldmm->mmap; mpnt; mpnt = mpnt->vm_next) {
struct file *file;
// VM_DONTCOPY 在 fork 系统调用执行时不复制
if (mpnt->vm_flags & VM_DONTCOPY) {
// 见下面
vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt));
continue;
}
charge = 0;
/*
* Don't duplicate many vmas if we've been oom-killed (for
* example)
*/
if (fatal_signal_pending(current)) {
retval = -EINTR;
goto out;
}
// VM_ACCOUNT
if (mpnt->vm_flags & VM_ACCOUNT) {
unsigned long len = vma_pages(mpnt);
if (security_vm_enough_memory_mm(oldmm, len)) /* sic */
goto fail_nomem;
charge = len;
}
tmp = vm_area_dup(mpnt);
if (!tmp)
goto fail_nomem;
retval = vma_dup_policy(mpnt, tmp);
if (retval)
goto fail_nomem_policy;
tmp->vm_mm = mm;
retval = dup_userfaultfd(tmp, &uf);
if (retval)
goto fail_nomem_anon_vma_fork;
if (tmp->vm_flags & VM_WIPEONFORK) {
/* VM_WIPEONFORK gets a clean slate in the child. */
tmp->anon_vma = NULL;
if (anon_vma_prepare(tmp))
goto fail_nomem_anon_vma_fork;
} else if (anon_vma_fork(tmp, mpnt))
goto fail_nomem_anon_vma_fork;
tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
tmp->vm_next = tmp->vm_prev = NULL;
file = tmp->vm_file;
if (file) {
struct inode *inode = file_inode(file);
struct address_space *mapping = file->f_mapping;
get_file(file);
if (tmp->vm_flags & VM_DENYWRITE)
atomic_dec(&inode->i_writecount);
i_mmap_lock_write(mapping);
if (tmp->vm_flags & VM_SHARED)
atomic_inc(&mapping->i_mmap_writable);
flush_dcache_mmap_lock(mapping);
/* insert tmp into the share list, just after mpnt */
vma_interval_tree_insert_after(tmp, mpnt,
&mapping->i_mmap);
flush_dcache_mmap_unlock(mapping);
i_mmap_unlock_write(mapping);
}
/*
* Clear hugetlb-related page reserves for children. This only
* affects MAP_PRIVATE mappings. Faults generated by the child
* are not guaranteed to succeed, even if read-only
*/
if (is_vm_hugetlb_page(tmp))
reset_vma_resv_huge_pages(tmp);
/*
* Link in the new vma and copy the page table entries.
*/
*pprev = tmp;
pprev = &tmp->vm_next;
tmp->vm_prev = prev;
prev = tmp;
__vma_link_rb(mm, tmp, rb_link, rb_parent);
rb_link = &tmp->vm_rb.rb_right;
rb_parent = &tmp->vm_rb;
// 这里就是 fork 写时复制（COW）在 copy_page_range 里面
mm->map_count++;
if (!(tmp->vm_flags & VM_WIPEONFORK))
retval = copy_page_range(mm, oldmm, mpnt);
if (tmp->vm_ops && tmp->vm_ops->open)
tmp->vm_ops->open(tmp);
if (retval)
goto out;
}
/* a new mm has just been created */
retval = arch_dup_mmap(oldmm, mm);
out:
up_write(&mm->mmap_sem);
flush_tlb_mm(oldmm);
up_write(&oldmm->mmap_sem);
dup_userfaultfd_complete(&uf);
fail_uprobe_end:
uprobe_end_dup_mmap();
return retval;
fail_nomem_anon_vma_fork:
mpol_put(vma_policy(tmp));
fail_nomem_policy:
vm_area_free(tmp);
fail_nomem:
retval = -ENOMEM;
vm_unacct_memory(charge);
goto out;
}
vm_stat_account
via：https://elixir.bootlin.com/linux/latest/source/mm/mmap.c#L3287
void vm_stat_account(struct mm_struct *mm, vm_flags_t flags, long npages)
{
mm->total_vm += npages;
// return (flags & (VM_EXEC | VM_WRITE | VM_STACK)) == VM_EXEC;
if (is_exec_mapping(flags))
mm->exec_vm += npages;
// return (flags & VM_STACK) == VM_STACK;
else if (is_stack_mapping(flags))
mm->stack_vm += npages;
// return (flags & (VM_WRITE | VM_SHARED | VM_STACK)) == VM_WRITE;
else if (is_data_mapping(flags))
mm->data_vm += npages;
}
copy_page_range
fork 的写时复制的核心
int copy_page_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
struct vm_area_struct *vma)
{
pgd_t *src_pgd, *dst_pgd;
unsigned long next;
// 获得 vma 的起始地址和 结束地址
unsigned long addr = vma->vm_start;
unsigned long end = vma->vm_end;
struct mmu_notifier_range range;
bool is_cow;
int ret;
/*
* Don't copy ptes where a page fault will fill them correctly.
* Fork becomes much lighter when there are big shared or private
* readonly mappings. The tradeoff is that copy_page_range is more
* efficient than faulting.
*/
/*
VM_HUGETLB 巨型页
VM_PFNMAP Page-ranges 管理没有 struct page，只有 PFN pages
VM_MIXEDMAP 可以包含 struct page 和 PFN pages
*/
if (!(vma->vm_flags & (VM_HUGETLB | VM_PFNMAP | VM_MIXEDMAP)) &&
!vma->anon_vma)
return 0;
// return !!(vma->vm_flags & VM_HUGETLB);
if (is_vm_hugetlb_page(vma))
return copy_hugetlb_page_range(dst_mm, src_mm, vma);
if (unlikely(vma->vm_flags & VM_PFNMAP)) {
/*
* We do not free on error cases below as remove_vma
* gets called on error from higher level routine
*/
ret = track_pfn_copy(vma);
if (ret)
return ret;
}
/*
* We need to invalidate the secondary MMU mappings only when
* there could be a permission downgrade on the ptes of the
* parent mm. And a permission downgrade will only happen if
* is_cow_mapping() returns true.
*/
// return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
is_cow = is_cow_mapping(vma->vm_flags);
if (is_cow) {
mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE,
0, vma, src_mm, addr, end);
mmu_notifier_invalidate_range_start(&range);
}
ret = 0;
dst_pgd = pgd_offset(dst_mm, addr);
src_pgd = pgd_offset(src_mm, addr);
do {
next = pgd_addr_end(addr, end);
if (pgd_none_or_clear_bad(src_pgd))
continue;
if (unlikely(copy_p4d_range(dst_mm, src_mm, dst_pgd, src_pgd,
vma, addr, next))) {
ret = -ENOMEM;
break;
}
} while (dst_pgd++, src_pgd++, addr = next, addr != end);
if (is_cow)
mmu_notifier_invalidate_range_end(&range);
return ret;
}
copy_namespaces
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/nsproxy.c#L149
/*
* 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;
int ret;
if (likely(!(flags & (CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC |
CLONE_NEWPID | CLONE_NEWNET |
CLONE_NEWCGROUP | CLONE_NEWTIME)))) {
if (likely(old_ns->time_ns_for_children == old_ns->time_ns)) {
get_nsproxy(old_ns);
return 0;
}
} else 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);
ret = timens_on_fork(new_ns, tsk);
if (ret) {
free_nsproxy(new_ns);
return ret;
}
tsk->nsproxy = new_ns;
return 0;
}
copy_io
via：https://elixir.bootlin.com/linux/v5.6.14/source/kernel/fork.c#L1476
static int copy_io(unsigned long clone_flags, struct task_struct *tsk)
{
#ifdef CONFIG_BLOCK
struct io_context *ioc = current->io_context;
struct io_context *new_ioc;
if (!ioc)
return 0;
/*
* Share io context with parent, if CLONE_IO is set
*/
if (clone_flags & CLONE_IO) {
ioc_task_link(ioc);
tsk->io_context = ioc;
} else if (ioprio_valid(ioc->ioprio)) {
new_ioc = get_task_io_context(tsk, GFP_KERNEL, NUMA_NO_NODE);
if (unlikely(!new_ioc))
return -ENOMEM;
new_ioc->ioprio = ioc->ioprio;
put_io_context(new_ioc);
}
#endif
return 0;
}
copy_thread_tls
via：https://elixir.bootlin.com/linux/v5.6.14/source/arch/x86/kernel/process.c#L125
nt copy_thread_tls(unsigned long clone_flags, unsigned long sp,
unsigned long arg, struct task_struct *p, unsigned long tls)
{
struct inactive_task_frame *frame;
struct fork_frame *fork_frame;
struct pt_regs *childregs;
int ret = 0;
childregs = task_pt_regs(p);
fork_frame = container_of(childregs, struct fork_frame, regs);
frame = &fork_frame->frame;
frame->bp = 0;
frame->ret_addr = (unsigned long) ret_from_fork;
p->thread.sp = (unsigned long) fork_frame;
p->thread.io_bitmap = NULL;
memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));
#ifdef CONFIG_X86_64
savesegment(gs, p->thread.gsindex);
p->thread.gsbase = p->thread.gsindex ? 0 : current->thread.gsbase;
savesegment(fs, p->thread.fsindex);
p->thread.fsbase = p->thread.fsindex ? 0 : current->thread.fsbase;
savesegment(es, p->thread.es);
savesegment(ds, p->thread.ds);
#else
p->thread.sp0 = (unsigned long) (childregs + 1);
/*
* Clear all status flags including IF and set fixed bit. 64bit
* does not have this initialization as the frame does not contain
* flags. The flags consistency (especially vs. AC) is there
* ensured via objtool, which lacks 32bit support.
*/
frame->flags = X86_EFLAGS_FIXED;
#endif
/* Kernel thread ? */
if (unlikely(p->flags & PF_KTHREAD)) {
memset(childregs, 0, sizeof(struct pt_regs));
kthread_frame_init(frame, sp, arg);
return 0;
}
frame->bx = 0;
*childregs = *current_pt_regs();
childregs->ax = 0;
if (sp)
childregs->sp = sp;
#ifdef CONFIG_X86_32
task_user_gs(p) = get_user_gs(current_pt_regs());
#endif
/* Set a new TLS for the child thread? */
if (clone_flags & CLONE_SETTLS)
ret = set_new_tls(p, tls);
if (!ret && unlikely(test_tsk_thread_flag(current, TIF_IO_BITMAP)))
io_bitmap_share(p);
return ret;
}
参考资料
via：https://www.cnblogs.com/qiuheng/p/5749366.html
via：https://www.jianshu.com/p/3035f2be3ef0
via：https://www.cnblogs.com/nufangrensheng/p/3509262.html
via：https://www.ibm.com/developerworks/cn/linux/l-cn-cncrrc-mngd-wkq/
via：https://www.jianshu.com/p/691d02380312
via：https://www.cnblogs.com/wanghetao/archive/2011/11/06/2237931.html
