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 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * Generic pidhash and scalable, time-bounded PID allocator
  *
  * (C) 2002-2003 Nadia Yvette Chambers, IBM
  * (C) 2004 Nadia Yvette Chambers, Oracle
  * (C) 2002-2004 Ingo Molnar, Red Hat
  *
  * pid-structures are backing objects for tasks sharing a given ID to chain
  * against. There is very little to them aside from hashing them and
  * parking tasks using given ID's on a list.
  *
  * The hash is always changed with the tasklist_lock write-acquired,
  * and the hash is only accessed with the tasklist_lock at least
  * read-acquired, so there's no additional SMP locking needed here.
  *
  * We have a list of bitmap pages, which bitmaps represent the PID space.
  * Allocating and freeing PIDs is completely lockless. The worst-case
  * allocation scenario when all but one out of 1 million PIDs possible are
  * allocated already: the scanning of 32 list entries and at most PAGE_SIZE
  * bytes. The typical fastpath is a single successful setbit. Freeing is O(1).
  *
  * Pid namespaces:
  *    (C) 2007 Pavel Emelyanov <xemul@openvz.org>, OpenVZ, SWsoft Inc.
  *    (C) 2007 Sukadev Bhattiprolu <sukadev@us.ibm.com>, IBM
  *     Many thanks to Oleg Nesterov for comments and help
  *
  */
 
 #include <linux/mm.h>
 #include <linux/export.h>
 #include <linux/slab.h>
 #include <linux/init.h>
 #include <linux/rculist.h>
 #include <linux/memblock.h>
 #include <linux/pid_namespace.h>
 #include <linux/init_task.h>
 #include <linux/syscalls.h>
 #include <linux/proc_ns.h>
 #include <linux/refcount.h>
 #include <linux/anon_inodes.h>
 #include <linux/sched/signal.h>
 #include <linux/sched/task.h>
 #include <linux/idr.h>
 #include <net/sock.h>
 #include <uapi/linux/pidfd.h>
 
 struct pid init_struct_pid = {
     .count      = REFCOUNT_INIT(1),
     .tasks      = {
         { .first = NULL },
         { .first = NULL },
         { .first = NULL },
     },
     .level      = 0,
     .numbers    = { {
         .nr     = 0,
         .ns     = &init_pid_ns,
     }, }
 };
 
 int pid_max = PID_MAX_DEFAULT;
 
 #define RESERVED_PIDS       300
 
 int pid_max_min = RESERVED_PIDS + 1;
 int pid_max_max = PID_MAX_LIMIT;
 
 /*
  * PID-map pages start out as NULL, they get allocated upon
  * first use and are never deallocated. This way a low pid_max
  * value does not cause lots of bitmaps to be allocated, but
  * the scheme scales to up to 4 million PIDs, runtime.
  */
 struct pid_namespace init_pid_ns = {
     .ns.count = REFCOUNT_INIT(2),
     .idr = IDR_INIT(init_pid_ns.idr),
     .pid_allocated = PIDNS_ADDING,
     .level = 0,
     .child_reaper = &init_task,
     .user_ns = &init_user_ns,
     .ns.inum = PROC_PID_INIT_INO,
 #ifdef CONFIG_PID_NS
     .ns.ops = &pidns_operations,
 #endif
 };
 EXPORT_SYMBOL_GPL(init_pid_ns);
 
 /*
  * Note: disable interrupts while the pidmap_lock is held as an
  * interrupt might come in and do read_lock(&tasklist_lock).
  *
  * If we don't disable interrupts there is a nasty deadlock between
  * detach_pid()->free_pid() and another cpu that does
  * spin_lock(&pidmap_lock) followed by an interrupt routine that does
  * read_lock(&tasklist_lock);
  *
  * After we clean up the tasklist_lock and know there are no
  * irq handlers that take it we can leave the interrupts enabled.
  * For now it is easier to be safe than to prove it can't happen.
  */
 
 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
 
 void put_pid(struct pid *pid)
 {
     struct pid_namespace *ns;
 
     if (!pid)
         return;
 
     ns = pid->numbers[pid->level].ns;
     if (refcount_dec_and_test(&pid->count)) {
         kmem_cache_free(ns->pid_cachep, pid);
         put_pid_ns(ns);
     }
 }
 EXPORT_SYMBOL_GPL(put_pid);
 
 static void delayed_put_pid(struct rcu_head *rhp)
 {
     struct pid *pid = container_of(rhp, struct pid, rcu);
     put_pid(pid);
 }
 
 void free_pid(struct pid *pid)
 {
     /* We can be called with write_lock_irq(&tasklist_lock) held */
     int i;
     unsigned long flags;
 
     spin_lock_irqsave(&pidmap_lock, flags);
     for (i = 0; i <= pid->level; i++) {
         struct upid *upid = pid->numbers + i;
         struct pid_namespace *ns = upid->ns;
         switch (--ns->pid_allocated) {
         case 2:
         case 1:
             /* When all that is left in the pid namespace
              * is the reaper wake up the reaper.  The reaper
              * may be sleeping in zap_pid_ns_processes().
              */
             wake_up_process(ns->child_reaper);
             break;
         case PIDNS_ADDING:
             /* Handle a fork failure of the first process */
             WARN_ON(ns->child_reaper);
             ns->pid_allocated = 0;
             break;
         }
 
         idr_remove(&ns->idr, upid->nr);
     }
     spin_unlock_irqrestore(&pidmap_lock, flags);
 
     call_rcu(&pid->rcu, delayed_put_pid);
 }
 
 struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
               size_t set_tid_size)
 {
     struct pid *pid;
     enum pid_type type;
     int i, nr;
     struct pid_namespace *tmp;
     struct upid *upid;
     int retval = -ENOMEM;
 
     /*
      * set_tid_size contains the size of the set_tid array. Starting at
      * the most nested currently active PID namespace it tells alloc_pid()
      * which PID to set for a process in that most nested PID namespace
      * up to set_tid_size PID namespaces. It does not have to set the PID
      * for a process in all nested PID namespaces but set_tid_size must
      * never be greater than the current ns->level + 1.
      */
     if (set_tid_size > ns->level + 1)
         return ERR_PTR(-EINVAL);
 
     pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
     if (!pid)
         return ERR_PTR(retval);
 
     tmp = ns;
     pid->level = ns->level;
 
     for (i = ns->level; i >= 0; i--) {
         int tid = 0;
 
         if (set_tid_size) {
             tid = set_tid[ns->level - i];
 
             retval = -EINVAL;
             if (tid < 1 || tid >= pid_max)
                 goto out_free;
             /*
              * Also fail if a PID != 1 is requested and
              * no PID 1 exists.
              */
             if (tid != 1 && !tmp->child_reaper)
                 goto out_free;
             retval = -EPERM;
             if (!checkpoint_restore_ns_capable(tmp->user_ns))
                 goto out_free;
             set_tid_size--;
         }
 
         idr_preload(GFP_KERNEL);
         spin_lock_irq(&pidmap_lock);
 
         if (tid) {
             nr = idr_alloc(&tmp->idr, NULL, tid,
                        tid + 1, GFP_ATOMIC);
             /*
              * If ENOSPC is returned it means that the PID is
              * alreay in use. Return EEXIST in that case.
              */
             if (nr == -ENOSPC)
                 nr = -EEXIST;
         } else {
             int pid_min = 1;
             /*
              * init really needs pid 1, but after reaching the
              * maximum wrap back to RESERVED_PIDS
              */
             if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
                 pid_min = RESERVED_PIDS;
 
             /*
              * Store a null pointer so find_pid_ns does not find
              * a partially initialized PID (see below).
              */
             nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
                           pid_max, GFP_ATOMIC);
         }
         spin_unlock_irq(&pidmap_lock);
         idr_preload_end();
 
         if (nr < 0) {
             retval = (nr == -ENOSPC) ? -EAGAIN : nr;
             goto out_free;
         }
 
         pid->numbers[i].nr = nr;
         pid->numbers[i].ns = tmp;
         tmp = tmp->parent;
     }
 
     /*
      * ENOMEM is not the most obvious choice especially for the case
      * where the child subreaper has already exited and the pid
      * namespace denies the creation of any new processes. But ENOMEM
      * is what we have exposed to userspace for a long time and it is
      * documented behavior for pid namespaces. So we can't easily
      * change it even if there were an error code better suited.
      */
     retval = -ENOMEM;
 
     get_pid_ns(ns);
     refcount_set(&pid->count, 1);
     spin_lock_init(&pid->lock);
     for (type = 0; type < PIDTYPE_MAX; ++type)
         INIT_HLIST_HEAD(&pid->tasks[type]);
 
     init_waitqueue_head(&pid->wait_pidfd);
     INIT_HLIST_HEAD(&pid->inodes);
 
     upid = pid->numbers + ns->level;
     spin_lock_irq(&pidmap_lock);
     if (!(ns->pid_allocated & PIDNS_ADDING))
         goto out_unlock;
     for ( ; upid >= pid->numbers; --upid) {
         /* Make the PID visible to find_pid_ns. */
         idr_replace(&upid->ns->idr, pid, upid->nr);
         upid->ns->pid_allocated++;
     }
     spin_unlock_irq(&pidmap_lock);
 
     return pid;
 
 out_unlock:
     spin_unlock_irq(&pidmap_lock);
     put_pid_ns(ns);
 
 out_free:
     spin_lock_irq(&pidmap_lock);
     while (++i <= ns->level) {
         upid = pid->numbers + i;
         idr_remove(&upid->ns->idr, upid->nr);
     }
 
     /* On failure to allocate the first pid, reset the state */
     if (ns->pid_allocated == PIDNS_ADDING)
         idr_set_cursor(&ns->idr, 0);
 
     spin_unlock_irq(&pidmap_lock);
 
     kmem_cache_free(ns->pid_cachep, pid);
     return ERR_PTR(retval);
 }
 
 void disable_pid_allocation(struct pid_namespace *ns)
 {
     spin_lock_irq(&pidmap_lock);
     ns->pid_allocated &= ~PIDNS_ADDING;
     spin_unlock_irq(&pidmap_lock);
 }
 
 struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
 {
     return idr_find(&ns->idr, nr);
 }
 EXPORT_SYMBOL_GPL(find_pid_ns);
 
 struct pid *find_vpid(int nr)
 {
     return find_pid_ns(nr, task_active_pid_ns(current));
 }
 EXPORT_SYMBOL_GPL(find_vpid);
 
 static struct pid **task_pid_ptr(struct task_struct *task, enum pid_type type)
 {
     return (type == PIDTYPE_PID) ?
         &task->thread_pid :
         &task->signal->pids[type];
 }
 
 /*
  * attach_pid() must be called with the tasklist_lock write-held.
  */
 void attach_pid(struct task_struct *task, enum pid_type type)
 {
     struct pid *pid = *task_pid_ptr(task, type);
     hlist_add_head_rcu(&task->pid_links[type], &pid->tasks[type]);
 }
 
 static void __change_pid(struct task_struct *task, enum pid_type type,
             struct pid *new)
 {
     struct pid **pid_ptr = task_pid_ptr(task, type);
     struct pid *pid;
     int tmp;
 
     pid = *pid_ptr;
 
     hlist_del_rcu(&task->pid_links[type]);
     *pid_ptr = new;
 
     for (tmp = PIDTYPE_MAX; --tmp >= 0; )
         if (pid_has_task(pid, tmp))
             return;
 
     free_pid(pid);
 }
 
 void detach_pid(struct task_struct *task, enum pid_type type)
 {
     __change_pid(task, type, NULL);
 }
 
 void change_pid(struct task_struct *task, enum pid_type type,
         struct pid *pid)
 {
     __change_pid(task, type, pid);
     attach_pid(task, type);
 }
 
 void exchange_tids(struct task_struct *left, struct task_struct *right)
 {
     struct pid *pid1 = left->thread_pid;
     struct pid *pid2 = right->thread_pid;
     struct hlist_head *head1 = &pid1->tasks[PIDTYPE_PID];
     struct hlist_head *head2 = &pid2->tasks[PIDTYPE_PID];
 
     /* Swap the single entry tid lists */
     hlists_swap_heads_rcu(head1, head2);
 
     /* Swap the per task_struct pid */
     rcu_assign_pointer(left->thread_pid, pid2);
     rcu_assign_pointer(right->thread_pid, pid1);
 
     /* Swap the cached value */
     WRITE_ONCE(left->pid, pid_nr(pid2));
     WRITE_ONCE(right->pid, pid_nr(pid1));
 }
 
 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
 void transfer_pid(struct task_struct *old, struct task_struct *new,
                enum pid_type type)
 {
     if (type == PIDTYPE_PID)
         new->thread_pid = old->thread_pid;
     hlist_replace_rcu(&old->pid_links[type], &new->pid_links[type]);
 }
 
 struct task_struct *pid_task(struct pid *pid, enum pid_type type)
 {
     struct task_struct *result = NULL;
     if (pid) {
         struct hlist_node *first;
         first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
                           lockdep_tasklist_lock_is_held());
         if (first)
             result = hlist_entry(first, struct task_struct, pid_links[(type)]);
     }
     return result;
 }
 EXPORT_SYMBOL(pid_task);
 
 /*
  * Must be called under rcu_read_lock().
  */
 struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
 {
     RCU_LOCKDEP_WARN(!rcu_read_lock_held(),
              "find_task_by_pid_ns() needs rcu_read_lock() protection");
     return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
 }
 
 struct task_struct *find_task_by_vpid(pid_t vnr)
 {
     return find_task_by_pid_ns(vnr, task_active_pid_ns(current));
 }
 
 struct task_struct *find_get_task_by_vpid(pid_t nr)
 {
     struct task_struct *task;
 
     rcu_read_lock();
     task = find_task_by_vpid(nr);
     if (task)
         get_task_struct(task);
     rcu_read_unlock();
 
     return task;
 }
 
 struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 {
     struct pid *pid;
     rcu_read_lock();
     pid = get_pid(rcu_dereference(*task_pid_ptr(task, type)));
     rcu_read_unlock();
     return pid;
 }
 EXPORT_SYMBOL_GPL(get_task_pid);
 
 struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
 {
     struct task_struct *result;
     rcu_read_lock();
     result = pid_task(pid, type);
     if (result)
         get_task_struct(result);
     rcu_read_unlock();
     return result;
 }
 EXPORT_SYMBOL_GPL(get_pid_task);
 
 struct pid *find_get_pid(pid_t nr)
 {
     struct pid *pid;
 
     rcu_read_lock();
     pid = get_pid(find_vpid(nr));
     rcu_read_unlock();
 
     return pid;
 }
 EXPORT_SYMBOL_GPL(find_get_pid);
 
 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
 {
     struct upid *upid;
     pid_t nr = 0;
 
     if (pid && ns->level <= pid->level) {
         upid = &pid->numbers[ns->level];
         if (upid->ns == ns)
             nr = upid->nr;
     }
     return nr;
 }
 EXPORT_SYMBOL_GPL(pid_nr_ns);
 
 pid_t pid_vnr(struct pid *pid)
 {
     return pid_nr_ns(pid, task_active_pid_ns(current));
 }
 EXPORT_SYMBOL_GPL(pid_vnr);
 
 pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
             struct pid_namespace *ns)
 {
     pid_t nr = 0;
 
     rcu_read_lock();
     if (!ns)
         ns = task_active_pid_ns(current);
     nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
     rcu_read_unlock();
 
     return nr;
 }
 EXPORT_SYMBOL(__task_pid_nr_ns);
 
 struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
 {
     return ns_of_pid(task_pid(tsk));
 }
 EXPORT_SYMBOL_GPL(task_active_pid_ns);
 
 /*
  * Used by proc to find the first pid that is greater than or equal to nr.
  *
  * If there is a pid at nr this function is exactly the same as find_pid_ns.
  */
 struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 {
     return idr_get_next(&ns->idr, &nr);
 }
 
 struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags)
 {
     struct fd f;
     struct pid *pid;
 
     f = fdget(fd);
     if (!f.file)
         return ERR_PTR(-EBADF);
 
     pid = pidfd_pid(f.file);
     if (!IS_ERR(pid)) {
         get_pid(pid);
         *flags = f.file->f_flags;
     }
 
     fdput(f);
     return pid;
 }
 
 /**
  * pidfd_get_task() - Get the task associated with a pidfd
  *
  * @pidfd: pidfd for which to get the task
  * @flags: flags associated with this pidfd
  *
  * Return the task associated with @pidfd. The function takes a reference on
  * the returned task. The caller is responsible for releasing that reference.
  *
  * Currently, the process identified by @pidfd is always a thread-group leader.
  * This restriction currently exists for all aspects of pidfds including pidfd
  * creation (CLONE_PIDFD cannot be used with CLONE_THREAD) and pidfd polling
  * (only supports thread group leaders).
  *
  * Return: On success, the task_struct associated with the pidfd.
  *     On error, a negative errno number will be returned.
  */
 struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
 {
     unsigned int f_flags;
     struct pid *pid;
     struct task_struct *task;
 
     pid = pidfd_get_pid(pidfd, &f_flags);
     if (IS_ERR(pid))
         return ERR_CAST(pid);
 
     task = get_pid_task(pid, PIDTYPE_TGID);
     put_pid(pid);
     if (!task)
         return ERR_PTR(-ESRCH);
 
     *flags = f_flags;
     return task;
 }
 
 /**
  * pidfd_create() - Create a new pid file descriptor.
  *
  * @pid:   struct pid that the pidfd will reference
  * @flags: flags to pass
  *
  * This creates a new pid file descriptor with the O_CLOEXEC flag set.
  *
  * Note, that this function can only be called after the fd table has
  * been unshared to avoid leaking the pidfd to the new process.
  *
  * This symbol should not be explicitly exported to loadable modules.
  *
  * Return: On success, a cloexec pidfd is returned.
  *         On error, a negative errno number will be returned.
  */
 int pidfd_create(struct pid *pid, unsigned int flags)
 {
     int fd;
 
     if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
         return -EINVAL;
 
     if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
         return -EINVAL;
 
     fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
                   flags | O_RDWR | O_CLOEXEC);
     if (fd < 0)
         put_pid(pid);
 
     return fd;
 }
 
 /**
  * pidfd_open() - Open new pid file descriptor.
  *
  * @pid:   pid for which to retrieve a pidfd
  * @flags: flags to pass
  *
  * This creates a new pid file descriptor with the O_CLOEXEC flag set for
  * the process identified by @pid. Currently, the process identified by
  * @pid must be a thread-group leader. This restriction currently exists
  * for all aspects of pidfds including pidfd creation (CLONE_PIDFD cannot
  * be used with CLONE_THREAD) and pidfd polling (only supports thread group
  * leaders).
  *
  * Return: On success, a cloexec pidfd is returned.
  *         On error, a negative errno number will be returned.
  */
 SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
 {
     int fd;
     struct pid *p;
 
     if (flags & ~PIDFD_NONBLOCK)
         return -EINVAL;
 
     if (pid <= 0)
         return -EINVAL;
 
     p = find_get_pid(pid);
     if (!p)
         return -ESRCH;
 
     fd = pidfd_create(p, flags);
 
     put_pid(p);
     return fd;
 }
 
 void __init pid_idr_init(void)
 {
     /* Verify no one has done anything silly: */
     BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_ADDING);
 
     /* bump default and minimum pid_max based on number of cpus */
     pid_max = min(pid_max_max, max_t(int, pid_max,
                 PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
     pid_max_min = max_t(int, pid_max_min,
                 PIDS_PER_CPU_MIN * num_possible_cpus());
     pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
 
     idr_init(&init_pid_ns.idr);
 
     init_pid_ns.pid_cachep = KMEM_CACHE(pid,
             SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
 }
 
 static struct file *__pidfd_fget(struct task_struct *task, int fd)
 {
     struct file *file;
     int ret;
 
     ret = down_read_killable(&task->signal->exec_update_lock);
     if (ret)
         return ERR_PTR(ret);
 
     if (ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS))
         file = fget_task(task, fd);
     else
         file = ERR_PTR(-EPERM);
 
     up_read(&task->signal->exec_update_lock);
 
     return file ?: ERR_PTR(-EBADF);
 }
 
 static int pidfd_getfd(struct pid *pid, int fd)
 {
     struct task_struct *task;
     struct file *file;
     int ret;
 
     task = get_pid_task(pid, PIDTYPE_PID);
     if (!task)
         return -ESRCH;
 
     file = __pidfd_fget(task, fd);
     put_task_struct(task);
     if (IS_ERR(file))
         return PTR_ERR(file);
 
     ret = receive_fd(file, O_CLOEXEC);
     fput(file);
 
     return ret;
 }
 
 /**
  * sys_pidfd_getfd() - Get a file descriptor from another process
  *
  * @pidfd:  the pidfd file descriptor of the process
  * @fd:     the file descriptor number to get
  * @flags:  flags on how to get the fd (reserved)
  *
  * This syscall gets a copy of a file descriptor from another process
  * based on the pidfd, and file descriptor number. It requires that
  * the calling process has the ability to ptrace the process represented
  * by the pidfd. The process which is having its file descriptor copied
  * is otherwise unaffected.
  *
  * Return: On success, a cloexec file descriptor is returned.
  *         On error, a negative errno number will be returned.
  */
 SYSCALL_DEFINE3(pidfd_getfd, int, pidfd, int, fd,
         unsigned int, flags)
 {
     struct pid *pid;
     struct fd f;
     int ret;
 
     /* flags is currently unused - make sure it's unset */
     if (flags)
         return -EINVAL;
 
     f = fdget(pidfd);
     if (!f.file)
         return -EBADF;
 
     pid = pidfd_pid(f.file);
     if (IS_ERR(pid))
         ret = PTR_ERR(pid);
     else
         ret = pidfd_getfd(pid, fd);
 
     fdput(f);
     return ret;
 }

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