btrfs: document what the spinlock unused_bgs_lock protects

[thirdparty/linux.git] / kernel / pid.c

// 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
#if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
        .memfd_noexec_scope = MEMFD_NOEXEC_SCOPE_EXEC,
#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);
}
EXPORT_SYMBOL_GPL(find_ge_pid);

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 pidfd;
        struct file *pidfd_file;

        pidfd = pidfd_prepare(pid, flags, &pidfd_file);
        if (pidfd < 0)
                return pidfd;

        fd_install(pidfd, pidfd_file);
        return pidfd;
}

/**
 * sys_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_create("pid",
                        struct_size_t(struct pid, numbers, 1),
                        __alignof__(struct pid),
                        SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT,
                        NULL);
}

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, NULL, 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;
}