// SPDX-License-Identifier: GPL-2.0
/* Watch queue and general notification mechanism, built on pipes
 *
 * Copyright (C) 2020 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * See Documentation/watch_queue.rst
 */

#define pr_fmt(fmt) "watchq: " fmt
#include <linux/module.h>
#include <linux/init.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/printk.h>
#include <linux/miscdevice.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/pagemap.h>
#include <linux/poll.h>
#include <linux/uaccess.h>
#include <linux/vmalloc.h>
#include <linux/file.h>
#include <linux/security.h>
#include <linux/cred.h>
#include <linux/sched/signal.h>
#include <linux/watch_queue.h>
#include <linux/pipe_fs_i.h>

MODULE_DESCRIPTION("Watch queue");
MODULE_AUTHOR("Red Hat, Inc.");
MODULE_LICENSE("GPL");

#define WATCH_QUEUE_NOTE_SIZE 128
#define WATCH_QUEUE_NOTES_PER_PAGE (PAGE_SIZE / WATCH_QUEUE_NOTE_SIZE)

static void watch_queue_pipe_buf_release(struct pipe_inode_info *pipe,
                                         struct pipe_buffer *buf)
{
        struct watch_queue *wqueue = (struct watch_queue *)buf->private;
        struct page *page;
        unsigned int bit;

        /* We need to work out which note within the page this refers to, but
         * the note might have been maximum size, so merely ANDing the offset
         * off doesn't work.  OTOH, the note must've been more than zero size.
         */
        bit = buf->offset + buf->len;
        if ((bit & (WATCH_QUEUE_NOTE_SIZE - 1)) == 0)
                bit -= WATCH_QUEUE_NOTE_SIZE;
        bit /= WATCH_QUEUE_NOTE_SIZE;

        page = buf->page;
        bit += page->index;

        set_bit(bit, wqueue->notes_bitmap);
}

// No try_steal function => no stealing
#define watch_queue_pipe_buf_try_steal NULL

/* New data written to a pipe may be appended to a buffer with this type. */
static const struct pipe_buf_operations watch_queue_pipe_buf_ops = {
        .release        = watch_queue_pipe_buf_release,
        .try_steal      = watch_queue_pipe_buf_try_steal,
        .get            = generic_pipe_buf_get,
};

/*
 * Post a notification to a watch queue.
 */
static bool post_one_notification(struct watch_queue *wqueue,
                                  struct watch_notification *n)
{
        void *p;
        struct pipe_inode_info *pipe = wqueue->pipe;
        struct pipe_buffer *buf;
        struct page *page;
        unsigned int head, tail, mask, note, offset, len;
        bool done = false;

        if (!pipe)
                return false;

        spin_lock_irq(&pipe->rd_wait.lock);

        if (wqueue->defunct)
                goto out;

        mask = pipe->ring_size - 1;
        head = pipe->head;
        tail = pipe->tail;
        if (pipe_full(head, tail, pipe->ring_size))
                goto lost;

        note = find_first_bit(wqueue->notes_bitmap, wqueue->nr_notes);
        if (note >= wqueue->nr_notes)
                goto lost;

        page = wqueue->notes[note / WATCH_QUEUE_NOTES_PER_PAGE];
        offset = note % WATCH_QUEUE_NOTES_PER_PAGE * WATCH_QUEUE_NOTE_SIZE;
        get_page(page);
        len = n->info & WATCH_INFO_LENGTH;
        p = kmap_atomic(page);
        memcpy(p + offset, n, len);
        kunmap_atomic(p);

        buf = &pipe->bufs[head & mask];
        buf->page = page;
        buf->private = (unsigned long)wqueue;
        buf->ops = &watch_queue_pipe_buf_ops;
        buf->offset = offset;
        buf->len = len;
        buf->flags = PIPE_BUF_FLAG_WHOLE;
        pipe->head = head + 1;

        if (!test_and_clear_bit(note, wqueue->notes_bitmap)) {
                spin_unlock_irq(&pipe->rd_wait.lock);
                BUG();
        }
        wake_up_interruptible_sync_poll_locked(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
        done = true;

out:
        spin_unlock_irq(&pipe->rd_wait.lock);
        if (done)
                kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
        return done;

lost:
        buf = &pipe->bufs[(head - 1) & mask];
        buf->flags |= PIPE_BUF_FLAG_LOSS;
        goto out;
}

/*
 * Apply filter rules to a notification.
 */
static bool filter_watch_notification(const struct watch_filter *wf,
                                      const struct watch_notification *n)
{
        const struct watch_type_filter *wt;
        unsigned int st_bits = sizeof(wt->subtype_filter[0]) * 8;
        unsigned int st_index = n->subtype / st_bits;
        unsigned int st_bit = 1U << (n->subtype % st_bits);
        int i;

        if (!test_bit(n->type, wf->type_filter))
                return false;

        for (i = 0; i < wf->nr_filters; i++) {
                wt = &wf->filters[i];
                if (n->type == wt->type &&
                    (wt->subtype_filter[st_index] & st_bit) &&
                    (n->info & wt->info_mask) == wt->info_filter)
                        return true;
        }

        return false; /* If there is a filter, the default is to reject. */
}

/**
 * __post_watch_notification - Post an event notification
 * @wlist: The watch list to post the event to.
 * @n: The notification record to post.
 * @cred: The creds of the process that triggered the notification.
 * @id: The ID to match on the watch.
 *
 * Post a notification of an event into a set of watch queues and let the users
 * know.
 *
 * The size of the notification should be set in n->info & WATCH_INFO_LENGTH and
 * should be in units of sizeof(*n).
 */
void __post_watch_notification(struct watch_list *wlist,
                               struct watch_notification *n,
                               const struct cred *cred,
                               u64 id)
{
        const struct watch_filter *wf;
        struct watch_queue *wqueue;
        struct watch *watch;

        if (((n->info & WATCH_INFO_LENGTH) >> WATCH_INFO_LENGTH__SHIFT) == 0) {
                WARN_ON(1);
                return;
        }

        rcu_read_lock();

        hlist_for_each_entry_rcu(watch, &wlist->watchers, list_node) {
                if (watch->id != id)
                        continue;
                n->info &= ~WATCH_INFO_ID;
                n->info |= watch->info_id;

                wqueue = rcu_dereference(watch->queue);
                wf = rcu_dereference(wqueue->filter);
                if (wf && !filter_watch_notification(wf, n))
                        continue;

                if (security_post_notification(watch->cred, cred, n) < 0)
                        continue;

                post_one_notification(wqueue, n);
        }

        rcu_read_unlock();
}
EXPORT_SYMBOL(__post_watch_notification);

/*
 * Allocate sufficient pages to preallocation for the requested number of
 * notifications.
 */
long watch_queue_set_size(struct pipe_inode_info *pipe, unsigned int nr_notes)
{
        struct watch_queue *wqueue = pipe->watch_queue;
        struct page **pages;
        unsigned long *bitmap;
        unsigned long user_bufs;
        unsigned int bmsize;
        int ret, i, nr_pages;

        if (!wqueue)
                return -ENODEV;
        if (wqueue->notes)
                return -EBUSY;

        if (nr_notes < 1 ||
            nr_notes > 512) /* TODO: choose a better hard limit */
                return -EINVAL;

        nr_pages = (nr_notes + WATCH_QUEUE_NOTES_PER_PAGE - 1);
        nr_pages /= WATCH_QUEUE_NOTES_PER_PAGE;
        user_bufs = account_pipe_buffers(pipe->user, pipe->nr_accounted, nr_pages);

        if (nr_pages > pipe->max_usage &&
            (too_many_pipe_buffers_hard(user_bufs) ||
             too_many_pipe_buffers_soft(user_bufs)) &&
            pipe_is_unprivileged_user()) {
                ret = -EPERM;
                goto error;
        }

        ret = pipe_resize_ring(pipe, nr_notes);
        if (ret < 0)
                goto error;

        pages = kcalloc(sizeof(struct page *), nr_pages, GFP_KERNEL);
        if (!pages)
                goto error;

        for (i = 0; i < nr_pages; i++) {
                pages[i] = alloc_page(GFP_KERNEL);
                if (!pages[i])
                        goto error_p;
                pages[i]->index = i * WATCH_QUEUE_NOTES_PER_PAGE;
        }

        bmsize = (nr_notes + BITS_PER_LONG - 1) / BITS_PER_LONG;
        bmsize *= sizeof(unsigned long);
        bitmap = kmalloc(bmsize, GFP_KERNEL);
        if (!bitmap)
                goto error_p;

        memset(bitmap, 0xff, bmsize);
        wqueue->notes = pages;
        wqueue->notes_bitmap = bitmap;
        wqueue->nr_pages = nr_pages;
        wqueue->nr_notes = nr_pages * WATCH_QUEUE_NOTES_PER_PAGE;
        return 0;

error_p:
        for (i = 0; i < nr_pages; i++)
                __free_page(pages[i]);
        kfree(pages);
error:
        (void) account_pipe_buffers(pipe->user, nr_pages, pipe->nr_accounted);
        return ret;
}

/*
 * Set the filter on a watch queue.
 */
long watch_queue_set_filter(struct pipe_inode_info *pipe,
                            struct watch_notification_filter __user *_filter)
{
        struct watch_notification_type_filter *tf;
        struct watch_notification_filter filter;
        struct watch_type_filter *q;
        struct watch_filter *wfilter;
        struct watch_queue *wqueue = pipe->watch_queue;
        int ret, nr_filter = 0, i;

        if (!wqueue)
                return -ENODEV;

        if (!_filter) {
                /* Remove the old filter */
                wfilter = NULL;
                goto set;
        }

        /* Grab the user's filter specification */
        if (copy_from_user(&filter, _filter, sizeof(filter)) != 0)
                return -EFAULT;
        if (filter.nr_filters == 0 ||
            filter.nr_filters > 16 ||
            filter.__reserved != 0)
                return -EINVAL;

        tf = memdup_user(_filter->filters, filter.nr_filters * sizeof(*tf));
        if (IS_ERR(tf))
                return PTR_ERR(tf);

        ret = -EINVAL;
        for (i = 0; i < filter.nr_filters; i++) {
                if ((tf[i].info_filter & ~tf[i].info_mask) ||
                    tf[i].info_mask & WATCH_INFO_LENGTH)
                        goto err_filter;
                /* Ignore any unknown types */
                if (tf[i].type >= sizeof(wfilter->type_filter) * 8)
                        continue;
                nr_filter++;
        }

        /* Now we need to build the internal filter from only the relevant
         * user-specified filters.
         */
        ret = -ENOMEM;
        wfilter = kzalloc(struct_size(wfilter, filters, nr_filter), GFP_KERNEL);
        if (!wfilter)
                goto err_filter;
        wfilter->nr_filters = nr_filter;

        q = wfilter->filters;
        for (i = 0; i < filter.nr_filters; i++) {
                if (tf[i].type >= sizeof(wfilter->type_filter) * BITS_PER_LONG)
                        continue;

                q->type                 = tf[i].type;
                q->info_filter          = tf[i].info_filter;
                q->info_mask            = tf[i].info_mask;
                q->subtype_filter[0]    = tf[i].subtype_filter[0];
                __set_bit(q->type, wfilter->type_filter);
                q++;
        }

        kfree(tf);
set:
        pipe_lock(pipe);
        wfilter = rcu_replace_pointer(wqueue->filter, wfilter,
                                      lockdep_is_held(&pipe->mutex));
        pipe_unlock(pipe);
        if (wfilter)
                kfree_rcu(wfilter, rcu);
        return 0;

err_filter:
        kfree(tf);
        return ret;
}

static void __put_watch_queue(struct kref *kref)
{
        struct watch_queue *wqueue =
                container_of(kref, struct watch_queue, usage);
        struct watch_filter *wfilter;
        int i;

        for (i = 0; i < wqueue->nr_pages; i++)
                __free_page(wqueue->notes[i]);

        wfilter = rcu_access_pointer(wqueue->filter);
        if (wfilter)
                kfree_rcu(wfilter, rcu);
        kfree_rcu(wqueue, rcu);
}

/**
 * put_watch_queue - Dispose of a ref on a watchqueue.
 * @wqueue: The watch queue to unref.
 */
void put_watch_queue(struct watch_queue *wqueue)
{
        kref_put(&wqueue->usage, __put_watch_queue);
}
EXPORT_SYMBOL(put_watch_queue);

static void free_watch(struct rcu_head *rcu)
{
        struct watch *watch = container_of(rcu, struct watch, rcu);

        put_watch_queue(rcu_access_pointer(watch->queue));
        atomic_dec(&watch->cred->user->nr_watches);
        put_cred(watch->cred);
}

static void __put_watch(struct kref *kref)
{
        struct watch *watch = container_of(kref, struct watch, usage);

        call_rcu(&watch->rcu, free_watch);
}

/*
 * Discard a watch.
 */
static void put_watch(struct watch *watch)
{
        kref_put(&watch->usage, __put_watch);
}

/**
 * init_watch - Initialise a watch
 * @watch: The watch to initialise.
 * @wqueue: The queue to assign.
 *
 * Initialise a watch and set the watch queue.
 */
void init_watch(struct watch *watch, struct watch_queue *wqueue)
{
        kref_init(&watch->usage);
        INIT_HLIST_NODE(&watch->list_node);
        INIT_HLIST_NODE(&watch->queue_node);
        rcu_assign_pointer(watch->queue, wqueue);
}

/**
 * add_watch_to_object - Add a watch on an object to a watch list
 * @watch: The watch to add
 * @wlist: The watch list to add to
 *
 * @watch->queue must have been set to point to the queue to post notifications
 * to and the watch list of the object to be watched.  @watch->cred must also
 * have been set to the appropriate credentials and a ref taken on them.
 *
 * The caller must pin the queue and the list both and must hold the list
 * locked against racing watch additions/removals.
 */
int add_watch_to_object(struct watch *watch, struct watch_list *wlist)
{
        struct watch_queue *wqueue = rcu_access_pointer(watch->queue);
        struct watch *w;

        hlist_for_each_entry(w, &wlist->watchers, list_node) {
                struct watch_queue *wq = rcu_access_pointer(w->queue);
                if (wqueue == wq && watch->id == w->id)
                        return -EBUSY;
        }

        watch->cred = get_current_cred();
        rcu_assign_pointer(watch->watch_list, wlist);

        if (atomic_inc_return(&watch->cred->user->nr_watches) >
            task_rlimit(current, RLIMIT_NOFILE)) {
                atomic_dec(&watch->cred->user->nr_watches);
                put_cred(watch->cred);
                return -EAGAIN;
        }

        spin_lock_bh(&wqueue->lock);
        kref_get(&wqueue->usage);
        kref_get(&watch->usage);
        hlist_add_head(&watch->queue_node, &wqueue->watches);
        spin_unlock_bh(&wqueue->lock);

        hlist_add_head(&watch->list_node, &wlist->watchers);
        return 0;
}
EXPORT_SYMBOL(add_watch_to_object);

/**
 * remove_watch_from_object - Remove a watch or all watches from an object.
 * @wlist: The watch list to remove from
 * @wq: The watch queue of interest (ignored if @all is true)
 * @id: The ID of the watch to remove (ignored if @all is true)
 * @all: True to remove all objects
 *
 * Remove a specific watch or all watches from an object.  A notification is
 * sent to the watcher to tell them that this happened.
 */
int remove_watch_from_object(struct watch_list *wlist, struct watch_queue *wq,
                             u64 id, bool all)
{
        struct watch_notification_removal n;
        struct watch_queue *wqueue;
        struct watch *watch;
        int ret = -EBADSLT;

        rcu_read_lock();

again:
        spin_lock(&wlist->lock);
        hlist_for_each_entry(watch, &wlist->watchers, list_node) {
                if (all ||
                    (watch->id == id && rcu_access_pointer(watch->queue) == wq))
                        goto found;
        }
        spin_unlock(&wlist->lock);
        goto out;

found:
        ret = 0;
        hlist_del_init_rcu(&watch->list_node);
        rcu_assign_pointer(watch->watch_list, NULL);
        spin_unlock(&wlist->lock);

        /* We now own the reference on watch that used to belong to wlist. */

        n.watch.type = WATCH_TYPE_META;
        n.watch.subtype = WATCH_META_REMOVAL_NOTIFICATION;
        n.watch.info = watch->info_id | watch_sizeof(n.watch);
        n.id = id;
        if (id != 0)
                n.watch.info = watch->info_id | watch_sizeof(n);

        wqueue = rcu_dereference(watch->queue);

        /* We don't need the watch list lock for the next bit as RCU is
         * protecting *wqueue from deallocation.
         */
        if (wqueue) {
                post_one_notification(wqueue, &n.watch);

                spin_lock_bh(&wqueue->lock);

                if (!hlist_unhashed(&watch->queue_node)) {
                        hlist_del_init_rcu(&watch->queue_node);
                        put_watch(watch);
                }

                spin_unlock_bh(&wqueue->lock);
        }

        if (wlist->release_watch) {
                void (*release_watch)(struct watch *);

                release_watch = wlist->release_watch;
                rcu_read_unlock();
                (*release_watch)(watch);
                rcu_read_lock();
        }
        put_watch(watch);

        if (all && !hlist_empty(&wlist->watchers))
                goto again;
out:
        rcu_read_unlock();
        return ret;
}
EXPORT_SYMBOL(remove_watch_from_object);

/*
 * Remove all the watches that are contributory to a queue.  This has the
 * potential to race with removal of the watches by the destruction of the
 * objects being watched or with the distribution of notifications.
 */
void watch_queue_clear(struct watch_queue *wqueue)
{
        struct watch_list *wlist;
        struct watch *watch;
        bool release;

        rcu_read_lock();
        spin_lock_bh(&wqueue->lock);

        /* Prevent new additions and prevent notifications from happening */
        wqueue->defunct = true;

        while (!hlist_empty(&wqueue->watches)) {
                watch = hlist_entry(wqueue->watches.first, struct watch, queue_node);
                hlist_del_init_rcu(&watch->queue_node);
                /* We now own a ref on the watch. */
                spin_unlock_bh(&wqueue->lock);

                /* We can't do the next bit under the queue lock as we need to
                 * get the list lock - which would cause a deadlock if someone
                 * was removing from the opposite direction at the same time or
                 * posting a notification.
                 */
                wlist = rcu_dereference(watch->watch_list);
                if (wlist) {
                        void (*release_watch)(struct watch *);

                        spin_lock(&wlist->lock);

                        release = !hlist_unhashed(&watch->list_node);
                        if (release) {
                                hlist_del_init_rcu(&watch->list_node);
                                rcu_assign_pointer(watch->watch_list, NULL);

                                /* We now own a second ref on the watch. */
                        }

                        release_watch = wlist->release_watch;
                        spin_unlock(&wlist->lock);

                        if (release) {
                                if (release_watch) {
                                        rcu_read_unlock();
                                        /* This might need to call dput(), so
                                         * we have to drop all the locks.
                                         */
                                        (*release_watch)(watch);
                                        rcu_read_lock();
                                }
                                put_watch(watch);
                        }
                }

                put_watch(watch);
                spin_lock_bh(&wqueue->lock);
        }

        spin_unlock_bh(&wqueue->lock);
        rcu_read_unlock();
}

/**
 * get_watch_queue - Get a watch queue from its file descriptor.
 * @fd: The fd to query.
 */
struct watch_queue *get_watch_queue(int fd)
{
        struct pipe_inode_info *pipe;
        struct watch_queue *wqueue = ERR_PTR(-EINVAL);
        struct fd f;

        f = fdget(fd);
        if (f.file) {
                pipe = get_pipe_info(f.file, false);
                if (pipe && pipe->watch_queue) {
                        wqueue = pipe->watch_queue;
                        kref_get(&wqueue->usage);
                }
                fdput(f);
        }

        return wqueue;
}
EXPORT_SYMBOL(get_watch_queue);

/*
 * Initialise a watch queue
 */
int watch_queue_init(struct pipe_inode_info *pipe)
{
        struct watch_queue *wqueue;

        wqueue = kzalloc(sizeof(*wqueue), GFP_KERNEL);
        if (!wqueue)
                return -ENOMEM;

        wqueue->pipe = pipe;
        kref_init(&wqueue->usage);
        spin_lock_init(&wqueue->lock);
        INIT_HLIST_HEAD(&wqueue->watches);

        pipe->watch_queue = wqueue;
        return 0;
}