Merge branch 'spi-5.1' into spi-linus

[sfrench/cifs-2.6.git] / fs / kernfs / file.c

/*
 * fs/kernfs/file.c - kernfs file implementation
 *
 * Copyright (c) 2001-3 Patrick Mochel
 * Copyright (c) 2007 SUSE Linux Products GmbH
 * Copyright (c) 2007, 2013 Tejun Heo <tj@kernel.org>
 *
 * This file is released under the GPLv2.
 */

#include <linux/fs.h>
#include <linux/seq_file.h>
#include <linux/slab.h>
#include <linux/poll.h>
#include <linux/pagemap.h>
#include <linux/sched/mm.h>
#include <linux/fsnotify.h>

#include "kernfs-internal.h"

/*
 * There's one kernfs_open_file for each open file and one kernfs_open_node
 * for each kernfs_node with one or more open files.
 *
 * kernfs_node->attr.open points to kernfs_open_node.  attr.open is
 * protected by kernfs_open_node_lock.
 *
 * filp->private_data points to seq_file whose ->private points to
 * kernfs_open_file.  kernfs_open_files are chained at
 * kernfs_open_node->files, which is protected by kernfs_open_file_mutex.
 */
static DEFINE_SPINLOCK(kernfs_open_node_lock);
static DEFINE_MUTEX(kernfs_open_file_mutex);

struct kernfs_open_node {
        atomic_t                refcnt;
        atomic_t                event;
        wait_queue_head_t       poll;
        struct list_head        files; /* goes through kernfs_open_file.list */
};

/*
 * kernfs_notify() may be called from any context and bounces notifications
 * through a work item.  To minimize space overhead in kernfs_node, the
 * pending queue is implemented as a singly linked list of kernfs_nodes.
 * The list is terminated with the self pointer so that whether a
 * kernfs_node is on the list or not can be determined by testing the next
 * pointer for NULL.
 */
#define KERNFS_NOTIFY_EOL                       ((void *)&kernfs_notify_list)

static DEFINE_SPINLOCK(kernfs_notify_lock);
static struct kernfs_node *kernfs_notify_list = KERNFS_NOTIFY_EOL;

static struct kernfs_open_file *kernfs_of(struct file *file)
{
        return ((struct seq_file *)file->private_data)->private;
}

/*
 * Determine the kernfs_ops for the given kernfs_node.  This function must
 * be called while holding an active reference.
 */
static const struct kernfs_ops *kernfs_ops(struct kernfs_node *kn)
{
        if (kn->flags & KERNFS_LOCKDEP)
                lockdep_assert_held(kn);
        return kn->attr.ops;
}

/*
 * As kernfs_seq_stop() is also called after kernfs_seq_start() or
 * kernfs_seq_next() failure, it needs to distinguish whether it's stopping
 * a seq_file iteration which is fully initialized with an active reference
 * or an aborted kernfs_seq_start() due to get_active failure.  The
 * position pointer is the only context for each seq_file iteration and
 * thus the stop condition should be encoded in it.  As the return value is
 * directly visible to userland, ERR_PTR(-ENODEV) is the only acceptable
 * choice to indicate get_active failure.
 *
 * Unfortunately, this is complicated due to the optional custom seq_file
 * operations which may return ERR_PTR(-ENODEV) too.  kernfs_seq_stop()
 * can't distinguish whether ERR_PTR(-ENODEV) is from get_active failure or
 * custom seq_file operations and thus can't decide whether put_active
 * should be performed or not only on ERR_PTR(-ENODEV).
 *
 * This is worked around by factoring out the custom seq_stop() and
 * put_active part into kernfs_seq_stop_active(), skipping it from
 * kernfs_seq_stop() if ERR_PTR(-ENODEV) while invoking it directly after
 * custom seq_file operations fail with ERR_PTR(-ENODEV) - this ensures
 * that kernfs_seq_stop_active() is skipped only after get_active failure.
 */
static void kernfs_seq_stop_active(struct seq_file *sf, void *v)
{
        struct kernfs_open_file *of = sf->private;
        const struct kernfs_ops *ops = kernfs_ops(of->kn);

        if (ops->seq_stop)
                ops->seq_stop(sf, v);
        kernfs_put_active(of->kn);
}

static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos)
{
        struct kernfs_open_file *of = sf->private;
        const struct kernfs_ops *ops;

        /*
         * @of->mutex nests outside active ref and is primarily to ensure that
         * the ops aren't called concurrently for the same open file.
         */
        mutex_lock(&of->mutex);
        if (!kernfs_get_active(of->kn))
                return ERR_PTR(-ENODEV);

        ops = kernfs_ops(of->kn);
        if (ops->seq_start) {
                void *next = ops->seq_start(sf, ppos);
                /* see the comment above kernfs_seq_stop_active() */
                if (next == ERR_PTR(-ENODEV))
                        kernfs_seq_stop_active(sf, next);
                return next;
        } else {
                /*
                 * The same behavior and code as single_open().  Returns
                 * !NULL if pos is at the beginning; otherwise, NULL.
                 */
                return NULL + !*ppos;
        }
}

static void *kernfs_seq_next(struct seq_file *sf, void *v, loff_t *ppos)
{
        struct kernfs_open_file *of = sf->private;
        const struct kernfs_ops *ops = kernfs_ops(of->kn);

        if (ops->seq_next) {
                void *next = ops->seq_next(sf, v, ppos);
                /* see the comment above kernfs_seq_stop_active() */
                if (next == ERR_PTR(-ENODEV))
                        kernfs_seq_stop_active(sf, next);
                return next;
        } else {
                /*
                 * The same behavior and code as single_open(), always
                 * terminate after the initial read.
                 */
                ++*ppos;
                return NULL;
        }
}

static void kernfs_seq_stop(struct seq_file *sf, void *v)
{
        struct kernfs_open_file *of = sf->private;

        if (v != ERR_PTR(-ENODEV))
                kernfs_seq_stop_active(sf, v);
        mutex_unlock(&of->mutex);
}

static int kernfs_seq_show(struct seq_file *sf, void *v)
{
        struct kernfs_open_file *of = sf->private;

        of->event = atomic_read(&of->kn->attr.open->event);

        return of->kn->attr.ops->seq_show(sf, v);
}

static const struct seq_operations kernfs_seq_ops = {
        .start = kernfs_seq_start,
        .next = kernfs_seq_next,
        .stop = kernfs_seq_stop,
        .show = kernfs_seq_show,
};

/*
 * As reading a bin file can have side-effects, the exact offset and bytes
 * specified in read(2) call should be passed to the read callback making
 * it difficult to use seq_file.  Implement simplistic custom buffering for
 * bin files.
 */
static ssize_t kernfs_file_direct_read(struct kernfs_open_file *of,
                                       char __user *user_buf, size_t count,
                                       loff_t *ppos)
{
        ssize_t len = min_t(size_t, count, PAGE_SIZE);
        const struct kernfs_ops *ops;
        char *buf;

        buf = of->prealloc_buf;
        if (buf)
                mutex_lock(&of->prealloc_mutex);
        else
                buf = kmalloc(len, GFP_KERNEL);
        if (!buf)
                return -ENOMEM;

        /*
         * @of->mutex nests outside active ref and is used both to ensure that
         * the ops aren't called concurrently for the same open file.
         */
        mutex_lock(&of->mutex);
        if (!kernfs_get_active(of->kn)) {
                len = -ENODEV;
                mutex_unlock(&of->mutex);
                goto out_free;
        }

        of->event = atomic_read(&of->kn->attr.open->event);
        ops = kernfs_ops(of->kn);
        if (ops->read)
                len = ops->read(of, buf, len, *ppos);
        else
                len = -EINVAL;

        kernfs_put_active(of->kn);
        mutex_unlock(&of->mutex);

        if (len < 0)
                goto out_free;

        if (copy_to_user(user_buf, buf, len)) {
                len = -EFAULT;
                goto out_free;
        }

        *ppos += len;

 out_free:
        if (buf == of->prealloc_buf)
                mutex_unlock(&of->prealloc_mutex);
        else
                kfree(buf);
        return len;
}

/**
 * kernfs_fop_read - kernfs vfs read callback
 * @file: file pointer
 * @user_buf: data to write
 * @count: number of bytes
 * @ppos: starting offset
 */
static ssize_t kernfs_fop_read(struct file *file, char __user *user_buf,
                               size_t count, loff_t *ppos)
{
        struct kernfs_open_file *of = kernfs_of(file);

        if (of->kn->flags & KERNFS_HAS_SEQ_SHOW)
                return seq_read(file, user_buf, count, ppos);
        else
                return kernfs_file_direct_read(of, user_buf, count, ppos);
}

/**
 * kernfs_fop_write - kernfs vfs write callback
 * @file: file pointer
 * @user_buf: data to write
 * @count: number of bytes
 * @ppos: starting offset
 *
 * Copy data in from userland and pass it to the matching kernfs write
 * operation.
 *
 * There is no easy way for us to know if userspace is only doing a partial
 * write, so we don't support them. We expect the entire buffer to come on
 * the first write.  Hint: if you're writing a value, first read the file,
 * modify only the the value you're changing, then write entire buffer
 * back.
 */
static ssize_t kernfs_fop_write(struct file *file, const char __user *user_buf,
                                size_t count, loff_t *ppos)
{
        struct kernfs_open_file *of = kernfs_of(file);
        const struct kernfs_ops *ops;
        ssize_t len;
        char *buf;

        if (of->atomic_write_len) {
                len = count;
                if (len > of->atomic_write_len)
                        return -E2BIG;
        } else {
                len = min_t(size_t, count, PAGE_SIZE);
        }

        buf = of->prealloc_buf;
        if (buf)
                mutex_lock(&of->prealloc_mutex);
        else
                buf = kmalloc(len + 1, GFP_KERNEL);
        if (!buf)
                return -ENOMEM;

        if (copy_from_user(buf, user_buf, len)) {
                len = -EFAULT;
                goto out_free;
        }
        buf[len] = '\0';        /* guarantee string termination */

        /*
         * @of->mutex nests outside active ref and is used both to ensure that
         * the ops aren't called concurrently for the same open file.
         */
        mutex_lock(&of->mutex);
        if (!kernfs_get_active(of->kn)) {
                mutex_unlock(&of->mutex);
                len = -ENODEV;
                goto out_free;
        }

        ops = kernfs_ops(of->kn);
        if (ops->write)
                len = ops->write(of, buf, len, *ppos);
        else
                len = -EINVAL;

        kernfs_put_active(of->kn);
        mutex_unlock(&of->mutex);

        if (len > 0)
                *ppos += len;

out_free:
        if (buf == of->prealloc_buf)
                mutex_unlock(&of->prealloc_mutex);
        else
                kfree(buf);
        return len;
}

static void kernfs_vma_open(struct vm_area_struct *vma)
{
        struct file *file = vma->vm_file;
        struct kernfs_open_file *of = kernfs_of(file);

        if (!of->vm_ops)
                return;

        if (!kernfs_get_active(of->kn))
                return;

        if (of->vm_ops->open)
                of->vm_ops->open(vma);

        kernfs_put_active(of->kn);
}

static vm_fault_t kernfs_vma_fault(struct vm_fault *vmf)
{
        struct file *file = vmf->vma->vm_file;
        struct kernfs_open_file *of = kernfs_of(file);
        vm_fault_t ret;

        if (!of->vm_ops)
                return VM_FAULT_SIGBUS;

        if (!kernfs_get_active(of->kn))
                return VM_FAULT_SIGBUS;

        ret = VM_FAULT_SIGBUS;
        if (of->vm_ops->fault)
                ret = of->vm_ops->fault(vmf);

        kernfs_put_active(of->kn);
        return ret;
}

static vm_fault_t kernfs_vma_page_mkwrite(struct vm_fault *vmf)
{
        struct file *file = vmf->vma->vm_file;
        struct kernfs_open_file *of = kernfs_of(file);
        vm_fault_t ret;

        if (!of->vm_ops)
                return VM_FAULT_SIGBUS;

        if (!kernfs_get_active(of->kn))
                return VM_FAULT_SIGBUS;

        ret = 0;
        if (of->vm_ops->page_mkwrite)
                ret = of->vm_ops->page_mkwrite(vmf);
        else
                file_update_time(file);

        kernfs_put_active(of->kn);
        return ret;
}

static int kernfs_vma_access(struct vm_area_struct *vma, unsigned long addr,
                             void *buf, int len, int write)
{
        struct file *file = vma->vm_file;
        struct kernfs_open_file *of = kernfs_of(file);
        int ret;

        if (!of->vm_ops)
                return -EINVAL;

        if (!kernfs_get_active(of->kn))
                return -EINVAL;

        ret = -EINVAL;
        if (of->vm_ops->access)
                ret = of->vm_ops->access(vma, addr, buf, len, write);

        kernfs_put_active(of->kn);
        return ret;
}

#ifdef CONFIG_NUMA
static int kernfs_vma_set_policy(struct vm_area_struct *vma,
                                 struct mempolicy *new)
{
        struct file *file = vma->vm_file;
        struct kernfs_open_file *of = kernfs_of(file);
        int ret;

        if (!of->vm_ops)
                return 0;

        if (!kernfs_get_active(of->kn))
                return -EINVAL;

        ret = 0;
        if (of->vm_ops->set_policy)
                ret = of->vm_ops->set_policy(vma, new);

        kernfs_put_active(of->kn);
        return ret;
}

static struct mempolicy *kernfs_vma_get_policy(struct vm_area_struct *vma,
                                               unsigned long addr)
{
        struct file *file = vma->vm_file;
        struct kernfs_open_file *of = kernfs_of(file);
        struct mempolicy *pol;

        if (!of->vm_ops)
                return vma->vm_policy;

        if (!kernfs_get_active(of->kn))
                return vma->vm_policy;

        pol = vma->vm_policy;
        if (of->vm_ops->get_policy)
                pol = of->vm_ops->get_policy(vma, addr);

        kernfs_put_active(of->kn);
        return pol;
}

#endif

static const struct vm_operations_struct kernfs_vm_ops = {
        .open           = kernfs_vma_open,
        .fault          = kernfs_vma_fault,
        .page_mkwrite   = kernfs_vma_page_mkwrite,
        .access         = kernfs_vma_access,
#ifdef CONFIG_NUMA
        .set_policy     = kernfs_vma_set_policy,
        .get_policy     = kernfs_vma_get_policy,
#endif
};

static int kernfs_fop_mmap(struct file *file, struct vm_area_struct *vma)
{
        struct kernfs_open_file *of = kernfs_of(file);
        const struct kernfs_ops *ops;
        int rc;

        /*
         * mmap path and of->mutex are prone to triggering spurious lockdep
         * warnings and we don't want to add spurious locking dependency
         * between the two.  Check whether mmap is actually implemented
         * without grabbing @of->mutex by testing HAS_MMAP flag.  See the
         * comment in kernfs_file_open() for more details.
         */
        if (!(of->kn->flags & KERNFS_HAS_MMAP))
                return -ENODEV;

        mutex_lock(&of->mutex);

        rc = -ENODEV;
        if (!kernfs_get_active(of->kn))
                goto out_unlock;

        ops = kernfs_ops(of->kn);
        rc = ops->mmap(of, vma);
        if (rc)
                goto out_put;

        /*
         * PowerPC's pci_mmap of legacy_mem uses shmem_zero_setup()
         * to satisfy versions of X which crash if the mmap fails: that
         * substitutes a new vm_file, and we don't then want bin_vm_ops.
         */
        if (vma->vm_file != file)
                goto out_put;

        rc = -EINVAL;
        if (of->mmapped && of->vm_ops != vma->vm_ops)
                goto out_put;

        /*
         * It is not possible to successfully wrap close.
         * So error if someone is trying to use close.
         */
        rc = -EINVAL;
        if (vma->vm_ops && vma->vm_ops->close)
                goto out_put;

        rc = 0;
        of->mmapped = true;
        of->vm_ops = vma->vm_ops;
        vma->vm_ops = &kernfs_vm_ops;
out_put:
        kernfs_put_active(of->kn);
out_unlock:
        mutex_unlock(&of->mutex);

        return rc;
}

/**
 *      kernfs_get_open_node - get or create kernfs_open_node
 *      @kn: target kernfs_node
 *      @of: kernfs_open_file for this instance of open
 *
 *      If @kn->attr.open exists, increment its reference count; otherwise,
 *      create one.  @of is chained to the files list.
 *
 *      LOCKING:
 *      Kernel thread context (may sleep).
 *
 *      RETURNS:
 *      0 on success, -errno on failure.
 */
static int kernfs_get_open_node(struct kernfs_node *kn,
                                struct kernfs_open_file *of)
{
        struct kernfs_open_node *on, *new_on = NULL;

 retry:
        mutex_lock(&kernfs_open_file_mutex);
        spin_lock_irq(&kernfs_open_node_lock);

        if (!kn->attr.open && new_on) {
                kn->attr.open = new_on;
                new_on = NULL;
        }

        on = kn->attr.open;
        if (on) {
                atomic_inc(&on->refcnt);
                list_add_tail(&of->list, &on->files);
        }

        spin_unlock_irq(&kernfs_open_node_lock);
        mutex_unlock(&kernfs_open_file_mutex);

        if (on) {
                kfree(new_on);
                return 0;
        }

        /* not there, initialize a new one and retry */
        new_on = kmalloc(sizeof(*new_on), GFP_KERNEL);
        if (!new_on)
                return -ENOMEM;

        atomic_set(&new_on->refcnt, 0);
        atomic_set(&new_on->event, 1);
        init_waitqueue_head(&new_on->poll);
        INIT_LIST_HEAD(&new_on->files);
        goto retry;
}

/**
 *      kernfs_put_open_node - put kernfs_open_node
 *      @kn: target kernfs_nodet
 *      @of: associated kernfs_open_file
 *
 *      Put @kn->attr.open and unlink @of from the files list.  If
 *      reference count reaches zero, disassociate and free it.
 *
 *      LOCKING:
 *      None.
 */
static void kernfs_put_open_node(struct kernfs_node *kn,
                                 struct kernfs_open_file *of)
{
        struct kernfs_open_node *on = kn->attr.open;
        unsigned long flags;

        mutex_lock(&kernfs_open_file_mutex);
        spin_lock_irqsave(&kernfs_open_node_lock, flags);

        if (of)
                list_del(&of->list);

        if (atomic_dec_and_test(&on->refcnt))
                kn->attr.open = NULL;
        else
                on = NULL;

        spin_unlock_irqrestore(&kernfs_open_node_lock, flags);
        mutex_unlock(&kernfs_open_file_mutex);

        kfree(on);
}

static int kernfs_fop_open(struct inode *inode, struct file *file)
{
        struct kernfs_node *kn = inode->i_private;
        struct kernfs_root *root = kernfs_root(kn);
        const struct kernfs_ops *ops;
        struct kernfs_open_file *of;
        bool has_read, has_write, has_mmap;
        int error = -EACCES;

        if (!kernfs_get_active(kn))
                return -ENODEV;

        ops = kernfs_ops(kn);

        has_read = ops->seq_show || ops->read || ops->mmap;
        has_write = ops->write || ops->mmap;
        has_mmap = ops->mmap;

        /* see the flag definition for details */
        if (root->flags & KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK) {
                if ((file->f_mode & FMODE_WRITE) &&
                    (!(inode->i_mode & S_IWUGO) || !has_write))
                        goto err_out;

                if ((file->f_mode & FMODE_READ) &&
                    (!(inode->i_mode & S_IRUGO) || !has_read))
                        goto err_out;
        }

        /* allocate a kernfs_open_file for the file */
        error = -ENOMEM;
        of = kzalloc(sizeof(struct kernfs_open_file), GFP_KERNEL);
        if (!of)
                goto err_out;

        /*
         * The following is done to give a different lockdep key to
         * @of->mutex for files which implement mmap.  This is a rather
         * crude way to avoid false positive lockdep warning around
         * mm->mmap_sem - mmap nests @of->mutex under mm->mmap_sem and
         * reading /sys/block/sda/trace/act_mask grabs sr_mutex, under
         * which mm->mmap_sem nests, while holding @of->mutex.  As each
         * open file has a separate mutex, it's okay as long as those don't
         * happen on the same file.  At this point, we can't easily give
         * each file a separate locking class.  Let's differentiate on
         * whether the file has mmap or not for now.
         *
         * Both paths of the branch look the same.  They're supposed to
         * look that way and give @of->mutex different static lockdep keys.
         */
        if (has_mmap)
                mutex_init(&of->mutex);
        else
                mutex_init(&of->mutex);

        of->kn = kn;
        of->file = file;

        /*
         * Write path needs to atomic_write_len outside active reference.
         * Cache it in open_file.  See kernfs_fop_write() for details.
         */
        of->atomic_write_len = ops->atomic_write_len;

        error = -EINVAL;
        /*
         * ->seq_show is incompatible with ->prealloc,
         * as seq_read does its own allocation.
         * ->read must be used instead.
         */
        if (ops->prealloc && ops->seq_show)
                goto err_free;
        if (ops->prealloc) {
                int len = of->atomic_write_len ?: PAGE_SIZE;
                of->prealloc_buf = kmalloc(len + 1, GFP_KERNEL);
                error = -ENOMEM;
                if (!of->prealloc_buf)
                        goto err_free;
                mutex_init(&of->prealloc_mutex);
        }

        /*
         * Always instantiate seq_file even if read access doesn't use
         * seq_file or is not requested.  This unifies private data access
         * and readable regular files are the vast majority anyway.
         */
        if (ops->seq_show)
                error = seq_open(file, &kernfs_seq_ops);
        else
                error = seq_open(file, NULL);
        if (error)
                goto err_free;

        of->seq_file = file->private_data;
        of->seq_file->private = of;

        /* seq_file clears PWRITE unconditionally, restore it if WRITE */
        if (file->f_mode & FMODE_WRITE)
                file->f_mode |= FMODE_PWRITE;

        /* make sure we have open node struct */
        error = kernfs_get_open_node(kn, of);
        if (error)
                goto err_seq_release;

        if (ops->open) {
                /* nobody has access to @of yet, skip @of->mutex */
                error = ops->open(of);
                if (error)
                        goto err_put_node;
        }

        /* open succeeded, put active references */
        kernfs_put_active(kn);
        return 0;

err_put_node:
        kernfs_put_open_node(kn, of);
err_seq_release:
        seq_release(inode, file);
err_free:
        kfree(of->prealloc_buf);
        kfree(of);
err_out:
        kernfs_put_active(kn);
        return error;
}

/* used from release/drain to ensure that ->release() is called exactly once */
static void kernfs_release_file(struct kernfs_node *kn,
                                struct kernfs_open_file *of)
{
        /*
         * @of is guaranteed to have no other file operations in flight and
         * we just want to synchronize release and drain paths.
         * @kernfs_open_file_mutex is enough.  @of->mutex can't be used
         * here because drain path may be called from places which can
         * cause circular dependency.
         */
        lockdep_assert_held(&kernfs_open_file_mutex);

        if (!of->released) {
                /*
                 * A file is never detached without being released and we
                 * need to be able to release files which are deactivated
                 * and being drained.  Don't use kernfs_ops().
                 */
                kn->attr.ops->release(of);
                of->released = true;
        }
}

static int kernfs_fop_release(struct inode *inode, struct file *filp)
{
        struct kernfs_node *kn = inode->i_private;
        struct kernfs_open_file *of = kernfs_of(filp);

        if (kn->flags & KERNFS_HAS_RELEASE) {
                mutex_lock(&kernfs_open_file_mutex);
                kernfs_release_file(kn, of);
                mutex_unlock(&kernfs_open_file_mutex);
        }

        kernfs_put_open_node(kn, of);
        seq_release(inode, filp);
        kfree(of->prealloc_buf);
        kfree(of);

        return 0;
}

void kernfs_drain_open_files(struct kernfs_node *kn)
{
        struct kernfs_open_node *on;
        struct kernfs_open_file *of;

        if (!(kn->flags & (KERNFS_HAS_MMAP | KERNFS_HAS_RELEASE)))
                return;

        spin_lock_irq(&kernfs_open_node_lock);
        on = kn->attr.open;
        if (on)
                atomic_inc(&on->refcnt);
        spin_unlock_irq(&kernfs_open_node_lock);
        if (!on)
                return;

        mutex_lock(&kernfs_open_file_mutex);

        list_for_each_entry(of, &on->files, list) {
                struct inode *inode = file_inode(of->file);

                if (kn->flags & KERNFS_HAS_MMAP)
                        unmap_mapping_range(inode->i_mapping, 0, 0, 1);

                if (kn->flags & KERNFS_HAS_RELEASE)
                        kernfs_release_file(kn, of);
        }

        mutex_unlock(&kernfs_open_file_mutex);

        kernfs_put_open_node(kn, NULL);
}

/*
 * Kernfs attribute files are pollable.  The idea is that you read
 * the content and then you use 'poll' or 'select' to wait for
 * the content to change.  When the content changes (assuming the
 * manager for the kobject supports notification), poll will
 * return EPOLLERR|EPOLLPRI, and select will return the fd whether
 * it is waiting for read, write, or exceptions.
 * Once poll/select indicates that the value has changed, you
 * need to close and re-open the file, or seek to 0 and read again.
 * Reminder: this only works for attributes which actively support
 * it, and it is not possible to test an attribute from userspace
 * to see if it supports poll (Neither 'poll' nor 'select' return
 * an appropriate error code).  When in doubt, set a suitable timeout value.
 */
__poll_t kernfs_generic_poll(struct kernfs_open_file *of, poll_table *wait)
{
        struct kernfs_node *kn = kernfs_dentry_node(of->file->f_path.dentry);
        struct kernfs_open_node *on = kn->attr.open;

        poll_wait(of->file, &on->poll, wait);

        if (of->event != atomic_read(&on->event))
                return DEFAULT_POLLMASK|EPOLLERR|EPOLLPRI;

        return DEFAULT_POLLMASK;
}

static __poll_t kernfs_fop_poll(struct file *filp, poll_table *wait)
{
        struct kernfs_open_file *of = kernfs_of(filp);
        struct kernfs_node *kn = kernfs_dentry_node(filp->f_path.dentry);
        __poll_t ret;

        if (!kernfs_get_active(kn))
                return DEFAULT_POLLMASK|EPOLLERR|EPOLLPRI;

        if (kn->attr.ops->poll)
                ret = kn->attr.ops->poll(of, wait);
        else
                ret = kernfs_generic_poll(of, wait);

        kernfs_put_active(kn);
        return ret;
}

static void kernfs_notify_workfn(struct work_struct *work)
{
        struct kernfs_node *kn;
        struct kernfs_super_info *info;
repeat:
        /* pop one off the notify_list */
        spin_lock_irq(&kernfs_notify_lock);
        kn = kernfs_notify_list;
        if (kn == KERNFS_NOTIFY_EOL) {
                spin_unlock_irq(&kernfs_notify_lock);
                return;
        }
        kernfs_notify_list = kn->attr.notify_next;
        kn->attr.notify_next = NULL;
        spin_unlock_irq(&kernfs_notify_lock);

        /* kick fsnotify */
        mutex_lock(&kernfs_mutex);

        list_for_each_entry(info, &kernfs_root(kn)->supers, node) {
                struct kernfs_node *parent;
                struct inode *inode;

                /*
                 * We want fsnotify_modify() on @kn but as the
                 * modifications aren't originating from userland don't
                 * have the matching @file available.  Look up the inodes
                 * and generate the events manually.
                 */
                inode = ilookup(info->sb, kn->id.ino);
                if (!inode)
                        continue;

                parent = kernfs_get_parent(kn);
                if (parent) {
                        struct inode *p_inode;

                        p_inode = ilookup(info->sb, parent->id.ino);
                        if (p_inode) {
                                fsnotify(p_inode, FS_MODIFY | FS_EVENT_ON_CHILD,
                                         inode, FSNOTIFY_EVENT_INODE, kn->name, 0);
                                iput(p_inode);
                        }

                        kernfs_put(parent);
                }

                fsnotify(inode, FS_MODIFY, inode, FSNOTIFY_EVENT_INODE,
                         kn->name, 0);
                iput(inode);
        }

        mutex_unlock(&kernfs_mutex);
        kernfs_put(kn);
        goto repeat;
}

/**
 * kernfs_notify - notify a kernfs file
 * @kn: file to notify
 *
 * Notify @kn such that poll(2) on @kn wakes up.  Maybe be called from any
 * context.
 */
void kernfs_notify(struct kernfs_node *kn)
{
        static DECLARE_WORK(kernfs_notify_work, kernfs_notify_workfn);
        unsigned long flags;
        struct kernfs_open_node *on;

        if (WARN_ON(kernfs_type(kn) != KERNFS_FILE))
                return;

        /* kick poll immediately */
        spin_lock_irqsave(&kernfs_open_node_lock, flags);
        on = kn->attr.open;
        if (on) {
                atomic_inc(&on->event);
                wake_up_interruptible(&on->poll);
        }
        spin_unlock_irqrestore(&kernfs_open_node_lock, flags);

        /* schedule work to kick fsnotify */
        spin_lock_irqsave(&kernfs_notify_lock, flags);
        if (!kn->attr.notify_next) {
                kernfs_get(kn);
                kn->attr.notify_next = kernfs_notify_list;
                kernfs_notify_list = kn;
                schedule_work(&kernfs_notify_work);
        }
        spin_unlock_irqrestore(&kernfs_notify_lock, flags);
}
EXPORT_SYMBOL_GPL(kernfs_notify);

const struct file_operations kernfs_file_fops = {
        .read           = kernfs_fop_read,
        .write          = kernfs_fop_write,
        .llseek         = generic_file_llseek,
        .mmap           = kernfs_fop_mmap,
        .open           = kernfs_fop_open,
        .release        = kernfs_fop_release,
        .poll           = kernfs_fop_poll,
        .fsync          = noop_fsync,
};

/**
 * __kernfs_create_file - kernfs internal function to create a file
 * @parent: directory to create the file in
 * @name: name of the file
 * @mode: mode of the file
 * @uid: uid of the file
 * @gid: gid of the file
 * @size: size of the file
 * @ops: kernfs operations for the file
 * @priv: private data for the file
 * @ns: optional namespace tag of the file
 * @key: lockdep key for the file's active_ref, %NULL to disable lockdep
 *
 * Returns the created node on success, ERR_PTR() value on error.
 */
struct kernfs_node *__kernfs_create_file(struct kernfs_node *parent,
                                         const char *name,
                                         umode_t mode, kuid_t uid, kgid_t gid,
                                         loff_t size,
                                         const struct kernfs_ops *ops,
                                         void *priv, const void *ns,
                                         struct lock_class_key *key)
{
        struct kernfs_node *kn;
        unsigned flags;
        int rc;

        flags = KERNFS_FILE;

        kn = kernfs_new_node(parent, name, (mode & S_IALLUGO) | S_IFREG,
                             uid, gid, flags);
        if (!kn)
                return ERR_PTR(-ENOMEM);

        kn->attr.ops = ops;
        kn->attr.size = size;
        kn->ns = ns;
        kn->priv = priv;

#ifdef CONFIG_DEBUG_LOCK_ALLOC
        if (key) {
                lockdep_init_map(&kn->dep_map, "kn->count", key, 0);
                kn->flags |= KERNFS_LOCKDEP;
        }
#endif

        /*
         * kn->attr.ops is accesible only while holding active ref.  We
         * need to know whether some ops are implemented outside active
         * ref.  Cache their existence in flags.
         */
        if (ops->seq_show)
                kn->flags |= KERNFS_HAS_SEQ_SHOW;
        if (ops->mmap)
                kn->flags |= KERNFS_HAS_MMAP;
        if (ops->release)
                kn->flags |= KERNFS_HAS_RELEASE;

        rc = kernfs_add_one(kn);
        if (rc) {
                kernfs_put(kn);
                return ERR_PTR(rc);
        }
        return kn;
}