Merge tag 'for-linus' of git://git.kernel.org/pub/scm/virt/kvm/kvm

[linux-stable.git] / fs / sysfs / file.c

// SPDX-License-Identifier: GPL-2.0
/*
 * fs/sysfs/file.c - sysfs regular (text) file implementation
 *
 * Copyright (c) 2001-3 Patrick Mochel
 * Copyright (c) 2007 SUSE Linux Products GmbH
 * Copyright (c) 2007 Tejun Heo <teheo@suse.de>
 *
 * Please see Documentation/filesystems/sysfs.rst for more information.
 */

#include <linux/module.h>
#include <linux/kobject.h>
#include <linux/slab.h>
#include <linux/list.h>
#include <linux/mutex.h>
#include <linux/seq_file.h>
#include <linux/mm.h>

#include "sysfs.h"

/*
 * Determine ktype->sysfs_ops for the given kernfs_node.  This function
 * must be called while holding an active reference.
 */
static const struct sysfs_ops *sysfs_file_ops(struct kernfs_node *kn)
{
        struct kobject *kobj = kn->parent->priv;

        if (kn->flags & KERNFS_LOCKDEP)
                lockdep_assert_held(kn);
        return kobj->ktype ? kobj->ktype->sysfs_ops : NULL;
}

/*
 * Reads on sysfs are handled through seq_file, which takes care of hairy
 * details like buffering and seeking.  The following function pipes
 * sysfs_ops->show() result through seq_file.
 */
static int sysfs_kf_seq_show(struct seq_file *sf, void *v)
{
        struct kernfs_open_file *of = sf->private;
        struct kobject *kobj = of->kn->parent->priv;
        const struct sysfs_ops *ops = sysfs_file_ops(of->kn);
        ssize_t count;
        char *buf;

        if (WARN_ON_ONCE(!ops->show))
                return -EINVAL;

        /* acquire buffer and ensure that it's >= PAGE_SIZE and clear */
        count = seq_get_buf(sf, &buf);
        if (count < PAGE_SIZE) {
                seq_commit(sf, -1);
                return 0;
        }
        memset(buf, 0, PAGE_SIZE);

        count = ops->show(kobj, of->kn->priv, buf);
        if (count < 0)
                return count;

        /*
         * The code works fine with PAGE_SIZE return but it's likely to
         * indicate truncated result or overflow in normal use cases.
         */
        if (count >= (ssize_t)PAGE_SIZE) {
                printk("fill_read_buffer: %pS returned bad count\n",
                                ops->show);
                /* Try to struggle along */
                count = PAGE_SIZE - 1;
        }
        seq_commit(sf, count);
        return 0;
}

static ssize_t sysfs_kf_bin_read(struct kernfs_open_file *of, char *buf,
                                 size_t count, loff_t pos)
{
        struct bin_attribute *battr = of->kn->priv;
        struct kobject *kobj = of->kn->parent->priv;
        loff_t size = file_inode(of->file)->i_size;

        if (!count)
                return 0;

        if (size) {
                if (pos >= size)
                        return 0;
                if (pos + count > size)
                        count = size - pos;
        }

        if (!battr->read && !battr->read_new)
                return -EIO;

        if (battr->read_new)
                return battr->read_new(of->file, kobj, battr, buf, pos, count);

        return battr->read(of->file, kobj, battr, buf, pos, count);
}

/* kernfs read callback for regular sysfs files with pre-alloc */
static ssize_t sysfs_kf_read(struct kernfs_open_file *of, char *buf,
                             size_t count, loff_t pos)
{
        const struct sysfs_ops *ops = sysfs_file_ops(of->kn);
        struct kobject *kobj = of->kn->parent->priv;
        ssize_t len;

        /*
         * If buf != of->prealloc_buf, we don't know how
         * large it is, so cannot safely pass it to ->show
         */
        if (WARN_ON_ONCE(buf != of->prealloc_buf))
                return 0;
        len = ops->show(kobj, of->kn->priv, buf);
        if (len < 0)
                return len;
        if (pos) {
                if (len <= pos)
                        return 0;
                len -= pos;
                memmove(buf, buf + pos, len);
        }
        return min_t(ssize_t, count, len);
}

/* kernfs write callback for regular sysfs files */
static ssize_t sysfs_kf_write(struct kernfs_open_file *of, char *buf,
                              size_t count, loff_t pos)
{
        const struct sysfs_ops *ops = sysfs_file_ops(of->kn);
        struct kobject *kobj = of->kn->parent->priv;

        if (!count)
                return 0;

        return ops->store(kobj, of->kn->priv, buf, count);
}

/* kernfs write callback for bin sysfs files */
static ssize_t sysfs_kf_bin_write(struct kernfs_open_file *of, char *buf,
                                  size_t count, loff_t pos)
{
        struct bin_attribute *battr = of->kn->priv;
        struct kobject *kobj = of->kn->parent->priv;
        loff_t size = file_inode(of->file)->i_size;

        if (size) {
                if (size <= pos)
                        return -EFBIG;
                count = min_t(ssize_t, count, size - pos);
        }
        if (!count)
                return 0;

        if (!battr->write && !battr->write_new)
                return -EIO;

        if (battr->write_new)
                return battr->write_new(of->file, kobj, battr, buf, pos, count);

        return battr->write(of->file, kobj, battr, buf, pos, count);
}

static int sysfs_kf_bin_mmap(struct kernfs_open_file *of,
                             struct vm_area_struct *vma)
{
        struct bin_attribute *battr = of->kn->priv;
        struct kobject *kobj = of->kn->parent->priv;

        return battr->mmap(of->file, kobj, battr, vma);
}

static loff_t sysfs_kf_bin_llseek(struct kernfs_open_file *of, loff_t offset,
                                  int whence)
{
        struct bin_attribute *battr = of->kn->priv;
        struct kobject *kobj = of->kn->parent->priv;

        if (battr->llseek)
                return battr->llseek(of->file, kobj, battr, offset, whence);
        else
                return generic_file_llseek(of->file, offset, whence);
}

static int sysfs_kf_bin_open(struct kernfs_open_file *of)
{
        struct bin_attribute *battr = of->kn->priv;

        if (battr->f_mapping)
                of->file->f_mapping = battr->f_mapping();

        return 0;
}

void sysfs_notify(struct kobject *kobj, const char *dir, const char *attr)
{
        struct kernfs_node *kn = kobj->sd, *tmp;

        if (kn && dir)
                kn = kernfs_find_and_get(kn, dir);
        else
                kernfs_get(kn);

        if (kn && attr) {
                tmp = kernfs_find_and_get(kn, attr);
                kernfs_put(kn);
                kn = tmp;
        }

        if (kn) {
                kernfs_notify(kn);
                kernfs_put(kn);
        }
}
EXPORT_SYMBOL_GPL(sysfs_notify);

static const struct kernfs_ops sysfs_file_kfops_empty = {
};

static const struct kernfs_ops sysfs_file_kfops_ro = {
        .seq_show       = sysfs_kf_seq_show,
};

static const struct kernfs_ops sysfs_file_kfops_wo = {
        .write          = sysfs_kf_write,
};

static const struct kernfs_ops sysfs_file_kfops_rw = {
        .seq_show       = sysfs_kf_seq_show,
        .write          = sysfs_kf_write,
};

static const struct kernfs_ops sysfs_prealloc_kfops_ro = {
        .read           = sysfs_kf_read,
        .prealloc       = true,
};

static const struct kernfs_ops sysfs_prealloc_kfops_wo = {
        .write          = sysfs_kf_write,
        .prealloc       = true,
};

static const struct kernfs_ops sysfs_prealloc_kfops_rw = {
        .read           = sysfs_kf_read,
        .write          = sysfs_kf_write,
        .prealloc       = true,
};

static const struct kernfs_ops sysfs_bin_kfops_ro = {
        .read           = sysfs_kf_bin_read,
};

static const struct kernfs_ops sysfs_bin_kfops_wo = {
        .write          = sysfs_kf_bin_write,
};

static const struct kernfs_ops sysfs_bin_kfops_rw = {
        .read           = sysfs_kf_bin_read,
        .write          = sysfs_kf_bin_write,
};

static const struct kernfs_ops sysfs_bin_kfops_mmap = {
        .read           = sysfs_kf_bin_read,
        .write          = sysfs_kf_bin_write,
        .mmap           = sysfs_kf_bin_mmap,
        .open           = sysfs_kf_bin_open,
        .llseek         = sysfs_kf_bin_llseek,
};

int sysfs_add_file_mode_ns(struct kernfs_node *parent,
                const struct attribute *attr, umode_t mode, kuid_t uid,
                kgid_t gid, const void *ns)
{
        struct kobject *kobj = parent->priv;
        const struct sysfs_ops *sysfs_ops = kobj->ktype->sysfs_ops;
        struct lock_class_key *key = NULL;
        const struct kernfs_ops *ops = NULL;
        struct kernfs_node *kn;

        /* every kobject with an attribute needs a ktype assigned */
        if (WARN(!sysfs_ops, KERN_ERR
                        "missing sysfs attribute operations for kobject: %s\n",
                        kobject_name(kobj)))
                return -EINVAL;

        if (mode & SYSFS_PREALLOC) {
                if (sysfs_ops->show && sysfs_ops->store)
                        ops = &sysfs_prealloc_kfops_rw;
                else if (sysfs_ops->show)
                        ops = &sysfs_prealloc_kfops_ro;
                else if (sysfs_ops->store)
                        ops = &sysfs_prealloc_kfops_wo;
        } else {
                if (sysfs_ops->show && sysfs_ops->store)
                        ops = &sysfs_file_kfops_rw;
                else if (sysfs_ops->show)
                        ops = &sysfs_file_kfops_ro;
                else if (sysfs_ops->store)
                        ops = &sysfs_file_kfops_wo;
        }

        if (!ops)
                ops = &sysfs_file_kfops_empty;

#ifdef CONFIG_DEBUG_LOCK_ALLOC
        if (!attr->ignore_lockdep)
                key = attr->key ?: (struct lock_class_key *)&attr->skey;
#endif

        kn = __kernfs_create_file(parent, attr->name, mode & 0777, uid, gid,
                                  PAGE_SIZE, ops, (void *)attr, ns, key);
        if (IS_ERR(kn)) {
                if (PTR_ERR(kn) == -EEXIST)
                        sysfs_warn_dup(parent, attr->name);
                return PTR_ERR(kn);
        }
        return 0;
}

int sysfs_add_bin_file_mode_ns(struct kernfs_node *parent,
                const struct bin_attribute *battr, umode_t mode, size_t size,
                kuid_t uid, kgid_t gid, const void *ns)
{
        const struct attribute *attr = &battr->attr;
        struct lock_class_key *key = NULL;
        const struct kernfs_ops *ops;
        struct kernfs_node *kn;

        if (battr->read && battr->read_new)
                return -EINVAL;

        if (battr->write && battr->write_new)
                return -EINVAL;

        if (battr->mmap)
                ops = &sysfs_bin_kfops_mmap;
        else if ((battr->read || battr->read_new) && (battr->write || battr->write_new))
                ops = &sysfs_bin_kfops_rw;
        else if (battr->read || battr->read_new)
                ops = &sysfs_bin_kfops_ro;
        else if (battr->write || battr->write_new)
                ops = &sysfs_bin_kfops_wo;
        else
                ops = &sysfs_file_kfops_empty;

#ifdef CONFIG_DEBUG_LOCK_ALLOC
        if (!attr->ignore_lockdep)
                key = attr->key ?: (struct lock_class_key *)&attr->skey;
#endif

        kn = __kernfs_create_file(parent, attr->name, mode & 0777, uid, gid,
                                  size, ops, (void *)attr, ns, key);
        if (IS_ERR(kn)) {
                if (PTR_ERR(kn) == -EEXIST)
                        sysfs_warn_dup(parent, attr->name);
                return PTR_ERR(kn);
        }
        return 0;
}

/**
 * sysfs_create_file_ns - create an attribute file for an object with custom ns
 * @kobj: object we're creating for
 * @attr: attribute descriptor
 * @ns: namespace the new file should belong to
 */
int sysfs_create_file_ns(struct kobject *kobj, const struct attribute *attr,
                         const void *ns)
{
        kuid_t uid;
        kgid_t gid;

        if (WARN_ON(!kobj || !kobj->sd || !attr))
                return -EINVAL;

        kobject_get_ownership(kobj, &uid, &gid);
        return sysfs_add_file_mode_ns(kobj->sd, attr, attr->mode, uid, gid, ns);
}
EXPORT_SYMBOL_GPL(sysfs_create_file_ns);

int sysfs_create_files(struct kobject *kobj, const struct attribute * const *ptr)
{
        int err = 0;
        int i;

        for (i = 0; ptr[i] && !err; i++)
                err = sysfs_create_file(kobj, ptr[i]);
        if (err)
                while (--i >= 0)
                        sysfs_remove_file(kobj, ptr[i]);
        return err;
}
EXPORT_SYMBOL_GPL(sysfs_create_files);

/**
 * sysfs_add_file_to_group - add an attribute file to a pre-existing group.
 * @kobj: object we're acting for.
 * @attr: attribute descriptor.
 * @group: group name.
 */
int sysfs_add_file_to_group(struct kobject *kobj,
                const struct attribute *attr, const char *group)
{
        struct kernfs_node *parent;
        kuid_t uid;
        kgid_t gid;
        int error;

        if (group) {
                parent = kernfs_find_and_get(kobj->sd, group);
        } else {
                parent = kobj->sd;
                kernfs_get(parent);
        }

        if (!parent)
                return -ENOENT;

        kobject_get_ownership(kobj, &uid, &gid);
        error = sysfs_add_file_mode_ns(parent, attr, attr->mode, uid, gid,
                                       NULL);
        kernfs_put(parent);

        return error;
}
EXPORT_SYMBOL_GPL(sysfs_add_file_to_group);

/**
 * sysfs_chmod_file - update the modified mode value on an object attribute.
 * @kobj: object we're acting for.
 * @attr: attribute descriptor.
 * @mode: file permissions.
 *
 */
int sysfs_chmod_file(struct kobject *kobj, const struct attribute *attr,
                     umode_t mode)
{
        struct kernfs_node *kn;
        struct iattr newattrs;
        int rc;

        kn = kernfs_find_and_get(kobj->sd, attr->name);
        if (!kn)
                return -ENOENT;

        newattrs.ia_mode = (mode & S_IALLUGO) | (kn->mode & ~S_IALLUGO);
        newattrs.ia_valid = ATTR_MODE;

        rc = kernfs_setattr(kn, &newattrs);

        kernfs_put(kn);
        return rc;
}
EXPORT_SYMBOL_GPL(sysfs_chmod_file);

/**
 * sysfs_break_active_protection - break "active" protection
 * @kobj: The kernel object @attr is associated with.
 * @attr: The attribute to break the "active" protection for.
 *
 * With sysfs, just like kernfs, deletion of an attribute is postponed until
 * all active .show() and .store() callbacks have finished unless this function
 * is called. Hence this function is useful in methods that implement self
 * deletion.
 */
struct kernfs_node *sysfs_break_active_protection(struct kobject *kobj,
                                                  const struct attribute *attr)
{
        struct kernfs_node *kn;

        kobject_get(kobj);
        kn = kernfs_find_and_get(kobj->sd, attr->name);
        if (kn)
                kernfs_break_active_protection(kn);
        else
                kobject_put(kobj);
        return kn;
}
EXPORT_SYMBOL_GPL(sysfs_break_active_protection);

/**
 * sysfs_unbreak_active_protection - restore "active" protection
 * @kn: Pointer returned by sysfs_break_active_protection().
 *
 * Undo the effects of sysfs_break_active_protection(). Since this function
 * calls kernfs_put() on the kernfs node that corresponds to the 'attr'
 * argument passed to sysfs_break_active_protection() that attribute may have
 * been removed between the sysfs_break_active_protection() and
 * sysfs_unbreak_active_protection() calls, it is not safe to access @kn after
 * this function has returned.
 */
void sysfs_unbreak_active_protection(struct kernfs_node *kn)
{
        struct kobject *kobj = kn->parent->priv;

        kernfs_unbreak_active_protection(kn);
        kernfs_put(kn);
        kobject_put(kobj);
}
EXPORT_SYMBOL_GPL(sysfs_unbreak_active_protection);

/**
 * sysfs_remove_file_ns - remove an object attribute with a custom ns tag
 * @kobj: object we're acting for
 * @attr: attribute descriptor
 * @ns: namespace tag of the file to remove
 *
 * Hash the attribute name and namespace tag and kill the victim.
 */
void sysfs_remove_file_ns(struct kobject *kobj, const struct attribute *attr,
                          const void *ns)
{
        struct kernfs_node *parent = kobj->sd;

        kernfs_remove_by_name_ns(parent, attr->name, ns);
}
EXPORT_SYMBOL_GPL(sysfs_remove_file_ns);

/**
 * sysfs_remove_file_self - remove an object attribute from its own method
 * @kobj: object we're acting for
 * @attr: attribute descriptor
 *
 * See kernfs_remove_self() for details.
 */
bool sysfs_remove_file_self(struct kobject *kobj, const struct attribute *attr)
{
        struct kernfs_node *parent = kobj->sd;
        struct kernfs_node *kn;
        bool ret;

        kn = kernfs_find_and_get(parent, attr->name);
        if (WARN_ON_ONCE(!kn))
                return false;

        ret = kernfs_remove_self(kn);

        kernfs_put(kn);
        return ret;
}
EXPORT_SYMBOL_GPL(sysfs_remove_file_self);

void sysfs_remove_files(struct kobject *kobj, const struct attribute * const *ptr)
{
        int i;

        for (i = 0; ptr[i]; i++)
                sysfs_remove_file(kobj, ptr[i]);
}
EXPORT_SYMBOL_GPL(sysfs_remove_files);

/**
 * sysfs_remove_file_from_group - remove an attribute file from a group.
 * @kobj: object we're acting for.
 * @attr: attribute descriptor.
 * @group: group name.
 */
void sysfs_remove_file_from_group(struct kobject *kobj,
                const struct attribute *attr, const char *group)
{
        struct kernfs_node *parent;

        if (group) {
                parent = kernfs_find_and_get(kobj->sd, group);
        } else {
                parent = kobj->sd;
                kernfs_get(parent);
        }

        if (parent) {
                kernfs_remove_by_name(parent, attr->name);
                kernfs_put(parent);
        }
}
EXPORT_SYMBOL_GPL(sysfs_remove_file_from_group);

/**
 *      sysfs_create_bin_file - create binary file for object.
 *      @kobj:  object.
 *      @attr:  attribute descriptor.
 */
int sysfs_create_bin_file(struct kobject *kobj,
                          const struct bin_attribute *attr)
{
        kuid_t uid;
        kgid_t gid;

        if (WARN_ON(!kobj || !kobj->sd || !attr))
                return -EINVAL;

        kobject_get_ownership(kobj, &uid, &gid);
        return sysfs_add_bin_file_mode_ns(kobj->sd, attr, attr->attr.mode,
                                          attr->size, uid, gid, NULL);
}
EXPORT_SYMBOL_GPL(sysfs_create_bin_file);

/**
 *      sysfs_remove_bin_file - remove binary file for object.
 *      @kobj:  object.
 *      @attr:  attribute descriptor.
 */
void sysfs_remove_bin_file(struct kobject *kobj,
                           const struct bin_attribute *attr)
{
        kernfs_remove_by_name(kobj->sd, attr->attr.name);
}
EXPORT_SYMBOL_GPL(sysfs_remove_bin_file);

static int internal_change_owner(struct kernfs_node *kn, kuid_t kuid,
                                 kgid_t kgid)
{
        struct iattr newattrs = {
                .ia_valid = ATTR_UID | ATTR_GID,
                .ia_uid = kuid,
                .ia_gid = kgid,
        };
        return kernfs_setattr(kn, &newattrs);
}

/**
 *      sysfs_link_change_owner - change owner of a sysfs file.
 *      @kobj:  object of the kernfs_node the symlink is located in.
 *      @targ:  object of the kernfs_node the symlink points to.
 *      @name:  name of the link.
 *      @kuid:  new owner's kuid
 *      @kgid:  new owner's kgid
 *
 * This function looks up the sysfs symlink entry @name under @kobj and changes
 * the ownership to @kuid/@kgid. The symlink is looked up in the namespace of
 * @targ.
 *
 * Returns 0 on success or error code on failure.
 */
int sysfs_link_change_owner(struct kobject *kobj, struct kobject *targ,
                            const char *name, kuid_t kuid, kgid_t kgid)
{
        struct kernfs_node *kn = NULL;
        int error;

        if (!name || !kobj->state_in_sysfs || !targ->state_in_sysfs)
                return -EINVAL;

        error = -ENOENT;
        kn = kernfs_find_and_get_ns(kobj->sd, name, targ->sd->ns);
        if (!kn)
                goto out;

        error = -EINVAL;
        if (kernfs_type(kn) != KERNFS_LINK)
                goto out;
        if (kn->symlink.target_kn->priv != targ)
                goto out;

        error = internal_change_owner(kn, kuid, kgid);

out:
        kernfs_put(kn);
        return error;
}

/**
 *      sysfs_file_change_owner - change owner of a sysfs file.
 *      @kobj:  object.
 *      @name:  name of the file to change.
 *      @kuid:  new owner's kuid
 *      @kgid:  new owner's kgid
 *
 * This function looks up the sysfs entry @name under @kobj and changes the
 * ownership to @kuid/@kgid.
 *
 * Returns 0 on success or error code on failure.
 */
int sysfs_file_change_owner(struct kobject *kobj, const char *name, kuid_t kuid,
                            kgid_t kgid)
{
        struct kernfs_node *kn;
        int error;

        if (!name)
                return -EINVAL;

        if (!kobj->state_in_sysfs)
                return -EINVAL;

        kn = kernfs_find_and_get(kobj->sd, name);
        if (!kn)
                return -ENOENT;

        error = internal_change_owner(kn, kuid, kgid);

        kernfs_put(kn);

        return error;
}
EXPORT_SYMBOL_GPL(sysfs_file_change_owner);

/**
 *      sysfs_change_owner - change owner of the given object.
 *      @kobj:  object.
 *      @kuid:  new owner's kuid
 *      @kgid:  new owner's kgid
 *
 * Change the owner of the default directory, files, groups, and attributes of
 * @kobj to @kuid/@kgid. Note that sysfs_change_owner mirrors how the sysfs
 * entries for a kobject are added by driver core. In summary,
 * sysfs_change_owner() takes care of the default directory entry for @kobj,
 * the default attributes associated with the ktype of @kobj and the default
 * attributes associated with the ktype of @kobj.
 * Additional properties not added by driver core have to be changed by the
 * driver or subsystem which created them. This is similar to how
 * driver/subsystem specific entries are removed.
 *
 * Returns 0 on success or error code on failure.
 */
int sysfs_change_owner(struct kobject *kobj, kuid_t kuid, kgid_t kgid)
{
        int error;
        const struct kobj_type *ktype;

        if (!kobj->state_in_sysfs)
                return -EINVAL;

        /* Change the owner of the kobject itself. */
        error = internal_change_owner(kobj->sd, kuid, kgid);
        if (error)
                return error;

        ktype = get_ktype(kobj);
        if (ktype) {
                /*
                 * Change owner of the default groups associated with the
                 * ktype of @kobj.
                 */
                error = sysfs_groups_change_owner(kobj, ktype->default_groups,
                                                  kuid, kgid);
                if (error)
                        return error;
        }

        return 0;
}
EXPORT_SYMBOL_GPL(sysfs_change_owner);

/**
 *      sysfs_emit - scnprintf equivalent, aware of PAGE_SIZE buffer.
 *      @buf:   start of PAGE_SIZE buffer.
 *      @fmt:   format
 *      @...:   optional arguments to @format
 *
 *
 * Returns number of characters written to @buf.
 */
int sysfs_emit(char *buf, const char *fmt, ...)
{
        va_list args;
        int len;

        if (WARN(!buf || offset_in_page(buf),
                 "invalid sysfs_emit: buf:%p\n", buf))
                return 0;

        va_start(args, fmt);
        len = vscnprintf(buf, PAGE_SIZE, fmt, args);
        va_end(args);

        return len;
}
EXPORT_SYMBOL_GPL(sysfs_emit);

/**
 *      sysfs_emit_at - scnprintf equivalent, aware of PAGE_SIZE buffer.
 *      @buf:   start of PAGE_SIZE buffer.
 *      @at:    offset in @buf to start write in bytes
 *              @at must be >= 0 && < PAGE_SIZE
 *      @fmt:   format
 *      @...:   optional arguments to @fmt
 *
 *
 * Returns number of characters written starting at &@buf[@at].
 */
int sysfs_emit_at(char *buf, int at, const char *fmt, ...)
{
        va_list args;
        int len;

        if (WARN(!buf || offset_in_page(buf) || at < 0 || at >= PAGE_SIZE,
                 "invalid sysfs_emit_at: buf:%p at:%d\n", buf, at))
                return 0;

        va_start(args, fmt);
        len = vscnprintf(buf + at, PAGE_SIZE - at, fmt, args);
        va_end(args);

        return len;
}
EXPORT_SYMBOL_GPL(sysfs_emit_at);

/**
 *      sysfs_bin_attr_simple_read - read callback to simply copy from memory.
 *      @file:  attribute file which is being read.
 *      @kobj:  object to which the attribute belongs.
 *      @attr:  attribute descriptor.
 *      @buf:   destination buffer.
 *      @off:   offset in bytes from which to read.
 *      @count: maximum number of bytes to read.
 *
 * Simple ->read() callback for bin_attributes backed by a buffer in memory.
 * The @private and @size members in struct bin_attribute must be set to the
 * buffer's location and size before the bin_attribute is created in sysfs.
 *
 * Bounds check for @off and @count is done in sysfs_kf_bin_read().
 * Negative value check for @off is done in vfs_setpos() and default_llseek().
 *
 * Returns number of bytes written to @buf.
 */
ssize_t sysfs_bin_attr_simple_read(struct file *file, struct kobject *kobj,
                                   struct bin_attribute *attr, char *buf,
                                   loff_t off, size_t count)
{
        memcpy(buf, attr->private + off, count);
        return count;
}
EXPORT_SYMBOL_GPL(sysfs_bin_attr_simple_read);