sysfs: use transient write buffer

There isn't much to be gained by keeping around kernel buffer while a
file is open especially as the read path planned to be converted to
use seq_file and won't use the buffer.  This patch makes
sysfs_write_file() use per-write transient buffer instead of
sysfs_open_file->page.

This simplifies the write path, enables removing sysfs_open_file->page
once read path is updated and will help merging bin file write path
which already requires the use of a transient buffer due to a locking
order issue.

As the function comments of flush_write_buffer() and
sysfs_write_buffer() are being updated anyway, reformat them so that
they're more conventional.

v2: Use min_t() instead of min() in sysfs_write_file() to avoid build
    warning on arm.  Reported by build test robot.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 52 insertions, 62 deletions

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index af6e9092a679..53cc096e6a1b 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -162,92 +162,82 @@ out:
 }
 
 /**
- *      fill_write_buffer - copy buffer from userspace.
- *      @of:            open file struct.
- *      @buf:           data from user.
- *      @count:         number of bytes in @userbuf.
+ * flush_write_buffer - push buffer to kobject
+ * @of: open file
+ * @buf: data buffer for file
+ * @count: number of bytes
  *
- *      Allocate @of->page if it hasn't been already, then copy the
- *      user-supplied buffer into it.
+ * Get the correct pointers for the kobject and the attribute we're dealing
+ * with, then call the store() method for it with @buf.
  */
-static int fill_write_buffer(struct sysfs_open_file *of,
-                             const char __user *buf, size_t count)
-{
-        int error;
-
-        if (!of->page)
-                of->page = (char *)get_zeroed_page(GFP_KERNEL);
-        if (!of->page)
-                return -ENOMEM;
-
-        if (count >= PAGE_SIZE)
-                count = PAGE_SIZE - 1;
-        error = copy_from_user(of->page, buf, count);
-
-        /*
-         * If buf is assumed to contain a string, terminate it by \0, so
-         * e.g. sscanf() can scan the string easily.
-         */
-        of->page[count] = 0;
-        return error ? -EFAULT : count;
-}
-
-/**
- *      flush_write_buffer - push buffer to kobject.
- *      @of:            open file
- *      @count:         number of bytes
- *
- *      Get the correct pointers for the kobject and the attribute we're
- *      dealing with, then call the store() method for the attribute,
- *      passing the buffer that we acquired in fill_write_buffer().
- */
-static int flush_write_buffer(struct sysfs_open_file *of, size_t count)
+static int flush_write_buffer(struct sysfs_open_file *of, char *buf,
+                              size_t count)
 {
         struct kobject *kobj = of->sd->s_parent->s_dir.kobj;
         const struct sysfs_ops *ops;
-        int rc;
+        int rc = 0;
 
-        /* need @of->sd for attr and ops, its parent for kobj */
-        if (!sysfs_get_active(of->sd))
+        /*
+         * Need @of->sd for attr and ops, its parent for kobj.  @of->mutex
+         * nests outside active ref and is just to ensure that the ops
+         * aren't called concurrently for the same open file.
+         */
+        mutex_lock(&of->mutex);
+        if (!sysfs_get_active(of->sd)) {
+                mutex_unlock(&of->mutex);
                 return -ENODEV;
+        }
 
         ops = sysfs_file_ops(of->sd);
-        rc = ops->store(kobj, of->sd->s_attr.attr, of->page, count);
+        rc = ops->store(kobj, of->sd->s_attr.attr, buf, count);
 
         sysfs_put_active(of->sd);
+        mutex_unlock(&of->mutex);
 
         return rc;
 }
 
 /**
- *      sysfs_write_file - write an attribute.
- *      @file:  file pointer
- *      @buf:   data to write
- *      @count: number of bytes
- *      @ppos:  starting offset
+ * sysfs_write_file - write an attribute
+ * @file: file pointer
+ * @user_buf: data to write
+ * @count: number of bytes
+ * @ppos: starting offset
  *
- *      Similar to sysfs_read_file(), though working in the opposite direction.
- *      We allocate and fill the data from the user in fill_write_buffer(),
- *      then push it to the kobject in flush_write_buffer().
- *      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.
+ * Copy data in from userland and pass it to the matching
+ * sysfs_ops->store() by invoking flush_write_buffer().
+ *
+ * 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 sysfs_write_file(struct file *file, const char __user *buf,
+static ssize_t sysfs_write_file(struct file *file, const char __user *user_buf,
                                 size_t count, loff_t *ppos)
 {
         struct sysfs_open_file *of = file->private_data;
-        ssize_t len;
+        ssize_t len = min_t(size_t, count, PAGE_SIZE - 1);
+        char *buf;
 
-        mutex_lock(&of->mutex);
-        len = fill_write_buffer(of, buf, count);
-        if (len > 0)
-                len = flush_write_buffer(of, len);
+        if (!len)
+                return 0;
+
+        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 */
+
+        len = flush_write_buffer(of, buf, len);
         if (len > 0)
                 *ppos += len;
-        mutex_unlock(&of->mutex);
+out_free:
+        kfree(buf);
         return len;
 }