[bug report] gpio: mockup: rework debugfs interface

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

Hello Bartosz Golaszewski,

The patch 2a9e27408e12: "gpio: mockup: rework debugfs interface" from
Jan 17, 2019, leads to the following static checker warning:

	drivers/gpio/gpio-mockup.c:223 gpio_mockup_debugfs_read()
	warn: maybe return -EFAULT instead of the bytes remaining?

drivers/gpio/gpio-mockup.c
    199 static ssize_t gpio_mockup_debugfs_read(struct file *file,
    200 					char __user *usr_buf,
    201 					size_t size, loff_t *ppos)
    202 {
    203 	struct gpio_mockup_dbgfs_private *priv;
    204 	struct gpio_mockup_chip *chip;
    205 	struct seq_file *sfile;
    206 	struct gpio_chip *gc;
    207 	char buf[3];
    208 	int val, rv;
    209 
    210 	if (*ppos != 0)
    211 		return 0;
    212 
    213 	sfile = file->private_data;
    214 	priv = sfile->private;
    215 	chip = priv->chip;
    216 	gc = &chip->gc;
    217 
    218 	val = gpio_mockup_get(gc, priv->offset);
    219 	snprintf(buf, sizeof(buf), "%d\n", val);
    220 
    221 	rv = copy_to_user(usr_buf, buf, sizeof(buf));
    222 	if (rv)
--> 223 		return rv;
    224 
    225 	return sizeof(buf) - 1;

The copy_to_user() function returns the number of bytes remaining to
be copied.  We probably want to return -EFAULT:

		if (copy_to_user(usr_buf, buf, sizeof(buf)))
			return -EFAULT;

But I would also question the sizeof(buf).  We should consider that
maybe the user only wants 1 byte and not write beyond the end of their
buffer.

	count = min(count, sizeof(buf));
	if (copy_to_user(usr_buf, buf, count))
		return -EFAULT;

Finally, on the success path, we should probably count the NUL
terminator?  (I forget the rules).


    226 }

regards,
dan carpenter