Re: [PATCH v5 01/11] driver: platform: Add helper for safer setting of driver_override

[Andy Shevchenko <andy.shevchenko@gmail.com>]

On Wed, Mar 16, 2022 at 5:06 PM Krzysztof Kozlowski
<krzysztof.kozlowski@canonical.com> wrote:

...

> +int driver_set_override(struct device *dev, const char **override,
> +                       const char *s, size_t len)
> +{
> +       const char *new, *old;
> +       char *cp;

> +       if (!dev || !override || !s)
> +               return -EINVAL;

Sorry, I didn't pay much attention on this. First of all, I would drop
dev checks and simply require that dev should be valid. Do you expect
this can be called when dev is invalid? I would like to hear if it's
anything but theoretical. Second one, is the !s requirement. Do I
understand correctly that the string must be always present? But then
how we NULify the override? Is it possible? Third one is absence of
len check. See below.

> +       /*
> +        * The stored value will be used in sysfs show callback (sysfs_emit()),
> +        * which has a length limit of PAGE_SIZE and adds a trailing newline.
> +        * Thus we can store one character less to avoid truncation during sysfs
> +        * show.
> +        */
> +       if (len >= (PAGE_SIZE - 1))
> +               return -EINVAL;

I would relax this to make sure we can use it if \n is within this limit.

> +       cp = strnchr(s, len, '\n');
> +       if (cp)
> +               len = cp - s;
> +
> +       new = kstrndup(s, len, GFP_KERNEL);

Here is a word about the len check.

> +       if (!new)

If len == 0, this won't trigger and you have something very
interesting as a result.

One way is to use ZERO_PTR_OR_NULL() another is explicitly check for 0
and issue a (different?) error code.

> +               return -ENOMEM;
> +
> +       device_lock(dev);
> +       old = *override;
> +       if (cp != s) {
> +               *override = new;
> +       } else {
> +               kfree(new);
> +               *override = NULL;
> +       }
> +       device_unlock(dev);
> +
> +       kfree(old);
> +
> +       return 0;
> +}

-- 
With Best Regards,
Andy Shevchenko