Re: [PATCH v2 RFC 2/7] kfuzztest: add user-facing API and data structures

[Alexander Potapenko <glider@google.com>]

> +       static ssize_t kfuzztest_write_cb_##test_name(struct file *filp, const char __user *buf, size_t len, \
> +                                                     loff_t *off)                                           \
> +       {                                                                                                    \
> +               test_arg_type *arg;                                                                          \
> +               void *buffer;                                                                                \
> +               int ret;                                                                                     \
> +                                                                                                             \
> +               buffer = kmalloc(len, GFP_KERNEL);                                                           \
> +               if (!buffer)                                                                                 \
> +                       return -ENOMEM;                                                                      \
> +               ret = simple_write_to_buffer(buffer, len, off, buf, len);                                    \
> +               if (ret < 0)                                                                                 \
> +                       goto out;                                                                            \
> +               ret = kfuzztest_parse_and_relocate(buffer, len, (void **)&arg);                              \
> +               if (ret < 0)                                                                                 \
> +                       goto out;                                                                            \
> +               kfuzztest_logic_##test_name(arg);                                                            \
> +               ret = len;                                                                                   \
> +out:                                                                                                         \
> +               kfree(buffer);                                                                               \
> +               return ret;                                                                                  \
> +       }                                                                                                    \
> +       static void kfuzztest_logic_##test_name(test_arg_type *arg)

simple_write_to_buffer() may write less than len bytes if it hits a
protected page.
You should check that `ret == len` and return -EFAULT if they differ.