Good design to expose debug info from kernel module

[Gilboa Davara]

On Thu, Mar 26, 2015 at 11:36 PM, Elazar Leibovich <elazarl at gmail.com> wrote:
> Hi,
>
> I'm writing a kernel module, and I want to expose some debug
> information about it.
>
> The debug information is often of the form of request-response.
>
> For example:
>
> - Hey module, what's up with data at 0xffffe8ff0040c000?
> - Cached, populated two hours ago.
>
> - Hey module, please invalidate data at 0xffffe8ff0002cb00
> - Sure thing.
>
> - Hey module, please record all accesses to 0xffffe8ff0006bbf0.
> - OK, ask me again for stats-5
> ...
> - Hey module, what's in stats-5?
> - So far, 41 accesses by 22 users.
>
> Now, the question is, what is a good design to expose this information.
>
> I think that the most reasonable way to interact with userspace is
> through a debugfs file.

In my view, I usually prefer using ioctl-over-character device with
*strongly* defined API / structures. Looking at the examples, your API
should be fairly simple:
typedef struct _debug_request_
{
    request_t request_type;
    void *request_pointer;
    answer_t answer_type;
    union answer_data {
          int __out error_code;
          int   __out next_answer_time;
          struct access_info {
                 int __out access_count;
                 int __out user_count;
          };
    };
} debug_request_t;

If you make sure you copy_from/to_user this structure in-and-out of
user-space, you get fairly robust interface in a fairly short and easy
to debug code.
(E.g. compared to having to parse user-supplied text string in kernel
mode or using debugfs [partial read/write, etc]).

- Gilboa