Re: Calling systemtap function from pure code

[Daniel Doron <danielmeirdoron at gmail dot com>]

I will explain....
i wanted to get as much info on kernel module insertion/removal but
the init_module/delete_module probe points do not provide enough
information. So , I thought about load_module() which does provide
adequate info via struct module (local var),  but this gets filled
along the way and I can;t really have access to it via systemtap. So I
thought the simplest way to access that struct is though notification
chain, i.e. use register_module_notifier() and extract the info from
the struct there and then print it via systemtap.
So I guess the right question would be how to access the STAP print
function/mechanism/buffer...from standard kernel code.

Thanks.

On Thu, Sep 14, 2017 at 12:22 AM, David Smith <dsmith@redhat.com> wrote:
> On Wed, Sep 13, 2017 at 9:41 AM, Daniel Doron <danielmeirdoron@gmail.com> wrote:
>> Hi,
>>
>> Is it possible to call a stap function from kernel code (guru mode)?
>
> No, not really.
>
> Yes, all stap language functions get translated to a C function.
> However, you can't call that function easily, for several reasons:
>
> 1) The stap language function name gets mangled when it gets
> translated to C. That mangling isn't documented and changes from time
> to time. The last time it changed was when we implemented function
> overloading. We have no plans to document the current mangling scheme
> (you could certainly figure it out looking at the generated output)
> and we reserve the right to change the mangling scheme in the future.
>
> 2) Even if you know the correct C function name for a stap language
> function, you can't easily call it. Function parameters aren't passed
> on the stack, they are passed in a context structure, which once again
> is undocumented and subject to change.
>
> I'm not really sure what you are really trying to do here, but if you
> need to call common code from a stap language function and a C
> function, I'd put that functionality in a C function, then write a
> stap language wrapper for it. Something like the following (untested):
>
> ====
> %{
>
> int internal_log_event(char *str)
> {
>     printk(KERN_INFO "name: %s\n", str);
>     return 0;
> }
>
> %}
>
> function log_event:long(str:string)
> %{
>     STAP_RETVALUE = internal_log_event(STAP_ARG_str);
> %}
>
> function call_me()
> {
>     log_event("hello world")
> }
>
> probe begin {
>     call_me()
> }
> ====
>
> Note that the STAP_ARG_ function argument prefix and STAP_RETVALUE
> item are documented and are supported.
>
> If this doesn't answer your question, you'll need to let us know what
> you are really trying to do.
>
> --
> David Smith
> Principal Software Engineer
> Red Hat