Re: Calling systemtap function from pure code

[Arkady <arkady dot miasnikov at gmail dot com>]

I am not sure that I understand the goal. Please provide more details
or rephrase the mail.

This is an example of a patch in the SystemTap which allows to add
code to the load/remove module flow
https://github.com/larytet/SystemTap/commit/6e4c99f96c9ecce9508f2c8612e8bace2ac91ae5

On Thu, Sep 14, 2017 at 10:20 AM, Daniel Doron
<danielmeirdoron@gmail.com> wrote:
> 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