Conditional dtrace based on stack

[Farhan Khan]

Is there a way to execute an if-statement only based on if a function exists in the stack?

Something like this:

fbt:kernel:ieee80211-whatever-here:entry{
    if (myfunction is in the stack) {
        // Do a print here
    }
}

I am trying to monitor a function call in the ieee80211 stack and only want to monitor it when it is called by a certain function or anywhere within a kernel module.

Thanks!

 

[ralphbsz]

Looking at it purely from the programming language point of view, ignoring dtrace functionality:

There is no standard C or C++ way to do this. Matter-of-fact, it is antithetical to good software design. In object-oriented programming, one could simulate it by using virtual functions and calling via different objects; in non-OO standard C, the same thing could be done by changing the function signatures and passing an extra flag along.

On the other hand, it is possible (although not in a standard-conforming way) for a function to examine the call stack. It's not even terribly difficult; most software engineers have at some point in their careers had to write a "dump stack trace" module which does exactly that. One of the complexities is that the stack only contains "numbers" (pointers, integers, and floats), and to translate it into human-readable stuff (like the string "myfunction"), one needs to decode the debugging information that's in the object code. Doable, but tedious.

Here is my suggestion: Modify "myfunction" to print a message on entry and exit. Then modify the ieee80211-whatever-here:entry function from able to also print a message. Now you have too many messages, because sometimes that function will be called without myfunction on the stack. That's OK, write a little awk script that filters the output to only look for the following pattern: myfunction enters, then iee80211-whatever-here prints, then myfunction exits.

 

[tobik@]

Is there a way to execute an if-statement only based on if a function exists in the stack?

Something like this:

fbt:kernel:ieee80211-whatever-here:entry{
    if (myfunction is in the stack) {
        // Do a print here
    }
}

I am trying to monitor a function call in the ieee80211 stack and only want to monitor it when it is called by a certain function or anywhere within a kernel module.

Thanks!

I don't really know DTrace that well, but can't you do something like this?

fbt:kernel:myfunction:entry
{
	self->myfunction_on_stack = 1
}

fbt:kernel:myfunction:return
{
	self->myfunction_on_stack = 0
}

fbt:kernel:ieee80211-whatever-here:entry
/self->myfunction_on_stack/
{
	// Do a print here
}

 

[Farhan Khan]

That's exactly it! Set a variable to true (1) and later check if its set. Thanks a lot!