Question

I’m trying to read the initial arguments that was passed into the function in my return probe. Unlike the entry probe, the argument variables(arg0,arg1,…) in the return probe do not contain the initial parameters, and I’m not sure how I would be able to retrieve those values.

Also, I would like to avoid storing the values in global variables because of the concurrency issues.

Answer 1

You can save the parameters in thread-local storage, e.g.

pid$target:foo:bar:entry
{
    self->arg0 = arg0;
    self->arg1 = arg1;
    /*
     * Avoid the race in which dtrace(1) attaches to the victim during
     * the window between the two probes.
     */
    self->trace = 1;
}

pid$target:foo:bar:return
/self->trace/
{
    printf("arg0 = 0x%x, arg1 = 0x%x\n", self->arg0, self->arg1);
    /* Deallocate the thread-local storage. */
    self->arg0 = 0;
    self->arg1 = 0;
}