Accessing probe info synchronously

[Frank Ch. Eigler]

Lasse Bigum <zenith@zenith.dk> writes:

> So my question is this: Is there any way to access the information
> that SystemTap probes directly from kernel space, without using
> relayfs?

In a sense, sure: the systemtap script "knows" the instant the
application is invoking a system call, and could take some action.

But because normal systemtap scripts are normally non-intrusive, they
will let the calling thread resume pretty soon (a couple of
microseconds probably).  This means that they likely won't be able to
communicate that knowledge to a slow/faraway (user-space) tool in time
for that tool to take control.

There have been experiments with systemtap scripts actively blocking
threads (by enqueuing a SIGSTOP signal), and this might be part of a
solution for your problem, but this area hasn't been thoroughly
explored.  (Hmm, maybe there is a way of supporting this generally via
prologue code in the syscall tapset...)

- FChE