Re: uprobes-via-utrace notes

[Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>]

On Mon, Jul 17, 2006 at 01:14:17AM -0700, Roland McGrath wrote:
> 
> > When the breakpoint is hit, kprobes shows no interest, and do_trap()
> > handles it.  utrace calls uprobes's report_signal callback with
> > action = SIGNAL_CORE and info.si_signo = SIGTRAP.  (If there's a gdb
> > breakpoint there, it looks like action could be SIGNAL_IGN by the
> > time uprobes sees it.  TBD)  
> 
> That's correct.  The way that multiple engines interact is something of an
> unresolved area in utrace.  As things stand, if ptrace was in the list
> first then it will get the callback first, tell the ptracer about it, and
> change the action to UTRACE_SIGNAL_IGN.  A "noninvasive" tracing engine
> would want to be the one to go first, and hide the signal from other engines.
> There isn't currently any good way to control the order of the list.

How about adding a "priority" to be indicated at the time of
utrace_attach(), something akin to what we have with the kernel
"notifier" infrastructure?

All engines that don't have such a priority indicated can have a default
priority and all engines with the same priority can then be invoked in
registered order.

Ananth