Re: [RFC][PATCH 1/3] ftrace/x86: Add dynamic allocated trampoline for ftrace_ops

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 04 Jul 2014 22:32:44 +0900
Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote:

> (2014/07/04 5:07), Steven Rostedt wrote:
> > +
> > +void arch_ftrace_update_trampoline(struct ftrace_ops *ops)
> > +{
> > +	unsigned char *new;
> > +	unsigned long start_offset;
> > +	unsigned long call_offset;
> > +	unsigned long offset;
> > +	unsigned long ip;
> > +	int ret;
> > +
> > +	if (ops->trampoline) {
> > +		/*
> > +		 * The ftrace_ops caller may set up its own trampoline.
> > +		 * In such a case, this code must not modify it.
> > +		 */
> > +		if (!(ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP))
> > +			return;
> 
> Just a question, what happen if the ftrace_ops caller sets up a trampoline which is
> not compatible to the ftrace's trampoline, and the ftrace_ops conflicts on a IP with other
> ftrace_ops? I guess in that case ftrace will use the loop callback on the IP, but since
> the trampoline is not compatible, the result will not be same, is that right? :)

If the caller sets up a trampoline, it must not set the ALLOC_TRAMP
flag. If you look at the comment about that flag it states this:

+ * ALLOC_TRAMP - A dynamic trampoline was allocated by the core code.
+ *            The arch specific code sets this flag when it allocated a
+ *            trampoline. This lets the arch know that it can update the
+ *            trampoline in case the callback function changes.
+ *            The ftrace_ops trampoline can be set by the ftrace users, and
+ *            in such cases the arch must not modify it. Only the arch ftrace
+ *            core code should set this flag.


That last line is important. Only the arch ftrace code (the one that
may modify it with arch_ftrace_update_trampoline should set the
ALLOC_TRAMP flag. That's how it knows if it can modify it or not.

The function_graph tracer sets up its own trampoline. Although it needs
to go through some hoops there because it shares the ftrace_ops with
the function tracer. Thus, it has to store the trampoline and this flag
before registering ftrace ops, and then it has to restore it when it
unregisters.

-- Steve