Re: [PATCH V4 08/13] ftrace: Add perf text poke events for ftrace trampolines

Peter Zijlstra
2020-04-01T10:10:21Z

On Wed, Mar 04, 2020 at 11:06:28AM +0200, Adrian Hunter wrote:
> Add perf text poke events for ftrace trampolines when created and when
> freed.

If I'm not mistaken that ends up like so:

static void ftrace_update_trampoline(struct ftrace_ops *ops)
{
+ unsigned long trampoline = ops->trampoline;
+
arch_ftrace_update_trampoline(ops);
+ if (ops->trampoline && ops->trampoline != trampoline &&
> + (ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP)) {
> + /* Add to kallsyms before the perf events */
+ ftrace_add_trampoline_to_kallsyms(ops);
> + perf_event_ksymbol(PERF_RECORD_KSYMBOL_TYPE_OOL,
> + ops->trampoline, ops->trampoline_size, false,
> + FTRACE_TRAMPOLINE_SYM);
> + /*
> + * Record the perf text poke event after the ksymbol register
> + * event.
> + */
> + perf_event_text_poke((void *)ops->trampoline, NULL, 0,
> + (void *)ops->trampoline,
> + ops->trampoline_size);
}
}

And afaict, that is wrong.

The thing is; arch_ftrace_update_trampoline() can actually *update* an
existing trampoline, as per the name. Yes it also creates a trampoline
if there isn't one already, but if there already is one, it will modify
it in-place.

I see the appeal of having this event in generic code; but I'm thinking
you'll need the update even in arch code anyway, at which point it'd
probably be easier to do all of this in arch code.