linux-kernel - Re: [PATCH 0/3] ftrace: Introduce PERMANENT ftrace

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Date:   Wed, 9 Oct 2019 13:22:34 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Joe Lawrence <joe.lawrence@...hat.com>
Cc:     Miroslav Benes <mbenes@...e.cz>, rostedt@...dmis.org,
        jikos@...nel.org, jpoimboe@...hat.com, mingo@...hat.com,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: [PATCH 0/3] ftrace: Introduce PERMANENT ftrace_ops flag

On Tue 2019-10-08 15:35:34, Joe Lawrence wrote:
> On Mon, Oct 07, 2019 at 10:17:11AM +0200, Miroslav Benes wrote:
> > Livepatch uses ftrace for redirection to new patched functions. It is
> > thus directly affected by ftrace sysctl knobs such as ftrace_enabled.
> > Setting ftrace_enabled to 0 also disables all live patched functions. It
> > is not a problem per se, because only administrator can set sysctl
> > values, but it still may be surprising.
> > 
> > Introduce PERMANENT ftrace_ops flag to amend this. If the
> > FTRACE_OPS_FL_PERMANENT is set, the tracing of the function is not
> > disabled. Such ftrace_ops can still be unregistered in a standard way.
> > 
> > The patch set passes ftrace and livepatch kselftests.
> > 
> > Miroslav Benes (3):
> >   ftrace: Make test_rec_ops_needs_regs() generic
> >   ftrace: Introduce PERMANENT ftrace_ops flag
> >   livepatch: Use FTRACE_OPS_FL_PERMANENT
> > 
> >  Documentation/trace/ftrace-uses.rst |  6 ++++
> >  Documentation/trace/ftrace.rst      |  2 ++
> >  include/linux/ftrace.h              |  8 +++--
> >  kernel/livepatch/patch.c            |  3 +-
> >  kernel/trace/ftrace.c               | 47 ++++++++++++++++++++++++-----
> >  5 files changed, 55 insertions(+), 11 deletions(-)
> > 
> > -- 
> > 2.23.0
> > 
> 
> Hi Miroslav,
> 
> I wonder if the opposite would be more intuitive: when ftrace_enabled is
> not set, don't allow livepatches to register ftrace filters and
> likewise, don't allow ftrace_enabled to be unset if any livepatches are
> already registered.  I guess you could make an argument either way, but
> just offering another option.  Perhaps livepatches should follow similar
> behavior of other ftrace clients (like perf probes?)

I am not sure that I understand it correctly.

ftrace_enables is a global flag. My expectation is that it can be
manipulated at any time. But it should affect only ftrace handlers
without FTRACE_OPS_FL_PERMANENT flag.

By other words, the handlers with FTRACE_OPS_FL_PERMANENT flag and
only these handlers should ignore the global flag.

To be even more precise. If a function has registered more ftrace
handlers then the global ftrace_enable setting shold affect only
the handlers without the flag.

Is this the plan, please?

Best Regards,
Petr