Re: [PATCH 0/3] ftrace: Introduce PERMANENT ftrace_ops flag
From: Petr Mladek
Date: Wed Oct 09 2019 - 07:22:38 EST
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?